7/31/2019 4049.EDIC Lecture 041010
1/20
1
ELECTRONIC DEVICES AND
INTEGRATED CIRCUITS (EL2006)
7/31/2019 4049.EDIC Lecture 041010
2/20
2
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 )
7/31/2019 4049.EDIC Lecture 041010
3/20
3
Todays Class
New Chapter Excess Carriers inSemiconductors : Introduction
Optical absorption Luminescence
7/31/2019 4049.EDIC Lecture 041010
4/20
4
EXCESS CARRIERS IN
SEMICONDUCTORS
Most semiconductor devices operate by the
creation of charge carriers in excess of the
thermal equilibrium values.
7/31/2019 4049.EDIC Lecture 041010
5/20
5
These excess carriers can
be created by
Optical
excitation
Electron
bombardment
or
Can be
injectedacross a
forward
biased pn
junction.
7/31/2019 4049.EDIC Lecture 041010
6/20
6
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.
7/31/2019 4049.EDIC Lecture 041010
7/20
7
OPTICAL ABSORPTION ( Contd.)
7/31/2019 4049.EDIC Lecture 041010
8/20
8
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 !!
7/31/2019 4049.EDIC Lecture 041010
9/20
9
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.)
7/31/2019 4049.EDIC Lecture 041010
10/20
10
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.
7/31/2019 4049.EDIC Lecture 041010
11/20
11
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
7/31/2019 4049.EDIC Lecture 041010
12/20
12
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
7/31/2019 4049.EDIC Lecture 041010
13/20
13
OPTICAL ABSORPTION ( Contd.)
E = hC /
= 1.24 /
7/31/2019 4049.EDIC Lecture 041010
14/20
14
Band gaps of some common semiconductors
relative to the optical spectrum
7/31/2019 4049.EDIC Lecture 041010
15/20
15
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
7/31/2019 4049.EDIC Lecture 041010
16/20
16
* 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
7/31/2019 4049.EDIC Lecture 041010
17/20
17
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
7/31/2019 4049.EDIC Lecture 041010
18/20
18
Carrier recombination and generation
Explanation of phenomenon of
phosphorescence
7/31/2019 4049.EDIC Lecture 041010
19/20
19
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
7/31/2019 4049.EDIC Lecture 041010
20/20
20
Thanks