Classification of Metals, Conductors and Semiconductors

Solids can be classified as metals, semiconductors or insulators based

on conductivity or resistivity and energy bands in electronics. This

helps us understand the band theory and the importance of valence and

conduction bands in solids. Let us also study the types of

semiconductor.

Classification Based on Conductivity

The relative values of electrical conductivity (σ) and resistivity

(ρ=1/σ) help classify solids as:

● Metals – These are solids which have very low resistivity or

very high conductivity). Hence,

σ ~ 102 – 108 S/m

ρ ~ 10-2 – 10-8 Ωm

● Insulators – These are solids which have very high resistivity or

very low conductivity. Hence,

σ ~ 10-11 – 10-19 S/m

ρ ~ 1011 – 1019 Ωm

● Semiconductors – These are solids which have resistivity or

conductivity values between those of metals and insulators.

Hence,

σ ~ 105 – 10-6 S/m

ρ ~ 10-5 – 106 Ωm

It is important to note that these values are indicative and could go

outside the range as well. Let’s shift our focus to semiconductors.

These can be of the following types:

Types of Semiconductor

1. Elemental semiconductors – available naturally like Silicon

(Si) and Germanium (Ge)

2. Compound semiconductors – made by compounding two or

more metals together. They are sub-divided into these

categories:

a. Inorganic; like CdS, GaAs, CdSe, etc.

b. Organic; like anthracene, doped phthalocyanine, etc.

c. Organic polymers; like polypyrrole, polyaniline,

polythiophene, etc.

Most semiconductor devices available are based on elemental and

compound inorganic semiconductors. Eventually, organic

semiconductors were used and semiconducting polymers were

developed.

Classification Based on Energy Bands

Bohr’s model of the atom states that the energy of an electron is

determined by the orbit in which it revolves. This is true for an

isolated atom. In a solid, atoms are close together. Therefore, electrons

from neighbouring atoms come very close even overlap at times.

Hence, the motion of an electron is different in a solid as compared to

an isolated atom.

In an atom, electrons in the innermost orbits, which are filled are

called Valence electrons. On the other hand, electrons in the outer

orbits that do not fill the shell completely are called Conduction

electrons.

In a crystal, every electron has a unique position and different pattern

of surrounding charges. Hence, each electron has a different energy

level. These energy levels with a continuous variation of energy form

the Energy bands. The energy band which includes the energy levels

of the valence electrons is called Valence band. Also, the energy band

above it is called Conduction band. If there is no external energy

supplied, then the valence electrons stay in the valence band.

Now, electrons move from the valence band to the conduction band

when the lowest level in the conduction band is lower than the highest

level in the valence band. Usually, the conduction band is empty.

Classification of Metals, Semiconductors, and Insulators

● In case of metallic conductors, conduction band overlaps on the

electrons in the valence band.

● In insulators, there is a large gap between both these bands.

Hence, the electrons in the valence band remain bound and no

free electrons are available in the conduction band.

● Semiconductors have a small gap between both these bands.

Some valence electrons gain energy from external sources and

cross the gap between the valence and conduction bands. By

this movement, they create a free electron in the conduction

band and a vacant energy level in the valence band for other

valence electrons to move. This creates the possibility of

conduction.

Let’s understand this better with the following diagrams and

explanations:

Case 1.

Ec denotes the lowest energy level of the conduction band and Ev

denotes the highest energy level of the valence band. The above

diagram describes a solid where the valence band is partially empty.

Hence, electrons from the lower energy levels can move to the higher

levels making conduction possible. Also, the resistivity of such solids

is low or the conductivity is high.

Case 2.

Ec and Ev are the same as case 1 and Eg is the energy gap. This

diagram describes a solid where the conduction and valence bands

overlap each other. Hence, electrons can easily move from the valence

to the conduction band. This makes a large number of electrons

available for conduction. Also, the resistivity of such solids is low or

the conductivity is high.

Case 3.

This diagram describes a solid where the energy gap (Eg) is very large

(>3 eV). Due to this large gap, electrons cannot be excited to move

from the valence to the conduction band by thermal excitation. Hence,

there are no free electrons in the conduction band and no conductivity.

These are insulators.

Case 4.

This diagram describes a solid where the energy gap is small (< 3 eV).

Since the gap is small, some electrons acquire enough energy even at

room temperature and enter the conduction band. These electrons can

move in the conduction band increasing the conductivity of the solid.

These are semiconductors. The resistivity of semiconductors is lower

than that of insulators but higher than that of metals.

Solved Examples for You

Question: Name the different types of semiconductors with examples.

Solution: Semiconductors are of the following types:

● The ones available naturally like Silicon and Germanium –

Elemental Semiconductors. Also, the ones made by

compounding two or more metals together – Compound

Semiconductors.

● Further, compound semiconductors are of three types –

inorganic (CdS, GaAs, etc.), organic (anthracene, etc.) and

organic polymers (polypyrrole, polyaniline, etc.).

Question: What are valence and conduction bands?

Solution: In a crystal, every electron has a different energy level.

These different energy levels along with the continuous variation of

energy are called Energy bands. The energy band which includes the

energy levels of the valence electrons is called the valence band. And

the band above it is called the conduction band.

Intrinsic Semiconductor

An Intrinsic Semiconductor is the purest form of a semiconductor,

elemental, without any impurities. Naturally available elements like

silicon and germanium are best examples of an Intrinsic

Semiconductor. Let’s know them in further more detail.

The Lattice Structure of Elements

They are also called diamond-like structures. In such structures, every

atom is surrounded by four neighbouring atoms. Now, both Si and Ge

have four valence electrons and in the crystalline structure, each atom

shares one of its valence electrons with each of its four neighbours.

Also, it takes one electron from each of its neighbours. This shared

pair of electron is called a Covalent bond or a Valence bond. This is

how the Si or Ge structure looks in two-dimensions with emphasis on

the covalent bond:

Also, the above image shows the structure with all bonds intact. This

is possible only at low temperatures. As the temperature increases and

more energy becomes available to the valence electrons, they break

away leading to an increase in conductivity of the element.

Now, the thermal energy ionizes only a few atoms. This ionization

creates a vacancy in the bond. When an electron, having charge –q,

gets excited due to the thermal energy, it breaks free from the bond.

This leaves a vacancy there with effective charge +q. This vacancy

with an effective positive electronic charge is a hole.

The hole also behaves like a free particle but with a positive charge. In

intrinsic semiconductors, the number of free electrons is equal to the

number of holes and is called the intrinsic carrier concentration.

Intrinsic Semiconductor – The Movement of Holes

Another interesting property of semiconductors is that like the

electrons, the holes move too. Consider the following image:

In the image above, you can see that an electron, after being excited

due to thermal energy breaks itself away from the bond, generating a

free electron. (Site1) A vacancy is created at the site from where the

electron releases itself. Now, imagine that an electron from Site 2, as

shown in the image, jumps to the hole or vacancy created in Site 1.

The hole will now have moved from Site1 to Site2 as shown in the

image below:

It is important to observe that the electron freed from Site 1 is not

involved in the movement of the hole. It moves independently like a

conduction electron contributing to electron current (Ie) under an

applied electric field. Also, movement of the hole is actually a

movement of bound electrons.

Under an electric field, these holes move towards the negative

potential generating hole current (Ih). Hence, the total current (I) is:

I = Ie + Ih

Another important thing to remember is that apart from the process of

the generation of free electrons and holes, a process of recombination

takes place simultaneously. In this process, the electrons recombine

with the holes. In the state of equilibrium, the rate of generation is

equal to the rate of recombination.

Intrinsic Semiconductor at T = 0K

At T = 0K, an intrinsic semiconductor will behave like an insulator.

Structurally, there is a small energy gap between the valence and

conduction bands in a semiconductor. When the temperature is low,

the electrons are not excited enough to jump to a higher energy state.

The image below explains how at T = 0K, the electrons stay in the

valence band and there is no movement to the conduction band.

When the temperature increases, at T > 0K, some electrons get

excited. These electrons jump from the valence to the conduction

band. Here is how it will look:

Solved Examples for You

Question: How many valence electrons does silicon have?

A. 1

B. 2

C. 3

D. 4

Solution: Option (D) Silicon has 4 valence electrons.

Question: If we add an impurity to a metal those atoms also deflect electrons.

Therefore,

A. Electrical and thermal conductivities both increase

B. Electrical and thermal conductivities both decrease

C. Thermal conductivity increases but electrical conductivity

decreases

D. Electrical conductivity increases but thermal conductivity

decreases

Solution: Option (B).

If the number of electrons increases, collisions between them also

increase thus increasing the thermal and electrical resistance. So,

electrical and thermal conductivities both decrease.

Extrinsic semiconductors

Semiconductors can be broadly classified into Intrinsic and Extrinsic

Semiconductors. Intrinsic Semiconductors start conducting at

temperatures above the room temperature, developing important

electronic devices using these can pose a problem. This led to a need

for improving the conductivity of intrinsic semiconductors. Let’s find

out more.

Extrinsic Semiconductors

After some experiments, scientists observed an increase in the

conductivity of a Semiconductor when a small amount of impurity

was added to it. These materials are Extrinsic Semiconductors or

impurity Semiconductors. Another term for these materials is ‘Doped

Semiconductor’. The impurities are dopants and the process – Doping.

An important condition to doping is that the amount of impurity added

should not change the lattice structure of the Semiconductor. To

achieve this the size of the dopant and Semiconductor atoms should be

the same.

Types of Dopants in Extrinsic Semiconductors

Crystals of Silicon and Germanium are doped using two types of

dopants:

1. Pentavalent (valency 5); like Arsenic (As), Antimony (Sb),

Phosphorous (P), etc.

2. Trivalent (valency 3); like Indium (In), Boron (B), Aluminium

(Al), etc.

The reason behind using these dopants is to have similarly sized atoms

as the pure semiconductor. Both Si and Ge belong to the fourth group

in the periodic table. Hence, the choice of dopants is from the third

and fifth group. This ensures that size of the atoms is not much

different from the fourth group. Hence, the trivalent and pentavalent

choices. These dopants give rise to two types of semiconductors:

1. n-type

2. p-type

n-type semiconductor

An n-type semiconductor is created when pure semiconductors, like Si

and Ge, are doped with pentavalent elements.

As can be seen in the image above, when a pentavalent atom takes the

place of a Si atom, four of its electrons bond with four neighbouring

Si atoms. However, the fifth electron remains loosely bound to the

parent atom. Hence, the ionization energy required to set this electron

free is very small. Thereby, this electron can move in the lattice even

at room temperature.

To give you a better perspective, the ionization energy required for

silicon at room temperature is around 1.1 eV. On the other hand, by

adding a pentavalent impurity, this energy drops to around 0.05 eV.

It is important to remember that the number of electrons made

available by the dopant atoms is independent of the ambient

temperature and primarily depends on the doping level. Also, as the

temperature rises, the Si atoms free some electrons and generate some

holes. But, the number of these holes is very small. Hence, at any

given point in time, the number of free electrons is much higher than

the number of holes. Also, due to recombination, the number of holes

reduce further.

In a nutshell, when a semiconductor is doped with a pentavalent atom,

electrons are the majority charge carriers. On the other hand, the holes

are the minority charge carriers. Therefore, such extrinsic

semiconductors are called n-type semiconductors. In an n-type

semiconductor,

Number of free electrons (ne) >> Number of holes (nh)

p-type semiconductor

A p-type semiconductor is created when trivalent elements are used to

dope pure semiconductors, like Si and Ge. As can be seen in the image

above, when a trivalent atom takes the place of a Si atom, three of its

electrons bond with three neighbouring Si atoms. However, there is no

electron to bond with the fourth Si atom.

This leads to a hole or a vacancy between the trivalent and the fourth

silicon atom. This hole initiates a jump of an electron from the outer

orbit of the atom in the neighbourhood to fill the vacancy. This creates

a hole at the site from where the electron jumps. In simple words, a

hole is now available for conduction.

It is important to remember that the number of holes made available

by the dopant atoms is independent of the ambient temperature and

primarily depends on the doping level. Also, as the temperature rises,

the Si atoms free some electrons and generate some holes. But, the

number of these electrons is very small. Hence, at any given point in

time, the number of holes is much higher than the number of free

electrons. Also, due to recombination, the number of free electrons

reduce further.

In a nutshell, when a semiconductor is doped with a trivalent atom,

holes are the majority charge carriers. On the other hand, the free

electrons are the minority charge carriers. Therefore, such extrinsic

semiconductors are called p-type semiconductors. In a p-type

semiconductor,

Number of holes (nh) >> Number of free electrons (ne)

Important note: The crystal maintains an overall charge neutrality. The

charge of additional charge carries is equal and opposite to that of the

ionized cores in the lattice.

Energy Bands of Extrinsic Semiconductors

In extrinsic semiconductors, a change in the ambient temperature

leads to the production of minority charge carriers. Also, the dopant

atoms produce the majority carriers. During recombination, the

majority carriers destroy most of these minority carriers. This leads to

a decrease in the concentration of the minority carriers.

Therefore, this affects the energy band structure of the semiconductor.

In such semiconductors, additional energy states exist:

● Energy state due to donor impurity (ED)

● Energy state due to acceptor impurity (EA)

The above energy band diagram is of n-type Si semiconductor. Here

you can see that the energy level of the donor (ED) is lower than that

of the conduction band (EC). Hence, electrons can move into the

conduction band with minimal energy (~0.01 eV). Also, at room

temperature, most donor atoms and very few Si atoms get ionized.

Hence, the conduction band has most electrons from the donor

impurities.

The above energy band diagram is of p-type Si semiconductor. Here

you can see that the energy level of the acceptor (EA) is higher than

that of the valence band (EV). Hence, electrons can move from the

valence band to the level Ea, with minimal energy. Also, at room

temperature, most acceptor atoms are ionized.

This leaves holes in the valence band. Hence, the valence band has

most holes from the impurities. The electron and hole concentration in

a semiconductor in thermal equilibrium is:

ne × nh = ni2

Do you know what is p-n Junction ?

Solved Examples for You

Question: What are Extrinsic Semiconductors?

Solution: Extrinsic Semiconductors are pure semiconductors which

conduct even at room temperature. This is achieved by adding

impurities to the pure semiconductor.

Question: Who are the major charge carriers in n-type and p-type

semiconductors?

Solution: In n-type semiconductors, electrons are the major charge

carriers. In p-type semiconductors, holes are the major charge carriers.

p-n Junction

Understanding the formation and functioning of a p-n Junction is

paramount to analyzing the working of semiconductor devices. Let’s

look at how a p-n Junction is formed and how it behaves under a bias

(or an externally applied voltage).

Formation of a p-n Junction

Let’s imagine a thin p-type silicon (p-Si) semiconductor wafer. Now,

we add a pentavalent impurity to it. Due to this, a small part of the

p-Si wafer is converted into an n-Si. Now, the wafer contains a

p-region and an n-region with a metallurgical junction between the

two. Two important processes take place during the formation of a p-n

Junction:

1. Diffusion

2. Drift

In an n-type semiconductor, the concentration of electrons is more

than that of holes. On the other hand, in a p-type semiconductor, the

concentration of holes is more than that of electrons.

When a p-n junction is being formed, holes diffuse from the p-side to

the n-side (p→n) while electrons diffuse from the n-side to the p-side

(n→p). This happens due to the concentration gradient across p and n

sides. This gives rise to a diffusion current across the junction. Let’s

look at both the scenarios:

Electron Diffuses from n→p

This diffusion leaves an ionized donor (or a positive charge) on the

n-side. This donor is bonded to the surrounding atoms and is

immobile. As more and more electrons start diffusing to the p-side, a

layer of positive charge (or positive space-charge region) in the n-side

of the junction is formed.

Learn Junction Transistor here in detail.

Hole Diffuses from p→n

This diffusion leaves an ionized acceptor (or a negative charge) on the

p-side. As more and more holes start diffusing to the n-side, a layer of

negative charge (or negative space-charge region) on the p-side of the

junction is formed.

Since the diffusion of electrons and holes across the junction depletes

the region of its free charges, these space charge regions together are

called the depletion region.

This process is depicted in the image above. The thickness of the

depletion region is merely around one-tenth of a micrometre. Also, an

electric field develops, directed from the p-side to the n-side of the

junction. This is because of the positive space-charge region on the

n-side and the negative space-charge region on the p-side of the

junction.

This electric field is responsible for the movement of electrons from

the p-side to the n-side and holes from the n-side to the p-side. This

motion of charged carriers due to the electric field is called drift.

Hence, a drift current starts which is opposite in direction to the

diffusion current. This is also seen in the image above.

Study Semiconductor Diode here in detail.

The Last Stages of Formation of a p-n Junction

When the diffusion starts, the diffusion current is large and the drift

current is very small. As diffusion continues, the space-charge regions

on either side of the junction start extending. This strengthens the

electric field and eventually the drift current. The process continues

till diffusion current = drift current. This is how a p-n junction is

formed.

Barrier Potential

In the state of equilibrium, there is no current in a p-n junction. A

difference of potential develops across the junction of the two regions

due to the loss of electrons by the n-region and the subsequent gain by

the p-region. The polarity of the potential opposes the further flow of

carriers to maintain the state of equilibrium. This is the Barrier

Potential.

Solved Examples for You

Question: Why is a photodiode is used in reverse bias?

A. Photocurrent flows only in reverse bias

B. Thermally generated current is large in reverse biased as

compared to the photocurrent

C. Photocurrent is large in forward bias as compared to the

forward bias current

D. Photocurrent is significant in reverse bias as compared to the

reverse bias current

Solution: Option (D)

When a photo-diode is reverse biased, the width of depletion layer

increases as compared to forward biased and a small reverse current

(dark current) flows through the diode. Now, when the light is incident

on the junction, electron-hole pairs are generated in depletion layer in

a big amount (due to broad depletion layer) and these charge carriers

can easily cross the barrier, hence contribute to current across the

diode. We can say that in reverse bias, diode changes the incident light

to current, more significantly due to broad depletion layer i.e.

photocurrent is significant in reverse bias as compared to the forward

bias current.

Question: The curve represents the I−V characteristics of which

optoelectronic device?

A. Solar Cell

B. Photo-diode

C. LED( Light Emitting Diode)

D. None of these

Solution: Option (A) Solar Cell

Question: From the adjacent circuit, the output voltage is

A. 10 V

B. 100 V

C. 90 V

D. 0 V

Solution: Here, Zener diode is connected parallel to the input voltage

source. Zener diode is known as a voltage regulating diode hence, the

output voltage will be equal to the voltage across Zener diode i.e. 10V

Special Purpose p-n Junction Diode

Let’s look at some p-n junction diodes, developed for specific

applications. We will cover Zener diode and Opto-electronic junction

devices including photodiodes, light emitting diode, and solar cells.

Zener Diode

C. Zener designed a p-n junction diode in reverse bias to operate in the

breakdown region. It is used as a voltage regulator. The symbol for a

Zener diode is:

It is made by doping both p and n sides of the junction heavily. This

leads to the formation of a very thin depletion region and an extremely

high electric field across the junction even for a small reverse bias

voltage (~5 V). Here is a quick look at the V-I characteristics of a

Zener diode:

In the above graph, you can see that as the reverse bias voltage (V)

reaches the breakdown voltage of the Zener diode (Vz), there is a large

change in current. Also, note that for a negligible change in the

reverse bias voltage, a large change in current is produced. Hence, in a

Zener diode, the voltage remains nearly constant even when there is a

large change in current. This property is used to regulate supply

voltages.

Why does the current increase suddenly?

This is really interesting. We know that the reverse current is created

due to the movement of the minority charge carriers. The electrons

from the p-side move to the n-side and the holes from the n-side move

to the p-side. As we increase the reverse bias voltage, the electric field

at the junction becomes stronger.

At V=Vz, the electric field is strong enough to pull valence electrons

from the host atoms on the p-side which are accelerated to n-side. This

explains the sudden increase in current. This process of emission of

electrons from the host atoms due to a high electric field is known as

Internal Field Emission or Field Ionization.

Zener Diode as a Voltage Regulator

By now we know that even if we use a rectifier to convert AC input

voltage, the output fluctuates too. A Zener diode is used to get

constant DC voltage from a DC unregulated output of a rectifier.

This is a circuit diagram of a Zener diode used as a voltage regulator.

Here the unregulated DC output of a rectifier is connected to it

through a series of resistance (Rs). Also, it is in reverse bias. Here is

how it works:

● An increase in input voltage increases the current through Rs

and the diode

● The voltage drop across Rs increases

● The voltage across the Zener diode does not change

This happens because, in the breakdown region, Zener voltage

remains constant despite the change in current.

● A decrease in the input voltage decreases the current through

Rs and the diode

● The voltage drop across Rs decreases

● The voltage across the Zener diode does not change

Hence, an increase/decrease of the voltage drop across the Rs does not

change the voltage across the Zener diode. Hence, it acts as a voltage

regulator.

Optoelectronic Junction Devices

Till now, you have read about how semiconductor diodes behave

under electrical inputs. Now, let’s look at how they react to

photo-excitation. These are optoelectronic devices or devices in which

carriers are generated by photons. Let’s look at three such devices:

1. Photodiodes to detect optical signals

2. Light Emitting Diodes (LEDs)

3. Solar cells

Photodiodes

This is a special p-n junction diode operated in reverse bias and

designed with a transparent window to allow light to fall on it. When

photons with energy (hv) greater than the energy gap of the

semiconductor fall on it, electron-hole pairs are generated. The diode

is designed in a manner that these electron-hole pairs are generated in

or near the depletion region.

Also, the electric field at the junction ensures that the electrons and

holes are separated before they recombine. Further, the direction of

the electric current is to ensure that the electrons reach the n-side and

holes reach the p-side. This gives rise to an EMF.

Current flows on connecting an external load. Also, the magnitude of

this photocurrent depends on the intensity of the incident light. The

direct correlation between the two is easily observed on application of

a reverse bias. Hence, a photodiode can detect optical signals. Here is

a circuit diagram of a photodiode:

Light emitting diode (LED)

A LED is a heavily doped p-n junction which emits spontaneous

radiation under forward bias. It is covered in a capsule with a

transparent cover allowing the emitted light to come out. Being

forward biased, electrons move from n to p-side and holes move from

p to n-side.

Also, the concentration of the minority carriers is higher near the

junction as compared to the equilibrium concentration. Hence, at the

junction, on either side, excess minority carriers recombine with the

majority carriers. This releases energy in the form of photons. The

photons emitted have energies equal to or less than the band gap.

Another thing to note here is that the intensity of the emitted light is

small when the forward current is small. The intensity of the emitted

light increases as this current increases and reached a maximum value.

Post this, an increase in forward current leads to a decrease in light

intensity.

Commercially available LEDs can emit red, yellow, orange, green and

blue light. LEDs are used extensively in remote controls, optical

communication, etc. Here are some advantages of LEDs over

incandescent low power lamps:

● Low operational voltage and less power.

● Fast action and no warm-up time required.

● The bandwidth of emitted light is 100 Å to 500 Å or in other

words, it is nearly (but not exactly) monochromatic.

● Long life and ruggedness.

● Fast on-off switching capability.

Solar Cell

A solar cell is a p-n junction which generates EMF when solar

radiation falls on it. The working principle is similar to the photodiode

except that no external bias is applied and the junction area is much

larger to enable solar radiation incidence. This is how a simple p-n

junction solar cell looks:

When sunlight falls on a solar cell, the EMF is generated due to these

three processes:

● Generation of electron-hole pairs due to light (with hν > Eg)

close to the junction

● Separation of electrons and holes due to the electric field of the

depletion region. Electrons are swept to n-side and holes to

p-side.

● Collection – the electrons reaching the n-side are collected by

the front contact and holes reaching p-side are collected by the

back contact. Thus p-side becomes positive and n-side becomes

negative giving rise to photo-voltage.

The above diagram shows an external load connected to a solar cell

leading to a flow of photocurrent. The V-I characteristics can be

graphically shown as follows:

The correlation is shown in the fourth quadrant because the solar cell

does not draw current but supplies it to the load. Also, remember that

sunlight is not always required for a solar cell. A light having a

frequency greater than the band gap suffices.

Solved Examples for You

Question: How does a Voltage regulator using Zener diode work?

Solution: The unregulated DC output of a rectifier is connected to a

Zener diode through a series of resistance (Rs). Also, it is in reverse

bias. Here is how it works:

● An increase in input voltage increases the current through Rs

and the diode

● The voltage drop across Rs increases

● The voltage across the Zener diode does not change

This happens because, in the breakdown region, Zener voltage

remains constant despite the change in current.

● A decrease in the input voltage decreases the current through

Rs and the diode

● The voltage drop across Rs decreases

● The voltage across the Zener diode does not change

Hence, an increase/decrease of the voltage drop across the Rs does not

change the voltage across the Zener diode. Hence, it acts as a voltage

regulator.

Semiconductor Diode

Imagine a p-n junction with metallic contacts at both the ends for

application of external voltage. This is a semiconductor diode.

This is the symbolical representation of a semiconductor diode:

In the image above, the arrow indicates the direction of current when

the diode is under forward bias. It is also important to note here that

the equilibrium barrier potential can be altered. This is achieved by

applying an external voltage across the diode. Depending on how this

voltage is applied, a semiconductor diode is a forward-bias or a

reverse-bias diode.

Semiconductor Diode under Forward Bias

In the image above, you can see that an external voltage is applied

across the semiconductor diode where the p-side of the diode is

connected to the positive terminal and the n-side is connected to the

negative terminal of the battery. This diode is forward biased.

Formation of a Forward Bias Diode

Since the resistance of the depletion region (the region where there are

no charges) is very high, the applied voltage drops primarily across

this region. The drop in voltage across the p and n side of the junction

is relatively negligible. Also, the direction of the applied voltage (V)

being opposite to that of the built-in potential (V0), the depletion

layer’s width decreases and the barrier height reduce.

If the applied voltage is small, then the barrier potential is reduced

marginally below the equilibrium value. This results in a small

number of carriers crossing the junction. Hence, the current is small.

On the other hand, for a significantly high value of voltage, more

carriers have the energy to cross the junction. This leads to a higher

current.

Another thing to note is that when the voltage is applied, some

electrons crossover to the p-side and some holes cross to the n-side.

Under forward bias, this process is the minority charge injection

process (refer to image below). Hence, the minority charge

concentration (electrons on the p-side are a minority and holes on the

n-side are a minority) is significantly higher at the junction boundary.

This concentration gradient, the injected electron diffuse from the

junction-end to the far-end of the p-side. Similarly, injected holes

diffuse to the far end of the n-side. This gives rise to current too.

The total diode forward current = Hole diffusion current + Electron

diffusion current (usually in mA)

Semiconductor Diode under Reverse Bias

In the image above, you can see that an external voltage is applied

across the semiconductor diode. The n-side of the diode connects to

the positive terminal and the p-side connects to the negative terminal

of the battery. This diode is a reverse-bias diode.

Formation of a Reverse Bias Diode

Since the resistance of the depletion region (the region where there are

no charges) is very high, the applied voltage drops primarily across

this region. The drop in voltage across the p and n side of the junction

is relatively negligible.

Also, the direction of the applied voltage (V) being the same as that of

the built-in potential (V0), the depletion layer’s width widens and the

barrier height increases. This leads to a suppression of the flow of

electron to the p-side and holes to the n-side. Hence, the diffusion

current decreases to a great extent.

Due to the direction of the electric field, the electrons in the p-side and

holes in the n-side are swept to their majority zones, if they come

close to the junction. This causes drift current. The drift current is

usually of a few μA. This current is very low even in the

forward-biased diode as compared to the current due to the injected

carriers.

Critical Value of Reverse Bias Voltage

Also, a small amount of voltage applied to the diode is sufficient to

sweep the minority charge carrier to the far side of the junction. This

diode reverses current is not dependent on the voltage but on the

concentration of the minority charge carriers on both sides of the

junction.

However, the current is independent up to a critical value of reverse

bias voltage – the Breakdown Voltage (Vbr). When the voltage applied

crosses Vbr, even a small change in the bias voltage causes a huge

change in current. There is also an upper limit of current for every

diode, beyond which it gets destroyed due to overheating. This is the

rated value of current.

Experimental Study of the V-I characteristics of a Semiconductor Diode

The above diagram shows a diode connected in forward bias. The

battery connects to the diode through a potentiometer enabling us to

change the voltage for the sake of the experiment. A milliammeter

measures the current.

This diagram shows a diode connected in reverse bias. The battery

connects to the diode through a potentiometer enabling us to change

the voltage for the sake of the experiment. A microammeter (since the

expected current is low) measures the current. Here is what we

observe:

As can be seen in the graph above, in the forward biased diode,

initially the current increases almost negligibly till a certain value is

reached. Post that, the current increases exponentially even for a small

increase in diode bias voltage. This voltage is the threshold voltage.

(~0.7 V for silicon diode and ~ 0.2 V for germanium diode)

In the reverse biased diode, the current is very small and almost

remains constant with a change in bias voltage. It is the Reverse

saturation current. In some cases, beyond the breakdown voltage, the

current increases suddenly.

Hence, from this experiment, we can conclude that the p-n junction

diode allows the flow of current only in one direction, i.e.

forward-bias. Also, the forward bias resistance is lower than the

reverse bias resistance.

Solved Examples for You

Question: Define a forward-bias diode

Solution: When you apply an external voltage across a p-n junction

diode such that:

● the positive terminal of the battery connects to the p-side and

● the negative terminal to the n-side

It is a forward-bias diode.

Question: Define a reverse-bias diode

Solution When you apply an external voltage across a p-n junction

diode such that:

● the positive terminal of the battery connects to the n-side and

● the negative terminal to the p-side

It is a reverse-bias diode.

Digital Electronics and Logic Gates

We live in a digital world. Understanding digital electronics is

necessary to make the most of digitization. In this article, we will try

to understand Logic gates which are the building blocks of digital

electronics.

Continuous or Analogue Signals

If you look at amplifiers or oscillators, the input signal used is a

continuous, time-varying current or voltage. These signals are

Continuous or Analogue Signals. They look like:

Now, it is possible to have an input signal with discrete values of

voltage. Or in simple words, an input signal which provides Level 0 or

Level 1 voltage. These signals are Digital Signals; refer to the diagram

below:

Binary numbers (0 and 1) are used to represent such signals. Only

these two levels of voltage are used in digital electronics. Both input

and output are either Level 0 or Level 1.

A classic example of digital signals: When you switch on a light in

your house, the output value is Level 1 which turns the light ON.

When you switch it off, the output value drops to Level 0, switching

the light OFF.

Logic Gates process digital signals in a specific manner and are used

in calculators, digital watches, etc. Let’s understand them better.

Logic Gates

For digital devices to function the way they do, a logic needs to be

established between the input and output voltages. This is achieved by

using a gate or a digital circuit that follows the logical relationship.

Since the control the flow of information based on a certain logic, they

are called logic gates.

Each logic gate is indicated by a symbol and has a truth table which

displays all possible input-output combinations. In short, the truth

tables help understand the behaviour of the logic gates. These gates

are made using semiconductor devices. The five most commonly used

logic gates are:

● NOT

● AND

● OR

● NAND

● NOR

Let’s study each one of them in detail.

1. NOT gate

A simple gate with one input and one output, a NOT gate simply

inverts the input signal. So, the output is ‘0’ when the input is ‘1’ and

vice-versa. Due to this property, a NOT gate is also known as an

inverter. Here is the commonly used symbol for a NOT gate:

And, the truth table for a NOT gate is as follows:

Input (A) Output (Y)

0 1

1 0

2. OR Gate

An OR gate has two or more inputs and one output. The logic of this

gate is that the output would be 1 when at least one of the inputs is 1.

Simply put, the output is high when any of the input is high. The

commonly used symbol for an OR gate is as follows:

And, the truth table for an OR gate is as follows:

Input Output

A B Y

0 0 0

0 1 1

1 0 1

1 1 1

3. AND gate

An AND gate also has two or more inputs and a single output. In this

gate, the output is 1 when all the inputs are 1. In other words, the

output is high when all the inputs are high. The most commonly used

symbol for an AND gate is as follows:

And the truth table for the AND gate is as follows:

Input Output

A B Y

0 0 0

0 1 0

1 0 0

1 1 1

4. NAND gate

A NAND gate is simply an AND gate followed by a NOT gate.

Confused? It’s simple. The output is 1 only when all inputs are NOT

1. Or the output is high when all the inputs are NOT high and at least

one of them is low. These are also called Universal gates since the

earlier three gates can be realized, y using the NAND gate. The

commonly used symbol for a NAND gate is as follows:

And, the truth table for a NAND gate is as follows:

Input Output

A B Y

0 0 1

0 1 1

1 0 1

1 1 0

As can be seen above, it is the opposite of an AND gate –

(NOT+AND = NAND).

5. NOR gate

A NOR gate is simply an OR gate followed by a NOT gate. The

output is 1 only when all inputs are 0. Or the output is high when all

the inputs are low. These are also called Universal gates since the

earlier three gates can be realized by using the NOR gate. The

commonly used symbol for a NOR gate is as follows:

And the truth table for a NOR gate is as follows:

Input Output

A B Y

0 0 1

0 1 0

1 0 0

1 1 0

As can be seen above, it is the opposite of an OR gate – (NOT+OR =

NOR).

Solved Examples for You

Question: Write the truth table for the NAND gate connected as given

below:

Solution: A NAND gate has two or more inputs and one output. In the

image above, the inputs are connected together. This means that when

‘A’ is low, all inputs for the NAND gate will be low and vice-versa.

Now, the original NAND truth table is as follows:

Input Output

A B Y

0 0 1

0 1 1

1 0 1

1 1 0

Since, inputs A and B are connected together, when A=0, both A, and

B are 0. Also, when A=1, both A and B are 1. Hence the truth table

will be as follows:

Input (A) Output (Y)

0 1

1 0

This is exactly the truth table for a NOT gate. Hence, a NAND gate

connected as shown in the example above works like a NOT gate.

Question:

Write the truth table for circuit given in the figure below consisting of

NOR gates and identify the logic operation (OR, AND, NOT) which

this circuit is performing.

Solution: The above image is a combination of two NOR gates. The

first gate functions like a normal NOR gate following the below

mentioned truth table:

Input Output

A B Y

0 0 1

0 1 0

1 0 0

1 1 0

In the second gate, the A and B inputs are connected together. So,

when A=0, both A and B are 0 and vice-versa. Hence the truth table

for this gate will be as follows:

Input (A) Output (Y)

0 1

1 0

This is the truth table for a NOT gate. Combining the two together, we

get a NOR gate followed by a NOT gate. So, the truth table for the

example cited above becomes:

Input Output

A B Y

0 0 0

0 1 1

1 0 1

1 1 1

This is the truth table for an OR gate. Hence, the diagram above uses

NOR gate to function like an OR gate.

Junction Transistor – Structure and Action

In 1951, William Schockley invented the first junction transistor with

two back to back p-n junctions. Over the years, different types of

transistors were invented and to differentiate the junction transistor

from the new ones it is now called Bipolar Junction Transistor (BJT).

In this chapter, since we are not talking about any other transistors we

will use the words ‘transistor’, ‘junction transistor’ and ‘Bipolar

Junction Transistor’ interchangeably.

Introduction to a Junction Transistor

A junction transistor has three doped regions – emitter, base, and

collector. These regions form two p-n junctions between them.

Depending on the number of n and p-type semiconductors in the

transistor, they are of two types:

● n-p-n transistor: A p-type semiconductor (base) separates two

segments of the n-type semiconductor (emitter and collector).

● p-n-p transistor: An n-type semiconductor (base) separates two

segments of the p-type semiconductor (emitter and collector.

As can be seen in both the figures above, all three segments have

different thickness and doping levels. The schematic symbols of both

these transistors are as follows:

The arrowhead shows the direction of the conventional current in the

transistor. Let’s understand the three segments in detail:

Emitter This segment is on one side of the transistor. It has a moderate size and is heavily doped causing it to supply a large number of carriers for the flow of current.

Base This segment is at the centre of the transistor. It is thin and lightly doped.

Collector This segment is also on one side of the transistor. It is larger than the emitter and is moderately doped. Hence, it collects most of the majority carriers supplied by the emitter.

Also, in the case of a junction transistor, the depletion regions are

formed at the emitter-base junction and the base-collector junction. To

understand the action of the transistor, it is important to consider the

nature of depletion regions formed at these junctions.

It is also important to note here that junction transistor was invented to

produce an enlarged copy of a signal – an amplifier. Eventually, it

became equally popular as a switch.

The Amplifying Capabilities of a Junction Transistor

A junction transistor works as an amplifier when,

1. The emitter-base junction is forward biased and

2. The base-collector junction is reverse biased.

The circuit diagram for the same is as shown below:

As can be seen above, the base is the common terminal for two power

supplies whose other terminals are connected to the emitter and

collector.

Some Terminologies

● The voltage between emitter and base = VEB

● The voltage between collector and base = VCB

● VEE = Power supply connected between the emitter and base

● VCC = Power supply connected between the collector and base

Let us now study the path of the current carriers in the junction

transistor where –

● The emitter-base junction is forward biased and

● The Base-collector junction is reverse biased

The emitter is heavily doped and has a large concentration of the

majority carriers. These majority carriers enter the base in large

numbers. Since the base is very thin and lightly doped, it has very few

majority carriers. Let’s look at the path of current in a p-n-p transistor:

p-n-p Transistor

The emitter has a large concentration of holes. The base, being an

n-type semiconductor will have electrons as its majority charge

carriers. When the majority carriers (holes) enter the base from the

emitter, they swamp the majority charge carriers of the base

(electrons). The base-collector junction is reverse biased.

Hence, these holes appear as minority carriers at the junction. Hence,

they can easily enter the collector (which is a p-type semiconductor).

The holes in the base can either:

● Move towards the base terminal to combine with the electrons

entering from outside or

● Cross the junction and enter the collector.

Since the base is very thin, most of the holes find themselves near the

base-collector junction (reverse biased). Hence, they cross over to the

collector rather than move to the base terminal.

Observations

The forward bias leads to a large current entering the emitter-base

junction. However, most of it diverts to the adjacent base-collector

junction. Hence, the current coming out of the base is a small fraction

of that entering the junction. The total current in a forward biased

diode is Ih + Ie … where Ih is the hole current and Ie is the electron

current.

The emitter current IE = Ih + Ie. However, the base current IB << Ih +

Ie. This is because a big part of IE goes to the collector instead of the

base terminal. Now, current enters the emitter from outside. Applying

Kirchhoff’s law:

IE = IC + IB

where IC is the current emerging from the collector terminal. Also, IC

is nearly equal to IE since IB is very small.

n-p-n Transistor

In an n-p-n transistor, current enters from the base to the emitter. The

description of the paths followed by the majority and minority charge

carriers is similar to that of the p-n-p transistor. However, the current

paths are exactly the opposite.

In an n-p-n transistor, electrons are the majority charge carriers,

supplied by the n-type emitter region. They cross the thin p-type base

region and are able to reach the collector to give the collector current,

IC.

Solved Examples for You

Question: Name and describe the three regions of a junction transistor

Solution: A junction transistor comprises three regions – emitter, base,

and collector. The emitter is on one side of the transistor. It is

moderate in size and heavily doped. Also, it supplies a large number

of majority charge carriers. The base lies at the centre of the transistor.

It is very thin and lightly doped. The collector is on the other side of

the transistor. It is large in size and moderately doped. Also, it collects

most of the majority carriers supplied by the emitter.

Junction Transistor – Circuit Configurations and Characteristics

In a junction transistor, the Emitter (E), Base (B) and Collector (C) are

the only three terminals available. Hence, in any circuit, one of these

terminals has to be common to both input and output connections.

Therefore, the junction transistor can be connected in either of these

configurations:

● CE or Common Emitter

● CB or Common Base

● CC or Common Collector

Among these, the junction transistor is most widely used in the

Common Emitter configuration. Also, the n-p-n Silicon transistors are

used more commonly than the p-n-p transistors. Hence, we will look

at the characteristics and configurations of an n-p-n Silicon Junction

Transistor in a CE configuration.

Characteristics of a CE Junction Transistor

In Common Emitter (CE) configuration, the emitter is the common

terminal. Hence, the input is between the base and the emitter while

the output is between the collector and the emitter. Two terms that you

must remember:

● Input characteristic – the variation of the base current (IB) with

the base-emitter voltage (VBE)

● Output characteristic – the variation of the collector current (IC)

with the collector-emitter voltage (VCE)

It is observed that the output characteristics are controlled by the input

characteristics. Hence, the collector current changes with the base

current. Let’s study them with the help of a circuit diagram shown

below:

Studying the Input Characteristics

Also, a curve is plotted between the base current (IB) and the

base-emitter voltage (VBE) to study the input characteristics of the

junction transistor in CE configuration. The collector-emitter voltage

(VCE) is kept at a fixed value to study the relation between IB and

VBE.

Since we intend to study the input characteristics when the transistor is

in an active state, VCE is maintained at a large value. The value chosen

is large enough to ensure reverse biasing of the base-collector

junction. For a Silicon transistor, VCE = 0.6-0.7 V. Also,

VCE = VCB + VBE

Hence, VCE has to be maintained at a value much larger than 0.7 V.

The approximate range of voltage is between 3 and 20 V. An increase

in the value of VCE appears as an increase in the value of VCB. Hence,

we get almost identical curves for various values of VCE. Also,

determining one input characteristic is sufficient to understand curve

as shown below:

Studying the Output Characteristics

To study the output characteristics, let’s plot a curve is between the

Collector current (IC) and the collector-emitter voltage (VCE). Also,

keep the base current (IB) at a steady value.

Now, if the base-emitter current (VBE) is increased by a small amount,

you can observe an increase in hole current from the emitter and

electron current from the base regions. Hence, IB and IC increase

proportionally. Or, if IB increases, IC increases too. So, keeping IB

constant and plotting IC against VCE, you can make the following

observations:

For every value of IB, the plot of IC versus VCE displays one output

characteristic.

Calculation of Important AC Parameters of Junction Transistors

Now, let’s calculate some important ac parameters of transistors using

the linear segments of the input and output characteristics.

Input Resistance (ri)

Input resistance (ri) is the ratio of change in the base-emitter voltage

(ΔVBE) to the subsequent change in base current (ΔIB) when the

collector-emitter voltage (VCE) is kept constant.

ri = (ΔVBE/ ΔIB)VCE

This is ac resistance or dynamic resistance and which is obvious in the

input characteristics since its value varies with the operating current in

the transistor.

Output Resistance (ro)

Output resistance (ro) is the ratio of change in the collector-emitter

voltage (ΔVCE) to the change in the collector current (ΔIC) when the

base current (IB) is kept constant.

ro = (ΔVCE/ ΔIC)IB

A close look at the output characteristics reveals that initially, IC

increases linearly for every small change in the value of VCE. The

reason is simple – the base-collector junction is not reverse biased and

the transistor is not active. On the contrary, the transistor is in a

saturation state.

Hence, the current is controlled by the supply voltage (VCC) in this

part of the characteristic. In this state VCC = VCE. As VCE increases

and reaches a value which is higher than that required to reverse bias

the junction, IC increases marginally with increasing VCE. This

reciprocal of the slope of the linear part of the output characteristics

offers the value of ro.

The bias of the base-collector junction primarily controls the output

resistance of the transistor. Also, the output resistance (ro) is very

high. This is because of the reverse-bias state of the diode. Hence, you

observe that initially, the resistance is very low when the transistor is

in a saturation state.

Current Amplification Factor (β)

Current Amplification Factor (β) is the ratio of change in the collector

current (IC) to the change in base current (IB) when the

collector-emitter voltage (VCE) is kept constant. Also, the transistor is

in an active state. Now, the small signal current gain is

βac = (ΔIC/ ΔIB)VCE

This has a large value. On the other hand, if we take a simple ratio of

IC and IB, we get βdc of the junction transistor.

βdc = IC/IB

It is important to note that IC and IB increase almost linearly. In simple

words, if IB=0, then IC=0. Hence, the values of βac and βdc are nearly

equal.

Solved Examples for You

Question: What are the input and output characteristics of a CE

Junction Transistor?

Solution: In a Common Emitter configuration of a Junction Transistor,

the emitter is the common terminal. Input is between the base and

emitter. The output is between the collector and emitter. Input

characteristics are the variation of base current (IB) with the

base-emitter voltage (VBE). Output characteristics are the variation of

collector current (IC) with the collector-emitter voltage (VCE).

Junction Transistor as a Device

A junction transistor can be used as a device application like a switch,

amplifier etc. depending on many parameters.

● The configuration used – CB (Common Base), CC (Common

Collector) or CE (Common Emitter)

● Reverse/Forward biasing of the E-B and B-C junctions

● Operation region – Cutoff, active or saturation

Our focus is the CE configuration and we will concentrate on the

biasing and operation region to understand how the device works.

Junction Transistor as a Switch

When a junction transistor is used in the cutoff or saturation state, it

acts as a switch. Let’s look at a base-biased transistor, as shown in the

figure below, in the CE configuration and analyze its behaviour to

understand the operation of a transistor as a switch.

Now, let’s apply Kirchhoff’s voltage rule to the input and output sides

of this circuit. It is easy to understand that

VBB = IBRB + VBE … (1) &

VCE = VCC – ICRC … (2)

Now, we will treat VBB as the input voltage (Vi) and VCE as the dc

output voltage (Vo). Therefore, we get,

Vi = IBRB + VBE

Vo = VCC – ICRC

Next, let’s observe the change in Vo as Vi increases from zero

A Silicon junction transistor remains in a state of Cutoff as long as Vi

is less than 0.6 V. Also, IC = 0. Hence, Vo = VCC. When Vi goes

beyond 0.6 V, the transistor moves into an active state. Also, IC > 0

and Vo decreases (since ICRC increases). Initially, IC increases almost

linearly with increasing Vi.

Also, Vo decreases linearly till its value falls below 1 V. Post this, the

change becomes non-linear and the transistor moves into the

saturation state. On increasing Vi further, Vo decreases but never

becomes zero. Here is a plot of Vo vs. Vi (also called the transfer

characteristics of a base-biased transistor):

In the above graph, we can see that the transition from Cutoff to

Active state and Active to Saturation state is not sharply defined. You

can see non-linear regions in these areas.

How to operate a junction transistor as a switch?

Two things to remember here:

1. If Vi is low and unable to forward-bias the transistor, then Vo is

high (=VCC).

2. If Vi is high enough to move the transistor into saturation, then

Vo is very low (~0).

Also, when a transistor is not conducting, it is switched off. On the

other hand, when it is in the saturation state, it is switched on. Putting

these elements together, imagine a transistor where the low and high

states are defined below and above certain voltage levels.

These levels correspond to the cutoff and saturation of the transistor.

In such a scenario, we can say that a low input switches the transistor

off and a high input switches it on. These circuits are designed so that

the transistor does not remain in an active state. This is how it can

operate as a switch.

Junction Transistor as an Amplifier

To use a junction transistor as an amplifier we will focus on the active

region of the Vo vs. Vi curve shown above. If you observe this region,

the linear part of the curve is the change of the output with the input.

However, the output decreases as input increases because,

Vo = VCC-ICRC AND NOT ICRC.

Now, if we consider ΔVo and ΔVi as small changes in the output and

input voltages, then the small signal gain of the amplifier (AV) is

Av = ΔVo/ ΔVi

If the VBB voltage has a fixed value corresponding to the mid-point of

the active region, then the circuit will behave like a CE amplifier

having a voltage gain of ΔV o/ ΔVi.

The voltage gain is expressed as follows:

We know that Vo = VCC – ICRC; hence, ΔVo = 0 – RCΔIC

Also, Vi = IBRB + VBE; hence ΔVi = RBΔIB + ΔVBE

However, ΔVBE is negligibly small as compared to RBΔIB in this

circuit. Hence, the voltage gain of this CE amplifier is

Av = – RCΔIC/RBΔIB = – βac(RC/RB) … since βac = ΔIC/ΔIB

The Configuration of a Junction Transistor as an Amplifier

It is important to fix the operating point of a junction transistor in the

middle of its active region to enable it to function as an amplifier. This

will ensure that the dc base current (IB) and collector current (IC) are

constant. Also, the dc voltage (VCC) remains constant. And, the

operating values of VCE and IB determine the operating point of the

amplifier.

Imagine a sinusoidal voltage (having amplitude vs) superimposed on a

dc base bias. If the sinusoidal source of the signal is connected in

series with the VBB supply, the base current will have sinusoidal

variations superimposed on IB. Hence, the collector current will also

have sinusoidal variations superimposed on IC.

Eventually, these will lead to a change in the value of Vo. AC

variations can be measured by blocking the dc voltages by using large

capacitors. Up until now, we have not spoken about ac signals. And

the fact is, amplifiers are used to amplify alternating signals. So, let’s

superimpose an ac input signal (vi) on the dc bias (VBB) as shown

below.

The output is measured between the collector and ground.

Let’s look at how the amplifier works.

Initially, let’s assume that vi = 0. Applying Kirchhoff’s law, for the

output loop we get

VCC = VCE + ICRL

And, for the input loop, we get,

VBB = VBE + IBRB

If vi is not zero, then

VBE + vi = VBE + IBRB + ΔIB (RB + ri)

Now, a change in VBE can be related to the input resistance ri and a

change in IB. Hence,

Vi = ΔIB (RB + ri) = rΔIB

A change in IB causes a change in IC. We define a parameter βac,

which is similar to the βdc defined earlier, as

βac = ΔIC/ΔIB = IC/IB

also known as ac current gain Ai. In the linear region of the output

characteristics, βac is nearly equal to βdc. Also, a change in IC (due to a

change in IB) affects a change in VCE. This causes the voltage to drop

across resistor RL since VCC is constant. So,

ΔVCC = ΔVCE + RLΔIC = 0 OR

ΔVCE = –RLΔIC

We also know that the change in VCE is the output voltage Vo. Hence,

we get

Vo = ΔVCE = – βacRLΔIB

And the voltage gain of the amplifier is

Av = Vo/Vi = ΔVCE/rΔIB = – βacRL/r

Please note that the negative sign means that the output voltage has an

opposite phase as of the input voltage. From the above calculations,

we can find the power gain Ap as follows:

Ap = βac × Av

Remember, a transistor is not a power generating device. The energy

for the higher ac power at the output is supplied by the battery.

Solved Examples for You

Question: What are the parameters based on which a junction

transistor is used as a device?

Solution: A junction transistor is used as a device based on the

configuration used, biasing of the junctions, and the operation region.

Configurations are Common Base, Collector, and Emitter. Biasing can

be forward or reverse. Operation region includes cutoff, active and

saturation regions.

Junction Transistor as a Feedback Amplifier and Transistor Oscillator

While using a junction transistor as an amplifier we give a sinusoidal

input to get an amplified signal at the output. In other words, in an

amplifier, it is important to provide an ‘ac input’ to sustain an ‘ac

output’. However, in an oscillator, you can get an ‘ac output’ without

the need for any external input signal. Or, the oscillator is

self-sustained.

Junction transistor as a Feedback Amplifier

Take an amplifier and return a portion of the output back to the input

in phase with the starting power as shown below.

Fig.1

This process is called ‘Positive Feedback’. To achieve this, you can

use inductive coupling (via mutual induction) or LC or RC networks.

There are different types of oscillators which use different coupling

methods to feedback the output to the input. Also, they use a resonant

circuit to get oscillation at a particular frequency.

Understanding the Oscillator Action

In the circuit given below, the feedback is given by inductive coupling

from one coil winding (T1) to another coil winding (T2).

Fig.2

Observe that both T1 and T2 are wound on the same core. This means

that they are inductively coupled through their mutual inductance.

Also, the base-emitter junction is forward biased and the

base-collector junction is reverse biased.

Now, assuming that the switch S1 is put on to apply a proper bias for

the first time, a surge of current flows through the transistor. This

current flows through the coil T2 (terminals number 3 and 4 in the

diagram above). Also, the current increases slowly and does not reach

full amplitude instantly as can be seen in the figure below. (Current

increases from X to Y)

Fig.3

Now, the inductive coupling between coils T2 and T1 causes a current

to flow in the emitter circuit.

Note: this is the FEEDBACK from input to output.

This positive feedback causes the emitter current (current in T1) to

increase too (from X to Y as can be seen in the figure below).

Fig.4

Now, the collector current (current in T2) acquires a value ‘Y’ when

the transistor is saturated. In simpler words, the maximum collector

current is flowing and it cannot increase any further. This causes the

magnetic field around T2 to stop increasing.

Since the magnetic field becomes static, feedback from T2 to T1 stops.

This eventually causes the emitter current to fall which results in a

decrease of the collector current from Y to Z. (Fig 3).

Now, a decreasing collector current results in a decaying magnetic

field around T2. Hence, T1 observes a decaying field in T2 which

results in a decrease in the emitter current until it reaches the value Z.

If it decreases further, the transistor is cut-off or IE and IC stop

flowing. Hence, the transistor reverts back to the starting point.

Summing up

This process keeps repeating itself. The transistor is first driven to

saturation, cut-off and then back to saturation. The time taken depends

on the constants of the tank or tuned circuit (inductance L of the coil

T2 and C connected parallel to it). The frequency of oscillation of the

oscillator depends on the resonance frequency (v) of the tuned circuit.

The circuit shown in Fig.2 has the tank or tuned circuit connected on

the collector side of the junction transistor. This device is called

Tuned Collector Oscillator. If it is connected to the base side, it will be

called Tuned Base Oscillator.

Solved Examples for You

Question: What is a positive feedback amplifier?

Solution: In an amplifier, when a part of the output is returned back to

the input, in phase with the starting power, then the amplifier is called

Positive Feedback Amplifier.

Application of a Junction Diode as a Rectifier

The concept of Direct Current and Alternating Current is not alien to

many. Devices, invariably have clear instructions about using AC/DC

current. However, there are times when you might have an AC power

point but need to connect a device that requires a DC. This is when a

Rectifier steps in. In simple words, a Rectifier converts Alternating

Current into Direct Current.

The Volt-Ampere characteristics of a junction diode explain how

current is passed through the diode only when it is ‘forward biased’.

Hence, if an alternating voltage is applied across a junction diode, then

the current will flow only in the part where it is forward biased. This

property of a junction diode can be used to rectify alternating voltage/

current. The circuit used for this purpose is a Rectifier.

A junction diode can be used as a rectifier in two ways:

Half-wave Rectifier

Look at the following diagram:

Source: Wikipedia

In the above circuit, an alternating voltage is applied across a junction

diode connected to a load in a series connection. In this case, a voltage

will appear across the load only during those half cycles of the AC

input when the diode is forward biased. This circuit, which rectifies

only one half of the input current is a Half-wave Rectifier.

The alternating current is supplied at points A and B. During the

alternating cycle, the diode is forward biased when the voltage at point

A is positive. This is when the diode conducts. On the other hand,

when the voltage at point A is negative, the diode is reverse biased

and it doesn’t conduct. For all practical purposes, the reverse

saturation current can be considered zero since it is negligible.

Hence, an output voltage is available only through one half of the

input cycle. Also, no current is available in the other half. Hence, the

output still varies between positive and zero but the negative cycle is

cut off and the output voltage is said to be rectified.

Full-wave Rectifier

Look at the following diagram:

Source: Wikipedia

In the above circuit, two junction diodes are connected to a load. This

circuit gives out in both positive and negative halves of the AC cycle.

Hence, it is a Full-wave Rectifier. In this circuit, the p-sides of both

the diodes are connected to the input. And, the n-sides are connected

together and connected to the load.

Also, the mid-point of the transformer is connected to the load to

complete the circuit. This mid-point of connection is also called

Center tap and hence, the transformer is called Center tap transformer.

The reason two diodes are connected is very simple. One diode

rectifies the voltage for one half of the cycle while the other diode

rectifies it for the other half. Therefore, the output between their

common terminals and the centre-tap of the transformer becomes a

full-wave rectifier output. Let’s see how this works-

If the voltage at point A is positive, then that at point B is negative. In

such a scenario, the diode D1 is forward biased while D2 is negatively

biased. Hence, D1 conducts while D2 blocks the current. Hence,

during the positive half of the input AC cycle, we get output current.

Subsequently, the voltage at point A becomes negative and that at

point B becomes positive. In such a scenario, D2 conducts while D1

blocks the current. Hence, we get an output current in the negative

half of the input AC cycle too.

Since the circuit rectifies both the halves of the input voltage, it is

called Full-wave Rectifier. But, the output is pulsating and not steady.

To derive a steady DC output, a capacitor is connected across the

output terminals (parallel to the load).

Role of a Capacitor

A capacitor filters out the AC ripple and provides pure DC output.

Here is how it works:

As can be seen in the circuit above, a capacitor is connected parallel to

the load. When the voltage across the capacitor rises, it gets charged.

It discharges only when a load is connected to it. On discharge, the

voltage across it falls.

As the AC cycle changes and the second diode kicks in, the capacitor

charges again to its peak value. This is again discharged due to the

presence of the load.

It is important to note here that the rate of discharge depends on the

inverse product of Capacitor and Resistance (or load). To increase the

discharge time and get a steady DC output, large capacitors are

connected. The idea is to obtain an output voltage close to the peak

voltage of the rectified current. You can find such filters in most

power supplies.

Solved Examples for You

Q1. What is a half-wave rectifier?

Ans. A half wave rectifier is a circuit which converts one half of the

alternating input voltage into direct voltage. This is achieved by using

a junction diode in series with a load. During the positive cycle of the

input alternating voltage, the diode is forward biased and conducts

current. Whereas during the negative cycle it shuts off and no current

is passed.

Q2. What is a full-wave rectifier?

Ans. A full wave rectifier is a circuit which converts the entire

alternating input voltage into direct voltage. This is achieved by using

two junction diodes. The p-sides of the diode are connected to the

input while the n-sides are connected together and along with the

centre, tap form the output.

During the positive cycle of the alternating voltage, one diode

conducts while the other doesn’t. This is reversed during the negative

cycle. Hence, both positive and negative cycles are converted into a

direct voltage.