Sharjah Indian School, Sharjah ELECTRONIC DEVICES - Class...

Sharjah Indian School, Sharjah

ELECTRONIC DEVICES - Class XII (Boys Wing) Page 01

Electronics is the fast developing branch of Physics. Before the discovery of transistors in

1948, vacuum tubes (thermionic valves) were used as the building blocks of electronic circuits.

The field of electronics gained a large momentum after the introduction of semi-conducting

devices.

Advantages of semi-conducting devices over vacuum tubes.

Small in size, consume less power, no need of a ‘warm-up’ time, have long life and high

reliability…

BAND THEORY OF SOLIDS

According to the Bohr atomic model, in an isolated atom the energy of any of its electrons

is decided by the orbit in which it revolves. But when the atoms come together to form a solid they

are close to each other. So the outer orbits of electrons from neighbouring atoms would come very

close or could even overlap. Hence the energy possessed by electrons in the outermost orbits of

atoms in a solid may be different rather than discrete. This range of energies possessed by the

electrons in the outermost orbit of atoms in a solid is called energy band. The energy band which includes the energy levels of the valence electrons is called the

valence band. The energy band above the valence band is called the conduction band. The energy

required to jump from valence and conduction bands is called Forbidden energy gap (Eg).

DISTINGUISH BETWEEN CONDUCTORS, INSULATORS AND SEMI CONDUCTORS

(A). Conductors:- Conduction and valence bands overlap. No energy gap.

(B) Insulators:- Conduction band is empty. Large energy gap. Hence no free electrons.

(C) Semiconductors:- Small energy gap. Easy for electrons to get transferred to

conduction band, even at room temperature.

(A) Conductors

(B) Insulators

(C) Semiconductors

INTRINSIC SEMICONDUCTORS

A semiconductor in its pure form is called an intrinsic semiconductor. Examples:

Germanium or Silicon.

In the structure of an intrinsic semiconductor, each atom makes co-valent bond with the

neighbouring atoms so that no free electrons are available in the structure at absolute zero

(figure1). But even at room temperature, due to thermal agitations, some co-valent bonds may be

broken and hence a few free electrons may be released in the structure. This creates vacancies of

electrons in some of the co-valent bonds and are called holes. (figure 2) – Next page dept. of physics, shrajah indian school, sharjah…dept.of physics. sharjah indian school sharjah dept.of physics. sharjah indian sch..



Figure 1 Figure 2

The presence of holes and free electrons contribute to the conductivity of a semiconductor.

As temperature increases, more electron- hole pairs are released, increasing the conductivity. Thus

the conductivity of a semiconductor increases with temperature.

In an intrinsic semiconductor, the number of free electrons is equal to the number of holes.

i.e., ne = nh = ni, called intrinsic carrier concentration.

The following diagrams represent the band diagrams of an intrinsic semi conductor at 0K and at

T>0K.

At T= 0 K At T > 0 K

EXTRINSIC SEMICONDUCTORS

The conductivity of a semiconductor can be increased by the addition of certain impurities

to the structure of a pure semiconductor. The process is called doping and the impurities are called

dopants.

(i) n-type semiconductors.

If pentavalent impurities such as, Arsenic, Antimony, Bismuth, Phosphorous… are added

to Ge or Si, Some atoms are displaced by the impurity atoms. Out of the five valence electrons,

four of them make co-valent bond with the neighbouring Ge atoms whereas the fifth electron

remains weakly bound to the parent atom. This increases the conductivity the semiconductor.

(figure 4)

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Figure 4

Conduction of electricity through such a semiconductor is due to the presence of such free

electrons, which are negatively charged. Thus such a semiconductor is called negative-type or n-

type semiconductor.

It shows that a pentavalent dopant is donating one extra electron for conduction and hence it is

called a donor impurity. The energy band diagram of n-type semiconductor is called donor level

diagram. (figure 5)

In n-type semiconductors, electrons are

the majority charge carriers and holes,

the minority carriers.

i.e., ne>>nh.

(ii) p-type semiconductors.

When trivalent impurities such as Ga, In, B…,. are added to the structure of a pure

semiconductor, each impurity atom can make co-valent bonds with the three neighbouring Ge

atoms whereas, the forth bond has a vacancy or a hole. (Figure 6)

This increases the conductivity of the semiconductor.

The charge carriers are Conduction Band

holes, which behave as

positive charges. Hence such

a semiconductor is called

positive type or p-type. oooooooooooooooooooo

The trivalent impurities are Acceptor Level

called acceptor impurities

and the energy band diagram Valence Band

is called acceptor level

diagram. (Figure 7)

Figure 06

P.N.Junction When a pure semiconductor (e.g: Si) is doped with penta-valent and trivalent impurities on both

the sides equally, we get a p-n junction. Two important processes during the formation of p-n

junction are diffusion and drift.

Holes diffuse from p-side to n-side (p n) and electrons diffuse from n-side to p-side (n

p). This motion of charge carriers gives rise to diffusion current across the junction. This leaves

behind an ionised donor (positive charge) on the n-side and an ionised acceptor (negative charge)

on the p-side. These are immobile.

As the process continues a layer of positive charge called positive space-charge region is

formed at the n-side of the junction and a layer of negative charge called negative space-charge

region on the p-side of the junction. Hence a neutral layer called depletion layer is developed at

the junction.

dept. of physics, shrajah indian school, sharjah…dept.of physics. sharjah indian school sharjah dept.of physics. sharjah indian sch..

Conduction Band

Donor Level

Valence Band

Figure 5

Figure 07



This further gives rise to an electric field (junction field) directed from positive charge

towards negative charge. Due to this field, electrons move from p-side to n-side and a hole from n-

side to p-side. The motion of charge carriers due to the electric field is called drift. This causes a

drift current,

opposite to the diffusion current. (Fig.6). Figure 06

The process of increasing the junction field and

hence the diffusion current continues until

both the currents become the same.

So, no net current is there at the p-n junction under equilibrium.

Due to the loss of electrons from the n-side

and gain of electrons from the p-side, positive

and negative potentials are developed respectively.

This potential prevents further movement of charge carriers across the junction. This is called

barrier potential.

SEMICONDUCTOR DIODE.

It is a p-n junction diode with metallic contacts to apply external voltage. The construction

and the circuit symbol of a semiconductor diode are shown below:

Figure 07

Forward Bias

When the p-side of the diode is connected to the positive and n-side to the negative terminal of a

battery, the diode is said to be forward biased.

During forward bias, the holes from p-side

and the electrons from the n-side are pushed

respectively, by positive and negative terminals

of the battery. Thus the width of the depletion layer

decreases. Under sufficient voltage,

more charge carriers gain sufficient energy

to cross the junction and hence more electron-hole Figure 8

combinations take place. This gives rise to a current, called forward bias current.

Reverse Bias

When the p-side of the diode is connected to the negative and n-side to the positive terminal of a

battery, the diode is said to be reverse biased.

During reverse bias, the holes from p-side

and the electrons from the n-side are attracted

respectively, by negative and positive terminals

of the battery. Thus the width of the depletion layer

increases. So, practically, no current flows through

the circuit. But a feeble current (in µA) flows through Fig 9

the junction due to the movement of the minority charge carriers. This current is called reverse

saturation current. It is independent of the applied voltage. As the voltage increases, the minority

charge carriers gain sufficient energy to break a large number of co-valent bonds at the junction,

releasing a large number of electron-hole pairs. This happens at a critical reverse bias voltage,

called “breakdown voltage or Zener voltage” and this phenomenon is called avalanche breakdown.

At this stage, the reverse current increases abruptly. Ordinary p-n diodes may get destroyed after

this voltage due to overheating.

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V-I characteristics of a diode.

The circuit diagram to study the V-I characteristics of a p-n diode is shown below:

Reverse bias Figure 10

Forward bias

For different values of voltages, corresponding values of current are measured under

forward bias and reverse bias and a graph is plotted between V and I as given in the figure 11.

In forward bias, the diode current increases

significantly after a given minimum voltage called

threshold voltage or cut-in voltage or knee voltage.

(0.2V for Ge and 0.7V for Si)

In reverse bias, only reverse saturation current (in µA)

is present. This is negligible and hence practically no

current flows in the circuit. But beyond a given reverse

voltage called break down voltage or Zener voltage, the

current suddenly increases. General purpose diodes get

destroyed when it crosses this voltage.

[thus an ideal diode offers zero resistance at forward

bias and infinite resistance at reverse bias.]

Dynamic resistance (Rd) is the ratio of the change in

voltage to the corresponding change in current.

i.e., Rd = V/ I.

RECTIFICATION It is the process of converting a.c into d.c.

Rectifier

a.c d.c

1. Half-Wave Rectifier. The following circuit diagram shows the use of a diode (D) as a half-wave rectifier. (figure 12)

The role of transformer:

(a) It reduces the voltage of a.c.

(b) It isolates output from the input.

During the positive half cycles of the input a.c at A, the diode

is forward biased and hence it conducts. But during the

negative half cycles at A, the diode is reverse biased and it

doesn’t conduct. Thus we get a pulsating d.c across the load

resistor (RL), as shown below: (figure 13)

The following

points may be noted:

To protect diode from reverse break down, the

peak ac voltage at the secondary should not exceed

the Zener voltage of the diode.

The efficiency of a half-wave rectifier is 40%

The frequency of the pulsating output dc is the

same as that of the input ac.



2. Full-Wave Rectifier. Two diodes D1 and D2and a centre-tap transformer can be used as a full-wave rectifier, as shown

below: (Figure 14)

The role of centre-tap transformer:

It acts as a potential divider

It isolates the output from the input

It reduces the voltage of a.c

During the positive cycles at A, the point B is

negative and vice-versa. Thus for positive half

cycles at A, D1 is forward biased and D2,

reverse biased. So, D1 conducts and D2 doesn’t.

But D2 conducts during the other half cycles.

Hence the for both the half-cycles, the current

flows through RL in the same direction. i.e, we

get a pulsating dc, as shown: (Figure 15)

Notes: (a) the efficiency is 80%

(b) the frequency of the pulsating dc

output is twice that of the input.

The output from this rectifier is further given to a filter circuit

consisting of a capacitor and a resistor ( Figure 16). This gives

out a smoothened dc, similar to that from a battery. (fig 17)

Special Purpose Diodes

ZENER DIODE

It is a reverse p-n junction with a sharp breakdown voltage. (Figure 18)

The depletion layer of a Zener Diode is very thin and the

junction field is extremely high, due to heavy doping.

At the reverse bias voltage, the feeble current is due to

the minority charge carriers. When the voltage becomes equal to

Vz, the junction field becomes sufficient to break co-valent

bonds, releasing electron-hole pairs. This causes the reverse

current to increase to a high value almost insignificant with the

change in voltage. The emission of electrons from the host atoms

due to the high electric field is known as internal field emission

or field ionisation.

The circuit symbol of Zener Diode is:

Zener Diode as a voltage regulator.

A Zener Diode can be used as a voltage regulator using

the circuit shown in figure 19 below: Any increase or decrease in

the input voltage results in the increase or decrease in the voltage

drop across the series resistor Rs, without any change in the

voltage across the Zener diode. Hence we get a constant output

voltage (equal to Zener voltage) across the load resistor RL.

The Zener diode and the series resistor Rs should be selected

according to the required output voltage.




Optoelectronic junction devices. The devices in which charge carriers are generated by photons are called optoelectronic

devices.

(i) Photodiode. It is reverse biased p-n junction used to convert light energy into electrical energy.

When light (photons) with energy h ν >Eg fall on a

photodiode, then electron-hole pairs are generated due to

the absorption of photons (Figure 20). The junction field

separates these electrons and holes. This gives rise to an

emf. When an external load is connected, current flows.

Thus we get an output. This photocurrent is directly

proportional to incident light intensity.

The V-I graph of a photodiode for different

intensities I1, I2, I3 and I4 are given in figure 21.

Photo diode is used to detect optical signals.

It is in reverse bias because, the fractional

change in minority charged carriers is greater than that

in majority charged carriers. So, reverse saturation

current is easily affected by the photons. So it is easier

to observe in the change in current with change in

intensity of light.

(Refer example.14.6)

(ii) Light Emitting Diode (LED)

It is a forward biased p-n junction used to convert electrical energy into light energy. In

forward bias, during electron-hole combinations, energy is released. In ordinary diodes, this energy

is in the invisible region. LED’s, made from specially semi-conducting compounds such as

Gallium Arsenide-Phosphide, releasing energy in the visible region during forward bias. The

energy of photons emitted is nearly equal to the energy gap (Eg) of the diode.

Symbol of

LED

(iii) Solar Cell

It is unbiased p-n junction used to convert light energy into electrical energy. When light

photons of energy, h > Eg, electron-hole pairs are generated. They are further separated by the

junction field. Holes are collected at the p-side and electrons at the n-side. This gives rise to a

voltage. Hence a photocurrent IL flows in the circuit. (Fig 22)

The V-I graph of a solar

cell is in the fourth

quadrant (Fig 23). This is

because the cell does not

draw any current, but it

supplies current to the load.

Notes: (i) Semiconductors with Eg nearly 1.5eV are ideal to

make solar cells. It has more solar conversion efficiency (refer Example 14.7).

(ii) The important criteria for the selection of a material for solar cell fabrication are (a) band gap

(b) high optical absorption (c) electrical conductivity, (d) availability of the raw material and

(v) cost. (iii) Solar cells are used to power electronic devices in satellites and also in some

calculators. dept. of physics, sharjah indian school, sharjah…dept.of physics. sharjah indian school sharjah. dept.of physics. sharjah indian sch.

In LED’s the frequency of

emitted photons depends on

the energy gap (Eg) of the

diode

The intensity of the light

emitted depends on the

forward bias current.

LED’s are used in

remote controls,

burglar alarms,

optical

communication

systems etc.

Advantages of LED’s over

conventional lamps

Can be switched on and

off rapidly.

Low operational voltage

No warm-up time needed

Long life ruggedness



TRANSISTORS These are another group of electronic components made from semiconductors. They are of two

types:- npn and pnp. The three segments of a transistor are emitter, base and collector. Schematic

representation and their symbols are given in figure 24.

The essential requirements for the

proper functioning of a transistor

are:

(i) Size:- Collector should be

largest.

(ii) Doping level:- Emitter should

be heavily doped.

(iii) Biasing:-The collector-base

junction should be reverse biased and

the emitter-base junction should be

forward biased.

Action of a transistor

An n-p-n transistor is connected in proper biasing as shown in figure 25:

The electrons from the n-side and the holes from the p-

side are repelled, respectively, by the negative and

positive terminals of the battery. Thus majority of the

electrons reach the collector. These excess electrons are

attracted by the positive near the collector. This creates

collector current (Ic). The deficiency so created at the

emitter is compensated by absorbing electrons from the

negative of the battery at the emitter side. This leads to

emitter current (IE). A few electron-hole combinations

taking place at the base leads to a feeble current from the

base, called base current (IB).

Applying Kirchoff’s junction rule at O, we get IE = IB + IC. But IB << IE. So, IE IC.

Configurations of a transistor.

1. Common Base (CB) configuration 2. Common Emitter (CE) configuration

Current gain Current gain

= IC /IE = IC /IB

A.C.Current gain A.C.Current gain

ac = IC /IE ac = IC /IB

3. Common Collector (CC) Configuration

Current gain

= IB / IE

A.C.Current gain

= IB / IE


It shows that > >

Due to the high value of current gain , the

CE configuration is widely preferred.

Due negligible gain , CC configuration is

seldom used in practice.

[ac is also referred as current amplification

factor.]



Characteristics of a transistor. The following circuit diagram (Figure 26) is used to study the characteristics of an n-p-n

transistor connected at the common emitter mode.

There are two types of characteristics:

(i) Input characteristics It is the graph between input current

IB and input voltage VBE at a constant

value of the output voltage VCE.

To plot the graph, different values of IB

and the corresponding values of VBE are

measured with the help of the rheostat at

the input side, keeping VCE the same

using the rheostat at the output side. The

experiment can be repeated, using

different values of VCE. The graph

obtained is shown in figure 27 below.

(i) Output characteristics It is the graph between output current IC and output voltage VCE at a constant value of the

base current IB.

To plot the graph, different values of IC and the corresponding values of VCE are measured with the

help of the rheostat at the output side, keeping IB the same using the rheostat at the input side. The

experiment can be repeated, using different values of IB. The graph obtained is shown in figure 28

below.

IC

(mA)

VCE

{Trans-conductance of a transistor is defined as the ratio of the change in collector current to the

corresponding change in base-emitter voltage. i.e. gm = Ic / VBE}

Transistor as an amplifier

Amplification is the process of increasing the strength of weak signals.

Amplifier

An n-p-n transistor in the common emitter mode can be used as an amplifier as shown in Fig29.

The small signal voltage is superposed on the dc bias voltage. This causes variation in the

base current. As a result, Ic also varies accordingly, producing corresponding

output voltage vo. We can measure the ac variations across the input and output terminals by

blocking the dc voltages by large capacitors.


Input resistance is the ratio of the

change in VBE to the corresponding

change in the base current IB.

ri = [VBE / IB]

Output resistance is the ratio of the

change in VCE to the corresponding

change in the collector current IC.

ro = VCE / IC



Assume the case when there is no signal voltage (vi = 0). Applying Kirchoff’s rule, to the output

loop,

VCC = VCE + ICRL………………..(1)

When there is a signal vi, it can be written as,

∆VCC = ∆VCE + ∆ICRL

But ∆VCC = 0, as Vcc is a constant (e.m.f)

Thus we get, ∆VCE = - ∆ICRL ………(2)

We know, ac = IC /IB

So, ∆VCE = - ac IB RL

The input signal causes a variation in IB, which gets

multiplied many times and obtained across RL as

the values of ac and RL are very high.

Further, the negative sign indicates that the output is out of phase ( i.e. phase diff 180o) with the

input

Why is there a phase difference of 180o between the input and output in the case of a common

emitter amplifier?

We have,

VCE = VCC - ICRL

During the positive half cycles of the input signals, the input voltage increases. Thus the input

current IB and the output current IC also increases. This will reduce the value of VCE, as VCC is a

constant. So the out becomes negative.

During the negative half cycles of the input, the input voltage decreases and thereby Ic also

decreases. This will increase VCE. So, the output becomes positive. This shows that there is a phase

difference of 180o () exists between the input and the output.

Saturation State:

We have,

VCE = VCC - ICRL

For large values of IC, ICRL can be greater than VCC. So VCE may become negative. Thus

the output side of the transistor becomes improperly biased. Hence it stops working. This state is

called saturation state.

Cut-off state

When the negative part of the input signals is dominating at the input voltage, the input of

the transistor becomes improperly biased. Again the transistor becomes improperly biased. At this

stage, the transistor stops working and is called saturation state.

The cut-off region and saturation region of a transistor are represented in the output characteristics

as shown below (Figure 31).

Ic

VCE

The region between saturation and cut-off is the region where the transistor actually works. This

region is called active region. dept. of physics, sharjah indian school, sharjah…dept.of physics. sharjah indian school sharjah. dept.of physics. sharjah indian sch.

Current gain ac = IC /IB

Voltage gain Av = ∆VCE / ∆VBE

=

iB

LC

RI

RI

= ac RL / Ri

Power gain = voltage gain x current

gain

= ac RL / Ri xac

= ac2RL / Ri


ELECTRONIC DEVICES - Class XII (Boys Wing) Page10

Transfer characteristics In the given circuit of a transistor in the common emitter mode (Fig32) ,

VCE = VCC - ICRL i.e. Vo = VCC - ICRL ...(1)

Let Vi be the input dc voltage. The transfer

characteristics of the transistor is shown in figure 33

When Vi is

low, Ic = 0.

So Vo is

high.

i.e.Vo = Vcc

So transistor

enters cut-off state.

When Vi is high, Ic is large and hence it enters saturation

state. We define low and high states to the cut-off and

saturation states respectively.

LOGIC GATES These are the circuits that follow definite logic relationship between the inputs and outputs

of the circuits.

Fundamental gates

1. OR gate

Meaning Logic Symbol Truth Table Boolean Expression

Realisation

2. AND gate

Meaning Logic Symbol Truth Table Boolean Expression

3. NOT gate It produces an inverted version of the input at its output. This is why it is also known

as an inverter.

Truth Table Boolean Expression Realisation

Logic Symbol A = Y


A + B = Y

A . B = Y


ELECTRONIC DEVICES - Class XII (Boys Wing) Page12

Combination of gates

1. NOR gate In this, the output of the OR gate is fed to the input of the NOT gate.

Logic Symbol Truth Table Boolean Expression

A + B = Y

2. NAND gate In this, the output of the AND gate is fed to the input of the NOT gate.

Logic Symbol Truth Table Boolean Expression

A . B = Y

NAND – A ‘a universal gate’ NAND gates can be used to make even the fundamental gates as shown below. Hence any required

input-output combinations are possible with NAND gates. Hence NAND gates are called ‘universal gates’.

NAND as NOT NAND as AND NAND as OR

Note: Similarly NOR gates can also be called ‘universal gates’, as they can also be used to make

fundamental gates. (Refer Exercises 14.18 & 14.19)

A question from the CBSE sample paper-2008

Input signals A and B are applied to the input terminals of the ‘dotted box’ set-up shown here. Let

Y be the final output signal from the box.

Draw the wave forms of the signals labelled as C1 and C2 within the box, giving (in brief) the

reasons for getting these wave forms. Hence draw the wave form of the final output signal Y. Give

reasons for your choice. What can we state (in words) as the relation between the final output

signal Y and the input signals A and B ?


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