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ELECTRONIC DEVICES AND

INTEGRATED CIRCUITS (EL2006)

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Last few classes# Drift of carriers in electricand magnetic fields Drift and resistance : Evaluating

expression for resistivity

Effect of temperature and dopingon mobility : Effect of impurityand lattice scattering

High field effects : Whathappens, if field is increasedbeyond a limit.

Hall Effect : Useful effect, whichhelps in getting to know a couple

of parameters

# Invariance of the Fermilevel at equilibrium

1..

.tw

LtwLR

21

111

f1 ( E) = f2 ( E )

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Todays Class

New Chapter Excess Carriers inSemiconductors : Introduction

Optical absorption Luminescence

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EXCESS CARRIERS IN

SEMICONDUCTORS

Most semiconductor devices operate by the

creation of charge carriers in excess of the

thermal equilibrium values.

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These excess carriers can

be created by

Optical

excitation

Electron

bombardment

or

Can be

injectedacross a

forward

biased pn

junction.

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OPTICAL ABSORPTION

* Good technique for measurement of band gap of amaterial

Two possible cases arise when light of some particular

wave length is shone on a sample.

h Eg h < Eg

Will be

abosrbed in SC

Will be transmitted

through SC.

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OPTICAL ABSORPTION ( Contd.)

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If a beam of photons with h > Eg falls on asemiconductor, then a predictable amount of absorption

is going to occur.

Ratio of transmitted light to incident light will depend on1. Photon wavelength

2. Thickness of sample

OPTICAL ABSORPTION ( Contd.)

We wish to find out a formal mathematical

relationship for this dependence !!

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Let us assume : -

Intensity of the incident photon beam directed on

the sample ( photons / cm2s ) = I0

Thickness of sample : l

Wavelength of light :

OPTICAL ABSORPTION ( Contd.)

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OPTICAL ABSORPTION ( Contd.)

As the beam passes

through the sample, itsintensity at a distance x

from the surface can be

calculated by considering

the probability of absorptionwithin any increment dx.

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Degradation of the intensity [dI (x) / dx] is

proportional to the intensity remaining at x :

-dI (x) / dx = . I (x)

OPTICAL ABSORPTION ( Contd.)

=> I (x) = I0 . e-

x

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It = I0 e-. l

is called the absorption coefficient (cm 1 ).

will depend upon photon wavelength and thematerial.

And hence the intensity of light transmitted

through the sample thickness l is

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OPTICAL ABSORPTION ( Contd.)

E = hC /

= 1.24 /

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Band gaps of some common semiconductors

relative to the optical spectrum

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When electron hole pairs are generated in asemiconductor, or when carriers are excited into

higher impurity levels, from which they fall to their

equilibrium states, light can be given off by the

material. Many of the semiconductors are wellsuited for light emission, particularly the compound

semiconductors with direct band gaps. This general

property of light emission is called

Luminescence

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* If carriers are

excited by

photon

absorption,

Luminescence (contd.)

cathodoluminescence

*If the excitedcarriers are

created by

high energy

electronbombardment

of the material,

electroluminescence

*If the

excitation

occurs by the

introduction ofa current into

a sample,

Photoluminescence

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Photoluminescence ( Contd.)

Fluorescence Phosphorescence

Persists for a longer

time . Main reason

behind thatrecombination occurs

through traps.

Applications : TVPicture tube

Material : ZnS

Fast process. Emission

of photons stops after

approx. 10-8 sec after

excitation is turned off.

Reason behind : Direct

recombination.

Applications :

Fluorescent lamp

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Carrier recombination and generation

Explanation of phenomenon of

phosphorescence

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NEXT CLASS

Carrier life time and photoconductivity

Direct recombination of electrons and holes

Indirect recombination: Trapping

Steady state carrier generation; quasi Fermi levels

Photoconductive devices

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Thanks