semi conductor Devices

classification of solids base on conductivity

metals 18 to 18 I m

semi conductor 105

to 156 I m

Insulators 15 to 189 I m

classification of solids based on band theory

For an isolated atom the energy of its electron

is decided
by its orbit it has definite energy
level

Inside a solid each e have different energy level

the collection of these energy levels are called
energy band

Valence band Range of valence e

energies possed by
conduction band Range of energies possed by
conduction e
 A solid is said to be
CB
Ec conducting

11 An electron from
Eg energy gap
VB jumps to CB
V E 08
UB
2 An electron
jumps
from Lower level
to higher level in
VB

Metals

case 1
CB is partially filled

i
and VB is partially empty

e jumps from lower

level to higher level in
VB
conduction takes place
 case 2

CB overlaps with VB
e can easily move from
VB to CB

i conduction takes place

Insulators
no e in CB
no conduction possible at
room temp

VB is fully filled and

CB is empty
 semiconductors

few é acquire enough

energy due to room temp

and jumps from VB to

CB
conduction takes place
VB is partially empty and
CB is partially filled at roomtemp

Eg carbon Eg Eg ae

Intrinsic semiconductor semiconductor in pure form

si

si c.si
I
S

Si and Ge have 4 valence e

and forms covalent bond with
At low temp no nearby atoms
bonds are broken
conduction process in semiconductors

As temp increases bonded e gains thermal energy

and becomes free
These e out
comes leaving a vacancy with effective
charge te this vacancy is called hole

hole behave as free particle with effective positive charge

for an intrinsic semiconductor

he ng n net no of free e

per unit volume

he no of holes

per unit volume

hit intrinsic charge carrier

concentration

in a semiconductor both holes and free e

are charge carriers

 I Ie If

I n u I he A vet hereAre

I eA here never

ke th Vf
e
Me
conductivity here nap

Me Me
Tae Tsi

conductivity increases with increase in Temp

Recombination At eqb
Rate of generation Rate of recombination
According to Band theory

A intrinsic semiconductor behaves like insulator at

OK

At temp OK
 process of adding impurity Doping

impurities are of two types

1 pentavalent impurity Donor impurities

5 Valente electrons

Donate extra free é

As P Sb

2 Trivalent impurity Acceptor impurity

3 valence e

Accepts é
In Ga Al B

majority e
Si penta N type
minority h electrically
neutral
si tri P type 1 may h
minority e
Extrinsic semiconductor
impure semiconductor

hi heng extrinsic semiconductor

Ge 3v

Si TV
 P N junction diode

P N

P N junction

1 I N
e e Due to concentration diff
00
h e diffusion occurs

p 0
e N A layer of negative
0
ions appear on P side

and positive ions on N

H
side near to function
If this region is called
B
depletion layer
when dopping increases width of depletion
layer decreases
V is called potential barrier

for Ge Vp 0.3107T for Si Vp 0.7Volt

There are two types of current in P N junction diode

1 Diffusion current due to diffusion

Idf current from P side to N side

2 Drift current Due to thermal collisions electron hole

Id pairs are continously created in

depletion region
e flow towards N side and holes flow towards
p side

i current from N side to P side

In steady state Idf Ida Inet 0

when no ext source is connected diode is called

unbaised
Biasing connecting emf source to P N junction diode

SEE

Zener breakdown Highly doped

when reverse bias is increased the electricfield at

the function also intreases
At a stage the high electricfield breaks down the
covalent bonds creating e hole pairs results in

large current flow

Avalanche breakdown At high reverse voltage due to

moderately doped
high É minority charge carriers
crosses junction with very high
velocities resulting collision that
breaks covalent bond
generating
 charge carriers high current

Dynamic resistance FB and RB do not obey ohm's law

For small change in applied voltage

9 HE

FITT THE D
forward bais Reverse bais
 ideal diode
Qn we Assume

find RAB

faint at VATVB b VAUB

Rectifier converts Ac into unidirectional current
pulsating DI

I
Soltz
Soltz

t look
Soltz

output frequency of full wave reitification is double

that of input signal

In half wave rectification the input and output signal

frequency is same
Zener diode designed to operate under reverse
bais in breakdown region

used as a voltage regulator

highly doped diode Depletion region is thin

air

so

too

when V V there is large change in current

without any change in voltage
 Zener diode as voltage regulator

Any increase Decrease in I 100A

input voltage results in

increase decrease of voltage drop 2A
without
flustifting IgA
across R any change
in voltage across Zener diode

if 2V Diode will not conduct

Iz
Vin I RstRL Vout IRL
Tout
Vin I tout Vin

if Vi Ve diode will conduct

I O

i it will allow all the extra current

to pass through it with voltage maintained at V2
ie Vout V2
find current through diode
on
in
200 ISVI ikr

I
 optoelectronic devices

1 photo diode detect optical signal

operated under reverse

Ehu bias
if hu Eg
e h pairs generated
Ein
e collected on N side

and he collected on P side

photocurrent α Intensity this gives rise to emf
2 Light emitting diode Rep converts electrical
energy
to light
heavily doped p n junction
used in forward bias

in recombination energy is released in form of photons

made
up of Gallium Arsenide phosphide

3 Solar cell photovoltaic device

ie converts optical
energy to electrical

energy

no external bais is
required

e h pairs are generated

due to incident light

of
separation e h
due to É of depletion
region

p side A
N side A
 made of materials with
high optical absorption

high electrical conductivity

on statement 1 Photodiodes are used to detect optical

signals used under reverse bias

statement 2 In reversed baised

a pn junction
NEET 2024 MI
diode the current is measured in

MA is due to minority charge

carrier
 Bipolar junition transistor
Transfer Resistor Transistor

transfer of current from a low resistance part to

high resistance part

P IPR Plow Phigh

two parts 1 P n junction in FB low resistance

2 p n junction in RB high resistanie

structure

Two types I npn transistor

n C
P n no

P
E
 2
Php transistor

P n P
E P P
y
E C n

common features

Emitter heavily doped moderate size

it supplies charge carriers into base

Base lightly doped very thin

charge carriers reach collector after passing
through base

collector moderately doped large size

collects charge carrier from base
 common emitter configuration

Ic

I
p R
RÉ In to
I
I
EÉE
I
For action of transistor
E B junction Forward baised input side
E C function Reverse baised output side

Base is at HP compared to emitter

Collettor should get more potential

I 95 to 98 time
IE
current gain β
1T
A Voltage gain β Re
1
negative sign shows that output voltage is 180 out of
phase
Power
gain Voltage gain β
 NOT 1 High ON
0 Low OFF
f Y
If
A 1

tap I
B

AND

A 1

Y ATB
Y
AI o
NAND
Logic gates Digital electronics

LOW 0 OFF
High I ON

NOT gate

org f
AND gate

ORgate
Y A

I a
iffy

Norgate

ai
thot
I

NAND gate A

ID ay
y 4
 Do
hot
gate

hot gate

Y ATB

fr gate

a
B

AID
 x
x
x x
x

A I É I A B