SEMICONDUCTOR PN JUNCTION THEORY

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Sub Topics1.1 Energy Band

1.2 Insulator and Semiconductor Materials

1.3 Intrinsic and Extrinsic Semiconductor

1.4 Formation of p-n Junction

1.5 Forward-biased and Reverse-biased P-N Junction

1.6 IV Characteristic of Forward-biased P-N Junction

1.7 IV Characteristic of Reverse-biased P-N Junction

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1.1 ENERGY BAND1.1.1 Atom

• All matter is made from atoms. • Atom - the smallest particles of an element that retains the

x-tic of that elements. • The configuration of electrons in an atom is the key factor

to differentiate a conductor, insulator or semiconductor.

• Atom consists of – nucleus at the centre – orbiting electrons.

• The nucleus carries most of the atom mass consist of: – Neutrons that are neutral and carry no charge. – Protons that are positively charged.

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1.1.2 Atomic Number and Weight

• Atomic number of an atom = no. of protons in the nucleus = no. of e- orbiting in a neutral atom. Example: no. of proton in Helium atom = 2, atomic number for Helium = 2.

• Atomic weight for an atom = no. of protons + no. of neutrons in the nucleus of an atom. Example: Helium atom have 2 protons and 2 neutrons; therefore the atomic weight for Helium is 4.

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1.1.3 Valence Shell and Valence e-

• Valence shell - The outermost shell in an atom• Valence electrons - e- in the valence shell • Every shell, which is located at a different

level, can contain max number of e-. • Basically the nth shell can contain max no of

electrons 2n2 e-. • The shell no 1 is the innermost and closest to the

nucleus.

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Fig 1.1 Bohr model of copper atom

1.1.3 Valence Shell and Valence e- (cont)

Fig 1.1 shows the copper atom with number of proton and neutron 29.

1st Shell (K) 2n2 =2(1)2 = 2 electrons

2nd Shell (L): 2n2 =2(2)2 = 8 electrons

3rd Shell (M): 2n2 =2(3)2 = 18 electrons

4th Shell (N): 1 electron

Total: 2+8+ 18+ 1 = 29 electrons

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1.1.3 Valence Shell and Valence e- (cont)

• Cu have 29 e- which fill up 4 shells.

• The no of valence e- in an atom has significant influence on the electrical properties of an element.

• The fewer the valence e- , the more distant the valence shell from the nucleus, the easier the valence e- to break away from the parent atom & become free e- with a min of external energy such as heat.

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1.1.3 Valence Shell and Valence e- (cont)

• Example, a single valence e- in the 4th shell (valence shell) of the Cu atom can break away easily at room temperature (25°C) create an abundance of free e-

in Cu which make it good conductor.

• Free e- are abundance in good conductors because of 2 reasons: – The single valence e- is in a higher level shell

farther away from the nucleus; hence it is not tightly bound by the +ve force from the nucleus.

– The atom tends to complete the outer shell by freeing the bound valence e-.

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Example 1-1

Suppose an outside force removes the valence e- from a Cu atom in fig 1-1. What is the net charge of the Cu atom? What is the net charge if an outside e- moves into the Cu valence orbit?

SolutionWhen the valence e- leaves, the net charge of the atom becomes +1. Whenever an atom loses 1 of its e-, it becomes positively charged. We call a positively charged atom a positive ion.

• When an outside e- moves into the Cu valence orbit, the net charge of the atom becomes -1. Whenever an atom has an extra e- in its valence orbit, we call the negatively charged atom a negative ion.

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1.1.4 Semiconductors and Covalent Bonding

• The best conductors (silver, copper, and gold) have 1 valence e-,

• The best insulators have 8 valence e-. • A semiconductor is an element with electrical

properties between conductor and insulator. The best semiconductors have 4 valence e-.

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1.1.4 Semiconductors and Covalent Bonding (cont.)

• Si and Ge are widely used in semiconductor fabrication industry. However, Si is the most popular for its tolerance to higher temperatures.

• Fig 1.2 shows that both Si & Ge atoms have 4 valence e-

in the outer shells.• They need 4 additional e- to complete the structure. • Atoms in semiconductor materials tend to share their 4

valence e- with the neighboring atoms. • This sharing of valence e- in semiconductor is called

covalent bonding which produce stable, tightly bond, and lattice structure called crystal.

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Fig 1.2 Valence electron: (a) Silicon, (b) Germanium

1.1.4 Semiconductors and Covalent Bonding (cont.)

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1.1.4 Semiconductors and Covalent Bonding (cont.)

• The covalent bonding of silicon is shown in Fig 1.3. The e- in the outer shell is shared to build up covalent bonding.

Figure 1.3 Covalent bonding in semiconductor crystal

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1.1.5 Energy Band• In the atomic structure, shells and subshells represent

distinct energy levels. • The more distant the shell or subshell from the

nucleus, the higher the energy level of the orbiting e-. • Hence, valence e- of atom posses the highest level of

energy and it can easily break away from the parent atom by acquiring a sufficient amount of external energy and becoming free e-.

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Fig 1.4 Energy band diagram; (a) insulator; (b) semiconductor; (c) conductor

1.1.5 Energy Band (cont.)• When atoms bond together to form molecules of matter, the

energy level which existed before for single atom will form energy band.

• Energy band diagram in Fig 1.4 illustrate the different energylevels to which the orbiting e- belong.

• The amount of energy that the valence e- must attain to beelevated to the next level (conduction band) is measured in electron volt. 1 eV = 1.6 x 10-19 Joules

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1.1.5 Energy Band (cont.)

• Forbidden band = Energy gap between the valence band and conduction band

• Valence e- can only leave the valence band toward conduction band if they have acquired sufficient amount of energy to jump across the forbidden band.

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1.1.5 Energy Band (cont.)

• Fig 1.4(a) - Forbidden band for insulator – wide e-

need large amount of energy to jump across the forbidden band which is impossible to make.

• Fig 1.4(b) - Forbidden band for semiconductor is smaller and relatively narrow e- can jump and make transition between valence band to conduction band after acquiring sufficient amount of energy. Eg: at room temperature (25°C) the energy gaps for Si is 1.1 e V and for Ge is O.67eV.

• Fig 1.4(c) - Energy gap for conductor almost nonexistent. The valence band and conduction band are overlapping and e- from valence band can move freely to conduction band.

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1.2 INSULATOR AND SEMICONDUCTOR MATERIALS

• Electrical insulator

– material/object which has no free e- to permit the e-

flow. – No current flow through the insulation material when

voltage is applied across an insulator. – Example: Silicon dioxide, Teflon, plastic materials.

• Semiconductor

– materials that neither good electrical conductors nor good electrical insulators.

– Ability to conduct electricity is intermediate. – Examples: Si, Ge.

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1.2 INSULATOR AND SEMICONDUCTOR MATERIALS (cont)• Previously, Ge was the only material suitable for making

semiconductor devices. But Ge had a fatal flaw (their excessive reverse current) that engineers could not overcome. Eventually, Si became practical and made Geobsolete in most electronic applications.

• Si atom has 14 protons and 14 electrons. Fig. 1.5 shown: - 1st orbit: 2 e-

- 2nd orbit: 8 e-

- valence orbit: 4 e-

In Fig. 1.5, the core has a net charge of +4 because it contains 14 protons in thenucleus and 10 e- in the 1st two orbits.

19Fig 1.5 Silicon atom

Example 1.2

What is the net charge of the silicon atom in Fig. 1.5: (a) If it loses 1 of its valence e-? (b) If it gains an extra e- in the valence

orbit?

Solution

(a) If it loses an e-, it becomes a +ve ion with a chargeof +1.

(b) If it gains an extra e-, it becomes a –ve ion with a charge of -1.

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1.3 INTRINSIC AND EXTRINSIC SEMICONDUCTOR

1.3.1 Intrinsic semiconductors

- Pure semiconductor. A Si crystal is an intrinsic semiconductor if every atom in the crystal is a Si atom. At room temperature, a Si crystal acts like an insulator because it has only a few free e- and holes produced by thermal energy.

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Flow Of Free Electrons- Fig 1.6 - Si crystal between charged

metallic plates.- Thermal energy has produced a free

e- and a hole. The free e- is in a largeorbit at the right end of the crystal.

- Because of the -ve charged plate,the free e- is repelled to the left.

- This free e- can move from one largeorbit to the next until it reaches the+ve plate.

Fig 1.6 Hole flow through a semiconductor

1.3.1 Intrinsic semiconductors(cont)

Flow Of Holes Hole at the left of fig 1.6 attracts the valence electron at point A. This causes the valence electron to move into the hole. When the valence electron at point A moves to the left, it creates a new hole at point A. The effect is the same as moving the original hole to the right. The new hole at point A can then attract and capture another valence electron. In this way, valence electrons can travel along the path shown by the arrows. This means the hole can move the opposite way, along path A-B-C-D-E-F, acting the same as a positive charge.

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Figure 1.6 Hole flow through a semiconductor

1.3.1 Intrinsic semiconductors (cont)

Two types of flow• Fig 1.7 shows an intrinsic semiconductor. It has the same

number of free e- and holes. This is because thermal energy produces free e- and holes in pairs.

• Applied voltage force the – free e- to flow left and – holes to flow right.

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Fig 1.7 Intrinsic semiconductor has equal number of free e- and holes

• When the free e- arrive at the left end of the crystal, they enter the external wire and flow to the +ve battery terminal.

1.3.1 Intrinsic semiconductors (cont)

• The free e- at the -ve battery terminal will flow to the right end of the crystal. At this point, they enter the crystal and recombine with holes that arrive at the right end of the crystal.

• A steady flow of free e- and holes occurs inside the semiconductor. There is no hole flow outside the semiconductor.

• Free e- & holes move in opposite directions.

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• Therefore current in a semiconductor is the combined effect of the 2 types of flow: the flow of free e- in 1 direction & the flow of holes in the other direction.

• Free e- and holes are called carriersbecause they carry a charge from 1 place to another.

1.3.2 Extrinsic semiconductors

• Doping - increase conductivity of a semiconductor. - adding impurity atoms to an intrinsic crystal to alter its

electrical conductivity. -A doped semiconductor called extrinsic semiconductor.

Increasing the free e-

• How to dope a Si crystal? • Melt a pure Si crystal breaks the covalent bonds and changes

the Si from a solid to a liquid. • Add pentavalent atoms to the molten silicon to increase the

number of free e-. Pentavalent atoms have 5 e- in the valence orbit. Examples: arsenic, antimony, and phosphorus.

• Pentavalent atoms will donate an extra e- to the Si crystal, and it call donor impurities.

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1.3.2 Extrinsic semiconductors (cont)

• Fig 1.8(a) - doped Si crystal appears after it cools down and reforms solid crystal structure.

• A pentavalent atom is in the center, surrounded by 4 Si atoms.

• The neighboring atoms share an e- with the central atom but left 1 extra e- . Each pentavalent atom has 5 valence e-. Since only 8 e- can fit into the valence orbit, the extra e- remains in a larger orbit as a free e-.

• Each pentavalent or donor atom in a Si crystal produces 1 free e-. This is how a manufacturer controls the conductivity of a doped semiconductor..

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Fig 1.8(a) Doping to get more free e-

• The more impurity that is added, the greater the conductivity. In this way, a semiconductor maybe lightly or heavily doped. A lightly doped semiconductor has a high resistance, whereas a heavily doped semiconductor has a low resistance

1.3.2 Extrinsic semiconductors (cont)

Increasing The Number Of Holes

• To get an excess holes - dope pure silicon crystal with trivalent atom which have 3 valence e-. Examples - aluminum, boron, and gallium.

• Fig 1.8(b) -trivalent atom in the center surrounded by 4 silicon atoms, each sharing 1 of its valence e-.

• Trivalent atom originally had only 3 valence e-

and each neighbor shares 1 e-, only 7 e- are in the valence orbit. This means that a hole exists in the valence orbit of each trivalent atom.

• Trivalent atom also called an acceptor atom because each hole it contributes can accept a free e- during recombination.

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Figure 1.8(b) doping to get more holes

Example 1.3A doped semiconductor has 10 billion Si atoms and 15 million pentavalent atoms. If the ambient temperature is 25°C, how many free e- and holes are there inside the semiconductor?

Solution Each pentavalent atom contributes 1 free e-. Therefore, the semiconductor has 15 million free e- produced by doping. There will be almost no holes by comparison because the only holes in the semiconductor are those produced by heat energy.

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1.3.2 Extrinsic semiconductors (cont)

2 Types of Extrinsic SemiconductorsA semiconductor can be doped to have an excess of free e-

or an excess of holes. Because of this, there are 2 types of doped semiconductors.

n-Type Semiconductor Si that has been doped with a pentavalent impurity is called an n-type semiconductor, where the n stands for -ve. Fig 1.9 shows an n-type semiconductor. – Since the free e- >> the holes ,

free e- - majority carriers holes - minority carriers

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1.3.2 Extrinsic semiconductors (cont)

• Applied voltage will move:- free e- to the left & - holes to the right.

• When a hole arrives at the right end of the crystal, 1 of the free e- from the external circuit enters the semiconductor & recombines with the hole.

• The free e- in Fig. 1.9 flow to the left end of the crystal, where they enter the wire and flow on to the +ve terminal of the battery.

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Fig 1.9 n-type semiconductor has many free e-

1.3.2 Extrinsic semiconductors (cont)

p-Type Semiconductor

– p-type semiconductor - Si doped with a trivalent impurity, p stands for +ve.

– Fig 1.10 shows a p-type semiconductor. – Since the holes >> the free e- ,

holes - majority carriers free e- - minority carriers

– Applied voltage will move:- free e- to the left - holes to the right.

– Fig. 1.10 - holes arriving at the right end of the crystal will recombine with free e- from the external circuit.

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Fig 1.10 p-type semiconductor has many holes

1.3.2 Extrinsic semiconductors (cont)

• There is also a flow of minority carriers in Fig. 1.10. • The free e- inside the semiconductor flow from right to

left.

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• Because there are so few minority carriers, they have almost no effect in this circuit.

• pn junction - border between p-type and n-type. • The pn junction has led to all kinds of inventions

including diodes, transistors, and integrated circuits. • Understanding the pn junction enables us to understand

semiconductor devices.

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1.4 THE UNBIASED DIODE

Fig 1.11 p-type semiconductor

The unbiased diod Each trivalent atom in a doped

silicon crystal produces 1 hole. Fig. 1-11 (a) – Illustrate a piece

of p-type semiconductor:• Each circled minus sign is the

trivalent atom, • and each plus sign is the

hole in its valence orbit.

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The unbiased diod (cont)• Each pentavalent atoms in a doped Si crystal produces

1 free e- of an n-type semiconductor

Fig 1.11 n-type semiconductor

• Each piece of semiconductor material is electrically neutral because the number of pluses and minuses is equal.

• Fig. 1-11(b) - Each circled plus sign represents a pentavalent atom, & each minus sign is the free e-.

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The unbiased diod (cont)• Fig. 1-12 – Product of a single crystal with p-type

material on 1 side and n-type on the other side. • The junction is the border where the p-type and the

n-type regions meet, and junction diode is another name for a pn crystal.

• The word diode is a contraction of two electrodes, where di stands for "two."

Fig 1.12 The pn junction

The Depletion Layer

• Because of their repulsion for each other, the free e- on the n side of Fig 1.12 tend to diffuse (spread) in all directions.

• Some of the free e- diffuse across the junction. • When a free e- enters the p region, it becomes a minority

carrier. • With so many holes around it, this minority carrier has a short

lifetime. • Soon after entering the p region, the free e- recombines with

a hole hole disappears and the free e- becomes a valence e-

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The Depletion Layer (cont)• Each time an e- diffuses across a junction, it creates a

pair of ions. When an e- leaves the n side, it leaves behind a pentavalent atom that is short 1 -ve charge; this pentavalent atom becomes a +ve ion. After the migrating e- falls into a hole on the p side, it makes a -ve ion out of the trivalent atom that captures it.

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The Depletion Layer (cont)

• Figure 1-13a - circled plus signs: +ve ions- circled minus signs: -ve ions.

• The ions are fixed in the crystal structure because of covalent bonding, and they cannot move around like free e-

and holes.

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Fig 1.13 (a) Creation of ions at junction; (b) depletion layer.

The Depletion Layer (cont)

• Each pair of +ve and -ve ions at the junction is called a dipole.

• Dipole means that 1 free e- and 1 hole have been taken out of circulation.

• As the number of dipoles builds up, the region near the junction is emptied of carriers.

• We call this charge-empty region the depletion layer (see Fig. 1-13b).

39Fig 1.13 (a) Creation of ions at junction; (b) depletion layer.

Barrier Potential• Each dipole has an electric field between the +ve and -ve ions.

• If additional free e- enter the depletion layer, the electric field tries to push these e- back into the n region.

• The strength of the electric field increases with each crossing e-

until equilibrium is reached. To a 1st approximation, this means that the electric field eventually stops the diffusion of e- across the junction.

• In Fig. 1-13a, the electric field between the ions is equivalent to a difference of potential called the barrier potential.

• At 25oC , the barrier potential- 0.3 V for Ge diodes- 0.7 V for Si diodes.

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1.5 FORWARD BIASED P-N JUNCTION • To forward bias the p-n junction, connect an external

DC source E: - +ve terminal connected to the p-type material - -ve terminal connected to n-type material

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Figure 1.14 Forward biasing the p-n junction with external source E

1.5 FORWARD BIASED P-N JUNCTION (cont)

• Since similar charges repel each other, the negatively charged e- in the n-type semiconductor are repelled by the -ve terminal of the voltage source.

• The holes in p-type semiconductor are repelled by the +veterminal connected to the junction.

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• As a result, the depletion region narrows and e- are force to diffuse across the junction and recombine with holes.

1.5 FORWARD BIASED P-N JUNCTION (cont)

• For every e- recombines with a hole, an e- leaves the covalent bond in the p-region and enters the +ve terminal of the source. Thus, the amount of current entering and leaving the external source is always equal.

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• The direction of conventional current flow (I) is considered opposite to the e-

flow, hence the current I leaves the +ve terminal of the DC source and return to the -ve terminal.

1.6 REVERSE BIASED P-N JUNCTION• In reverse-bias, the p-n junction is biased with external DC

source, E but with an opposite polarity compared with the forward bias.

• The +ve terminal is connected to the n- type and the -veterminal is connected to the p-type as shown in Fig 1.15.

44Fig 1.15 Reverse biasing p-n junction with external source

• There are very few e- present in the p-type semiconductor and very few holes present in the n-type semiconductor.

• The minority carriers, e- in p-type semiconductor are called minority e- and the minority carriers in n-type semiconductor are referred as minority holes.

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1.6 REVERSE BIASED P-N JUNCTION (cont)

- the holes in p-type semiconductor are attracted to the -veterminal of the external source away from the junction.

• When the p-n junction is reverse-biased, - the negatively charged majority e- in n-type

semiconductor are attracted to the +veterminal of the external source.

• Attraction of majority charge carriers will widen the depletion region and thus: – the majority e- cannot diffuse across the junction and

recombine with holes. – The minority carriers which are actually fwd-biased establish a

minority current flow that is <<< than the current flow under fwd-biased. This current flow is referred as reverse saturation current Is (or IR ).

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1.6 REVERSE BIASED P-N JUNCTION (cont)

• If the source voltage is increased in the reverse-biased circuit, a point will be reached that the p-n junction will allow a substantial current flow. The voltage at this reverse current flow occurs is called the reverse breakdown voltage VBR

1.7 IV CHARACTERISTIC OF FORWARD BIASED PN JUNCTION

1 simple example of pn junction is diode. The IV x-tics of fwd-biased pn junction can be obtained by constructing simple circuit as shown in Fig 1.16.

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Fig 1.16 Fwd-biased voltage

• When fwd-biased voltage is applied across a diode, there is fwd current (IF). The resistor in circuit is used to limit the fwd current to a value that will not overheat the diode and cause damage.

• As the supply voltage increases from 0V to max value, the IV characteristic obtained is shown in Fig 1.17.

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Fig 1.17 IV x-tic of fwd-biased pn junction

1.7 IV CHARACTERISTIC OF FORWARD BIASED PN JUNCTION (cont)

• If 0V across the diode no current flow.

• As fwd-biased voltage gradually increases forward current & the voltage across the diode gradually increases.

• When the fwd-biased voltage is increased to a value where the voltage across the diode reaches approximately 0.7V (barrier potential), the fwd current begins to increase rapidly.

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1.7 IV CHARACTERISTIC OF FORWARD BIASED PN JUNCTION (cont)

• As the supply voltage continues increases, the current continue to increase rapidly but the voltage across the diode increases only gradually above 0.7V. This is due to the dynamic internal resistance of the semiconductor material.

1.8 IV CHARACTERISTIC OF REVERSE BIASED PN JUNCTION

The IV x-tic for reverse-biased pn junction can be obtained by constructing a circuit as shown in Fig 1.18.

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Fig 1.18 Reverse-biased voltage

• When a reverse-biased voltage is applied across a diode, there is only extremely small reverse current (IR) through the pn junction. When the diode is supplied with 0V, there is no reverse current.

• As the reverse-biased voltage is increases, there is very small reverse current and voltage across the diode (VR).

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1.8 IV CHARACTERISTIC OF REVERSE BIASED PN JUNCTION (cont)

• When the applied bias voltage is increased to a value where the reverse voltage across the diode reaches the breakdown voltage (VBR), the reverse current begins to increase rapidly as shown in Fig 1.19.

• As the bias voltage continues increases, the reverse current continues to increase rapidly but the voltage across the diode increases very little above the VBR. Breakdown is not a normal mode of operation for most pn junction.

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1.8 IV CHARACTERISTIC OF REVERSE BIASED PN JUNCTION (cont)

Fig 1.19 IV x-tic of reversed-biased pn junction