semicondutor devices

8/2/2019 Semicondutor Devices

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Physics ofSemiconductor Devices


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Formation of PN - Junction

When a P-type Semiconductor is joined together

with an N-type Semiconductor a PN junction is

formed. And it is also known as a Semiconductor

Diode.

Semiconductor diodes are widely used in Rectifiers

which converts input AC signal into DC outputsignal.


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P N

----

---- +

+

+

+

+

+

+

+

Space charge region

(OR)

Depletion region

JunctioJunctio

nn

Ionized donorsIonized donors

Potential barrier(VPotential barrier(V00))

Potential barrier widthPotential barrier width

(W)(W)


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Depletion Region & Space Charge

The diffusing majority carriers from the two regionsrecombine near the junction and disappear.

The uncompensatedAcceptor and Donor ions set up anElectric field which halts majority carrier Diffusion and

causes minority carrier Drift.

The two kinds of majority carriers diffusing across thejunction meet each other near the junction and undergo

recombination's, leavingnegative ions on the P-side andpositive ions on the N-side of the junction.

This distribution of Positive and Negative Charges is calledSpace charge.


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N P

Cathode Anode

_+

Diode Symbol


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Fermi level

Depletion region

PN - junction

EFn

Valence band

Conduction band

NP

EFp

E

eVB

eVB

Ev

Ec

Ev

Ec

Energy level diagram


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V - I Characteristics of PN Junction

The diode can be operated in two different ways, as Forward and

Reverse bias.

When positive terminal of the battery is connected to the P-type& negative terminal is to the N-type of the PN-junction diode,

known the diode is kept in forward bias.

When negative terminal of the battery is connected to the P-type& positive terminal is to the N-type of the PN-junction diode,

known the diode is kept in reverse bias.


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Forward Bias

Reverse Bias V

I

Current

Reversebreak down

current

Forward Current

Knee Voltage


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The region between knee voltage & breakdown

voltage is known as non-ohmic region.

Above the knee & breakdown voltage the currentincreases.

Breakdown voltage is due to thermally broken

covalent bonds.Diode is conducting in forward bias &

non-conducting in reverse bias.


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A Rectifier is an electronic circuit which

converts alternating current to direct current

(OR) unidirectional current.

Rectifiers are mainly three types1.Half wave rectifiers

2.Full wave rectifiers

3.Bridge rectifiers


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An electronic circuit which converts

alternating voltage (OR) current for

half the period of input cycle hence

it is named as half-wave rectifier.


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Half Wave Rectifier

A.C Input

Pulsated

D .C Output

transformer

B

RL

rf


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The ratio of D.C power output to applied A.C

power input is known as rectifier efficiency.


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&

Therefore,


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since,


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An electronic circuit which converts

alternating voltage (OR) current into

pulsating voltage (OR) current duringboth half cycle of input is known as

full-wave rectifier.


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Full Wave Rectifier

D .C Output

A

B

Center tapped

transformer

rf

rf

RL


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The ratio of D.C power output to applied A.Cpower input is known as rectifier efficiency.


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&

Therefore,


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2

3

2

2

3

2

)2(2Where

)1().........exp(

)exp()2(2

h

kTmN

kT

EENn

kT

EE

h

kTmn

eC

cFC

cFe

!

!

!

T

T

2

3

2

2

3

2

)2(2Where

)2).......(exp(

)exp()2(2

h

kTmN

kT

EENp

kT

EE

h

kTmp

hv

FVv

FVe

!

!

!

T

T

Consider Intrinsic Semiconductor

Electron Concentration Holes Concentration

Equation 1 & 2 holds good for both intrinsic and extrinsic semiconductors

under Thermal equilibrium condition.


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)exp(

,,

kT

EENn

EEEEnn

cnFnCn

cncFnFn

!

!!!

For N type Material

)exp(

,,

kT

EENn

EEEEnn

cpFp

Cp

cpcFpFp

!

!!!

For P type Material

Electron Concentration

)3.().........exp(

)exp(

)exp(

)exp(

)exp(

kT

eVnn

kT

eV

n

n

kT

EE

kT

EEkT

EE

n

n

Bnp

B

p

n

cncp

cpFp

cnFn

p

n

!

!

p

!


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)exp(

,,

kT

EENp

EEEEpp

Fpvp

vp

vpvFpFp

!

!!!

For N type Material

)exp(

,,

kT

EENp

EEEEpp

Fnvnvn

vnvFnFn

!

!!!

For P type Material

Hole Concentration

)4().........exp(

)exp(

)exp(

)exp(

)exp(

kT

eVpp

kT

eV

p

p

kT

EE

kT

EEkT

EE

p

p

Bpn

B

p

n

vpvn

Fpvp

Fnvn

p

n

!

!

p

!


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}1){exp(

)exp(

)exp().exp(

))(

exp(

!(

!(

!(

!(

kT

eVnn

kT

eV

nnn

kT

eV

kT

eVnnn

kT

VVennn

pp

ppp

Bnpp

Bnpp


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}1{11 !(!TK

eV

ppeBencnci

}1{22 !(!TK

eV

nnhBepcpci


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}1){( 21 !! TK

eV

npheBepcnciiI

}1){( 21 !!

TK

eV

npheBepcnciiI


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I0 is called reverse saturation current

021 )( IpcncI np !!

1

KT

eV

e

1)exp(0

!

KT

eVII


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1)exp(0

!

KT

eV

II F


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LEDs are typically made of compound

semiconductors (OR) direct band gap

semiconductors like gallium arsenide.


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LED is a highly doped

diode


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substrate

VF

N

POhmic

Contacts(Al)

Sio2

Photons


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+ -

CathodeAnode


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V

Current

I(MA)


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+ -

CathodeAnode


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P

N

i

Photons

Electron hole

pair

VR

Figure shows thereverse bias of

p-i-n diode.


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N+

Layer 1

Layer 2

Layer 3

Layer 4

P

i

P+

VRPhotons

Electron holepair

N+ - heavily doped N-region

P+ - heavily doped P-region

P - lightly doped P-region


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