MODULE 2

SEMICONDUCTORS

Energy Levels :
Each orbit represents each energy level in an atom.
Orbit energy level increases as the distance from the nucleus increases.
Electrons in the outermost orbit is having higher energies.
As the distance increases, the attraction of nucleus over the outermost electrons reduces.
Energy Bands : when atoms bond together to form a solid, the electron in any orbit can have range of energies due to the influence of neighboring atoms.
The range of energies possessed by an electron in a solid is known as Energy band.

VALENCE BAND :

The band which is occupied by the valence electrons is known as valence band.
Valence band has the electrons of the highest energy.
Partially or completely filled electrons – never be empty.
Valence electrons are loosely attached to the nucleus.

CONDUCTION BAND :

At room temperature, the loosely attached electrons can move, called as free electrons.
The band which is occupied by the free electrons, who are responsible for conduction is known as conduction band.
Partially filled or empty.

FORBIDDEN ENERGY GAP :

The gap between the conduction band and valence band is called forbidden energy gap (Eg)
An electron can be lifted from the valence band to the conduction band by applying an energy which is greater than forbidden energy gap.

Classification of solids based on energy bands:

Electrical conductivity of a semiconductor increases with rise in temperature or by applying some electric field.

Intrinsic semiconductor

A semiconductor in its pure form is called intrinsic semiconductor.
The phenomenon of conduction in a semiconductor can be explained with the help of crystalline structure –

-When atoms bond together to form molecules, each atoms acquire eight electrons in the outermost shell to become stable.

In the case of Ge or Si semiconductors have only four valence electrons, so each atom requires four more electrons to become stable.
Leads to the formation of COVALENT BOND by sharing electrons between neighboring atoms. Thus forms the crystalline structure of intrinsic semiconductor

Crystalline structure of intrinsic semiconductor

intrinsic semiconductor…

At absolute zero temperature, all valence electrons are tightly bound to the parent atom, no free electrons are available for conduction. So intrinsic semiconductor act as perfect insulator at absolute zero temperature.
At room temperature, covalent bonds break down, free electrons are available and conductivity increases.
Shows negative temperature coefficient.

Concept of Hole

When a covalent bond is broken due to thermal energy, the removal of one electron leaves a vacancy in the covalent bond. This vacancy is called a Hole. {And it possess positive charge.}
Concentration of free electrons = Concentration of Holes in intrinsic semiconductor.
The process of occupying a free electron in a hole is called electron-hole recombination.

EXTRINSIC SEMICONDUCTOR

A semiconductor in its impure form is called Extrinsic semiconductor.

Doping :

The process of adding impurities to a semiconductor to increase conduction is known as doping. A doped semiconductor is called extrinsic semiconductor.

N type semiconductor

Semiconductors formed by doping pentavalent impurity atoms are known as n-type semiconductors.
Pentavalent impurity atoms – Phosphorus , Arsenic & Antimony – Donor atoms.
Creates large number of free electrons in the semiconductor crystal.
Whether a semiconductor is intrinsic or doped with impurity, it remains electrically neutral.

N type semiconductor formation

Donor ions-Which are immovable & do not contribute to the conduction of current in n-type semiconductor.

Current conduction is due to the movement of electrons and holes.

Majority carriers – Electrons (Negative charge)
Minority carriers – Holes (Positive charge)
Donor Ions (Positive charge)

P-TYPE SEMICONDUCTOR

Semiconductors formed by doping trivalent impurity atoms are known as p-type semiconductors.
Trivalent impurity atoms – Boron, Aluminium, Gallium – Acceptor atoms
creates a large number holes in the semiconductor crystal.

Acceptor Ions : After accepting electrons, the impurity atom becomes negatively charged ion known as acceptor ions. Which are immovable & do not contribute to the conduction of current in p-type semiconductor.
Current conduction is due to holes and electrons. Majority carriers – Holes (Positive charge)

Minority carriers – Free Electrons (Negative charge) Acceptor ion – Negative charge

Addition of small amount of donor or acceptor impurity produce large number of charge carriers in an extrinsic semiconductors.
Conductivity of extrinsic semiconductor > intrinsic semiconductor at room temperature.

SEMICONDUCTOR DIODES

P-n junction
- By joining the piece of a p-type semiconductor to a piece of n-type semiconductor, a p-n junction is formed.
- It is the basic building block of many semiconductor devices and is called semiconductor diode, p-n junction diode or crystal diode.
- Important characteristics of a p-n junction is its ability to conduct current only in one direction & offers high resistance in other direction.

a) p-n junction with no external voltage

a) p-n junction with no external voltage….

Holes from the p-side diffuse into n-side where they combine with free electrons.
Free electrons diffuse from n-side to p-side to combine with holes.
Each recombination depletes the holes and free electrons near the junction & contains only the immobile ions & devoid of free carriers is called Depletion region or Space charge region.
The electric field formed in the depletion region creates a potential difference across the junction is called Barrier potential. (0.7V for Si, 0.3V for Ge)

b. Forward biased p-n junction.

b. Forward biased p-n junction….

The holes get repel from +ve terminal and electrons from –ve terminal will move towards the junction & width of the depletion region is reduced.
The potential barrier is reduced & eliminated completely at a particular voltage. As a result, more majority carriers diffuse across the junction.
The junction offers low resistance to the current flow and the magnitude of current depends on the applied voltage.

c. Reverse biased p-n junctions.

c. Reverse biased p-n junctions.

The majority carriers move away from junction & the width of the depletion region is increased
Barrier potential is increased & the junction offers high resistance.
No current flows due to majority carriers.
A small current due to minority carriers known as Reverse saturation current flows through the junction.
Reverse saturation current increases with increase in temperature.

V-I CHARACTERISTICS OF A P-N JUNCTION DIODE

FORWARD CHARACTERISTICS REVERSE CHARACTERISTICS

Cut-in or Knee voltage :

The voltage at which the current starts to conduct rapidly in a forward biased p-n junction diode.
(Vk of Si=0.7V)(Vk of Ge= 0.3V)
Breakdown voltage :
The reverse voltage at which the current increase sharply in reverse biased p-n junction diode.

V-I characteristics of a p-n Diode

Diode Current Equation

Io = Reverse saturation current or leakage current due to flow of minority carriers.

V = Applied voltage(positive for forward bias & negative for reverse bias)

Ƞ= a constant, 1 for germanium and 2 for silicon.

q = absolute value of electron charge

K = Boltzmann’s constant; k = 1.38 x 10-23 J/K

T = absolute temperature

VT = kT/q ( volt-equivalent of temperature or thermal voltage) At room temperature, T = 300k, VT = 26mv

Effect of temperature on V-I characteristics of silicon diode

Reduction in the cut in voltage takes place with increase in temperature. {Therefore, at the same forward voltage VF1, a larger current IF1 flows through the diode at increased temperature.}
The breakdown voltage increases with increase in temperature.
Reverse saturation current increases with increase in temperature.

EFFECT OF TEMPERATURE ON DIODE CHARACTERISTICS

The diode current is a function of temperature. When temperature increases, the exponent will decrease and hence the diode current should also decrease.

EFFECT OF TEMPERATURE ON FORWARD CHARACTERISTICS

The characteristics curve of a silicon diode shifts to the left at the rate of -2.5mv per degree centigrade when the temperature increases & shifts to the right when the temperature decreases.

Ex:If the temperature increases from room temperature (25° C) to 75° C, the voltage drop across the diode will be (75-25) x 2.5 mV = 125 mV.

EFFECT OF TEMPERATURE ON REVERSE CHARACTERISTICS

Eg :Consider an increase of temperature from 25 °C to 85 °C, where the reverse saturation current at 25 °C is 100 nA.

The temperature increases by 60 °C (25 °C to 85 °C), which is 6 x 10.

Hence the reverse saturation current would increase by a factor of 26 = 64. Hence the reverse saturation current at 85 °C will be 100 nA x 64 = 6400 nA.

Effect of temperature on forward & reverse characteristics of a diode

Diode Equivalent Circuits

Equivalent circuit is a circuit consisting the components, to represent the characteristics of a device.
Equivalent circuits are used to simplify the analysis and design
To analyze the diode circuits,2 models namely constant drop model and piecewise linear model are discussed here.

IDEAL DIODE :

A diode is said to be ideal if it acts as a perfect conductor(with no voltage across it) when forward biased and as a perfect insulator(with no current through it) when reverse biased.
It acts as a switch. The switch is closed when forward biased and open when reverse biased.

V-I characteristics of ideal diode

The Constant-Voltage-Drop Model

- This model considers the cut-in voltage of diode.

(For ex: a Si diode doesn’t conduct current until the voltage across diode reaches 0.7V ).

This model assumes that the voltage drop across the diode remains constant and is independent of the current.

Piecewise-linear model

In this approximation current variation is considered and it is represented by a line segment having finite slope.

The slope of the line is due to the existence of a resistance, rD in the equivalent circuit. rD =

Piecewise-linear model of the diode forward characteristic and its equivalent circuit representation.

Static & Dynamic resistances of a Diode

A real diode does not behave as a perfect conductor, when it is forward biased and as a perfect insulator when reverse biased.

Static Resistance of a Diode

The resistance offered by the diode to an dc signal, when it is forward biased is called the dc or static resistance of the diode.

It is given by the ratio of dc voltage across the diode to the dc current flowing through it.

RD = VD /ID (graph)

SPECIFICATION PARAMETERS OF A DIODE

Peak Inverse Voltage – It is the maximum reverse biasing voltage that the diode can withstand with out breakdown.
Maximum Forward Current – It is the maximum current that the diode can sustain when it is forward biased.
Exceeding this limit cause excessive heat to be generated in the diode and leads to permanent failure.
Maximum Reverse Current – Data sheets specifies the reverse current for different values of reverse voltage & ambient temperature. If go beyond that current may damage the device.

TRANSISTORS…

A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power.
It is composed of semiconductor material with at least three terminals for connection to an external circuit.
A voltage or current applied to one pair of the transistor’s terminals changes the current through another pair of terminals.
Because the controlled (output) power can be higher than the controlling (input) power, a transistor can amplify a signal.

TRANSISTORS…

TRANSISTOR

Transistor transfers the input signal from a low resistance circuit to a high resistance circuit, therefore, it is called the TRANSFER RESISTOR

CONSTRUCTION OF BJT

BJT is a three terminal or a three layer semiconductor device.

BJT consists of 3 semiconductor regions:

The direction of arrow in the symbol represents the direction of the emitter current(opposite to electron current)
Thin base area helps to reduce recombination of electrons and holes in base.
To reduce the heat developed by power dissipation, area of collector is large.

Modes of operation of BJT

OPERATION OF A NPN TRANSISTOR

Some of the electrons that are diffusing through the base region will combine with holes to constitute base current IB.
The proportion of electrons “lost” through this recombination process will be quite small since the base is thin and lightly doped.
Because the collector is more positive than base most of the diffusing electrons will reach the boundary of the collector.
They will thus get “collected” to constitute the collector current IC.
Applying Kirchhoff’s first law, total current flowing into the transistor must be equal to the total current flowing out of it.

IE = IB + IC

Note: the direction of conventional current is taken opposite to direction of flow of electrons.

CONFIGURATIONS OF A TRANSISTOR

Depending on the common terminal between the input and the output circuits of a transistor, there are three types of transistor configurations. They are :
Common Base configuration
Common Emitter configuration
Common Collector configuration
As the common terminal is grounded, these configurations are also called as :

Grounded Base – Grounded Emitter – Grounded Collector

Comparison between transistor configurations

Common-Emitter Configuration

In CE configuration, emitter terminal is common to both input and output signals.
The input is applied between base and emitter & output is taken between collector and emitter.

Common-emitter configuration of pnp transistor

CE ac current gain (CE short circuit current gain) :

It is defined as the ratio of change in collector current to change in base current for a constant collector to emitter voltage.
Denoted as β or βac = ΔIC/ΔIB ……… VCE remaining constant.

Relationship between α and β

Transistor Characteristics

Used for studying behavior of transistors.
Two types of characteristics

Input Characteristics:

Input voltage Vs input current|output voltage constant.

Output characteristics:

Output voltage Vs output current |input current constant.

Circuit set up to plot CE chara. of npn transistor

Input Characteristics

- In CE, the curve drawn between base current IB and base-to- emitter voltage VBE for a given value of collector-emitter voltage VCE gives the input characteristics.

Early Effect/Base width modulation

- When the reverse bias voltage at collector increases, the depletion region at the collector-base junction gets widened.
- As the base is lightly doped, depletion region penetrates deep in to the base region than the collector region.
- The effect of reducing the base width is called Early Effect or base width modulation.
- Thus there is a lesser chance for recombination within the “smaller” base region & base current decreases as VCE increases.

Early Effect/Base width modulation

Output characteristics

The output set relates an output current (IC) to an output voltage (VCE) for various of level of input current (IB ).
The graph consists of 3 important regions; saturation, cut- off and active.

Cut off region

- The region for IB =0 is called cut off region.
- Both junctions of the transistors are reverse biased.
- Transistor is OFF.Transistors used to operate as switches
- Collector current at IB=0 is the reverse leakage current ICEO.(collector to emitter current when base is open).

Saturation region

- Collector as well as emitter junctions are forward biased.
- It is called saturation region because the collector IC does not depend up on the input current IB.