lecture 2 introduction to electronics rabie a. ramadan [email protected]

Lecture 2

Introduction to Electronics Introduction to Electronics

Rabie A. [email protected]

http://www.rabieramadan.org/classes/2014/electronics/

mailto:[email protected]

http://www.rabieramadan.org/classes/2014/electronics/

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Signal Amplification

• Introduce the most fundamental signal processing function employed in every electronic circuit


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• Transducers produce signals within a range of microvolt or millivolt

• signals are too small for reliable processing

• processing is much easier if the signal magnitude is made larger.

Detecting wirelessly signature

of a virus is impossible

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• Care must be exercised in the amplification of a signal, so that the information contained in the signal is not changed and no new information is introduced.

• when we feed the signal to an amplifier, we want the output signal of the amplifier to be an exact replica of that at the input, except of course for having larger magnitude.

• Any change in waveform is considered to be distortiondistortion and is obviously undesirable.

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• An amplifier that preserves the details of the signal waveform is characterized by the relationship

• where Vi, and Vo are the input and output Signals, respectively, and A is a constant representing the magnitude of amplification, known as amplifier gain.

• a linear relationship, hence the amplifier it describes is a linear amplifier.

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• The amplifiers discussed so far are primarily intended to operate on very small input signals.

• Their purpose is to make the signal magnitude larger and therefore are thought of as voltage amplifiers.

• The preamplifier in the home stereo system is an example of a voltage amplifier.

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power amplifier

• power amplifier : an amplifier may provide only a modest amount of voltage gain but substantial current gain.

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Amplifier Circuit Symbol

• This common terminal is used as a reference point and is called the circuit groundcircuit ground

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Voltage Gain

• A linear amplifier accepts an input signal Vi(t)and provides at the output, across a load resistance RL an output signal V0(t)

• magnified replica of Vi(t)

• The Voltage gain AVoltage gain Avv of the amplifier is defined by:

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Power Gain and Current Gain

• the Signal power. an Important feature that distinguishes an amplifier from a Transformer.

• Transformer : although the voltage delivered to the load could be greater than the voltage feeding the input side (the primary): – the power delivered to the load (from the secondary side of the

Transformer) is less than or at most equal to the power supplied by the Signal. source.

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Power Gain and Current Gain

• On the other hand. an amplifier provides the load With power greater than that obtained from the signal source.

• The power gain of the amplifier:The power gain of the amplifier:

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Semiconductors

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What you will learn

• The basic properties of semiconductors and in particular silicon which is the material used to make most of today's electronic circuits. '

• How doping a pure silicon crystal dramatically changes its electrical conductivity, which is the fundamental idea underlying the use of semiconductors In the implementation of electronic devices.

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• The two mechanisms by which current flows in semiconductors: drift and diffusion of charge carriers.

• The structure and operation of the pn Junction; a basic semiconductor structure that Implements the diode and plays a dominant role in transistors.

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Semiconductors

• The most significant property of semiconductors is that:– their conductivity can be varied over a very wide range

through the introduction of controlled amounts of impurity atoms into the semiconductor crystal in a process called doping.doping.

Categories of Solids

• There are three categories of solids, based on their conducting properties:

–conductors

–semiconductors

–insulators

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Electrical Resistivity and Conductivity of Selected Materials

at 293 Kelvin

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Reviewing the previous table reveals that:

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• The electrical conductivity at room temperature is quite different for each of these three kinds of solids

– Metals and alloys have the highest conductivities

– followed by semiconductors

– and then by insulators

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Intrinsic Semiconductors

• Semiconductors are materials whose conductivity lies between that of conductors, such as copper , and insulators such as glass

• There are two kinds of semiconductors:

– single-element semiconductors, such as germanium and Silicon

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• Compound semiconductors are useful in special electronic Circuit applications as well as in applications that involve light, such as Iight-emitting diodes (LEDs).

• Of the two elemental semiconductors, germanium was used in the fabrication of very early transistors (late 1940s, early 1950s).

• It was quickly supplanted, however, with silicon, on which today’s integrated-circuit (IC) technology is almost entirely based.

Atomic Structure• Atoms go around the nucleolus in their orbits –

discrete distances • Each orbit has some energy level • The closer the orbit to the nucleus the less energy it

has • Group of orbits called shell• Electrons on the same shell have similar energy level• Valence shell is the outmost shell • Valence shell has valence electrons ready to be

freed • Number of electrons (Ne) on each shell (n)

– First shell has 2 electrons– Second shell has 8 electrons (not shown here)

Ne = 2n2

Semiconductors • Remember the further away from the nucleus

the less energy is required to free the electrons • Germanium is less stable

– Less energy is required to make the electron to jump to the conduction band

• When atoms combine to form a solid, they arrange themselves in a symmetrical patterns

• Semiconductor atoms (silicon) form crystals• Intrinsic crystals have no impurities

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• A silicon atom has four valence electrons, and thus it requires another four to complete its outermost shell.

• This is achieved by sharing one of its valence electrons with each of its four neighboring atoms

Electronic Materials

A presentation of eSyst.org

• The goal of electronic materials is to generate and control the flow of an electrical current.

• Electronic materials include:1. Conductors: have low resistance which allows

electrical current flow2. Insulators: have high resistance which suppresses

electrical current flow3. Semiconductors: can allow or suppress electrical

current flow

Conductors


• Good conductors have low resistance so electrons flow through them with ease.

• Best element conductors include:– Copper, silver, gold, aluminum, & nickel

• Alloys are also good conductors:– Brass & steel

• Good conductors can also be liquid:– Salt water

Conductor Atomic Structure


• The atomic structure of good conductors usually includes only one electron in their outer shell. – It is called a valence electron. – It is easily striped from the

atom, producing current flow.

Copper Atom

Insulators


• Insulators have a high resistance so current does not flow in them.

• Good insulators include:– Glass, ceramic, plastics, & wood

• Most insulators are compounds of several elements.

• The atoms are tightly bound to one another so electrons are difficult to strip away for current flow.

Semiconductors


• Semiconductors are materials that essentially can be conditioned to act as good conductors, or good insulators, or any thing in between.

• Common elements such as carbon, silicon, and germanium are semiconductors.

• Silicon is the best and most widely used semiconductor.

Semiconductor Valence Orbit


• The main characteristic of a semiconductor element is that it has four electrons in its outer or valence orbit.

Crystal Lattice Structure


• The unique capability of semiconductor atoms is their ability to link together to form a physical structure called a crystal lattice.

• The atoms link together with one another sharing their outer electrons.

• These links are called covalent bonds.

2D Crystal Lattice Structure

3D Crystal Lattice Structure


Semiconductors can be Insulators


• If the material is pure semiconductor material like silicon, the crystal lattice structure forms an excellent insulator since all the atoms are bound to one another and are not free for current flow.

• Good insulating semiconductor material is referred to as intrinsic.

• Since the outer valence electrons of each atom are tightly bound together with one another, the electrons are difficult to dislodge for current flow.

• Silicon in this form is a great insulator.• Semiconductor material is often used as an insulator.

Doping


• To make the semiconductor conduct electricity, other atoms called impurities must be added.

• “Impurities” are different elements. • This process is called doping.

Semiconductors can be Conductors


• An impurity, or element like arsenic, has 5 valence electrons.

• Adding arsenic (doping) will allow four of the arsenic valence electrons to bond with the neighboring silicon atoms.

• The one electron left over for each arsenic atom becomes available to conduct current flow.

Resistance Effects of Doping


• If you use lots of arsenic atoms for doping, there will be lots of extra electrons so the resistance of the material will be low and current will flow freely.

• If you use only a few boron atoms, there will be fewer free electrons so the resistance will be high and less current will flow.

• By controlling the doping amount, virtually any resistance can be achieved.

Another Way to Dope


• You can also dope a semiconductor material with an atom such as boron that has only 3 valence electrons.

• The 3 electrons in the outer orbit do form covalent bonds with its neighboring semiconductor atoms as before. But one electron is missing from the bond.

• This place where a fourth electron should be is referred to as a hole. • The hole assumes a positive charge so it can attract electrons from

some other source.• Holes become a type of current carrier like the electron to support

current flow.

Types of Semiconductor Materials


• The silicon doped with extra electrons is called an “N type” semiconductor. – “N” is for negative, which is the charge of an

electron.

• Silicon doped with material missing electrons that produce locations called holes is called “P type” semiconductor. – “P” is for positive, which is the charge of a hole.

Current Flow in N-type Semiconductors


• The DC voltage source has a positive terminal that attracts the free electrons in the semiconductor and pulls them away from their atoms leaving the atoms charged positively.

• Electrons from the negative terminal of the supply enter the semiconductor material and are attracted by the positive charge of the atoms missing one of their electrons.

• Current (electrons) flows from the positive terminal to the negative terminal.

Current Flow in P-type Semiconductors


• Electrons from the negative supply terminal are attracted to the positive holes and fill them.

• The positive terminal of the supply pulls the electrons from the holes leaving the holes to attract more electrons.

• Current (electrons) flows from the negative terminal to the positive terminal.

• Inside the semiconductor current flow is actually by the movement of the holes from positive to negative.

In Summary


• In its pure state, semiconductor material is an excellent insulator.• The commonly used semiconductor material is silicon.• Semiconductor materials can be doped with other atoms to add

or subtract electrons.• An N-type semiconductor material has extra electrons.• A P-type semiconductor material has a shortage of electrons with

vacancies called holes.• The heavier the doping, the greater the conductivity or the lower

the resistance.• By controlling the doping of silicon the semiconductor material

can be made as conductive as desired.

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Recombination Process

• Thermal generation results in free electrons and holes in equal numbers and hence equal concentrations, where concentration refers to the number of charge carriers per unit volume(cm 3).

• The free electrons and holes move randomly through the silicon crystal structure, and in the process some electrons may fill some of the holes.

• This process, called recombination ,results in the disappearance of free electrons and holes.

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• In thermal equilibrium, the recombination rate is equal to the generation rate, and one can conclude that the concentration of free electrons n is equal to the concentration of holes p.

• where denotes the number of free electrons and holes in a unit volume (cm 3 ) of intrinsic silicon at a given temperature.

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• Results from semiconductor physics gives ni a

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Example

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hole and free- electron concentration

• Finally, it is useful for future purposes to express the product of the hole and free- electron concentration as :

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Doped Semiconductors

• Concentrations are far too small for silicon to conduct appreciable current at room temperature.

• Also, the carrier concentrations and hence the conductivity are strong functions of temperature,

• Not a desirable property in an electronic device.

• Fortunately, a method was developed to change the carrier concentration in a semiconductor crystal substantially and in a precisely controlled manner.

• This process is known as doping, and the resulting silicon is referred to as doped silicon

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Doped Semiconductors

• Doping involves introducing impurity atoms into the silicon crystal in sufficient numbers to substantially increase the concentration of either free electrons or holes but with little or no change in the crystal properties of silicon.

• To increase the concentration of free electrons, n , silicon is doped with an element with a valence of 5, such as phosphorus. – The resulting doped silicon is then said to be of n type

• . To increase the concentration of holes, p , silicon is doped with an element having a valence of 3, such as boron, – the resulting doped silicon is said to be of p type

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n Type

• A silicon crystal doped with phosphorus impurity. The dopant (phosphorus) atoms replace some of the silicon atoms in the crystal structure.

• Since the phosphorus atom has five electrons in its outer shell, four of these electrons form covalent bonds with the neighboring atoms, and the fifth electron becomes a free electron.

• Thus each phosphorus atom donates a free electron to the silicon crystal, and the phosphorus impurity is called a donor

• . It should be clear, though, that no holes are generated by this process.

• The positive charge associated with the phosphorus atom is a bound charge that does not move through the crystal

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n Type

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P Type

• To obtain p -type silicon in which holes are the majority charge carriers, a trivalent impurity such as boron is used.

• a silicon crystal doped with boron.

• Note that the boron atoms replace some of the silicon atoms in the silicon crystal structure.

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P type

lecture 2 introduction to electronics rabie a. ramadan [email protected]

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