2. Contents
Introduction
Basic requirements of a photodetector
Types of photodetector
(a)-Photoconductive photodetector
(b)-Junction photodetector
Application of photodector
Reference
6. Photoconductive Detector
Photoconductive photodetector widely known as light dependent
Resistor (LDR). On light incidence the electrical conductivity and
Hence the resistance of the device changes from several mega ohm (in
dark condition) to few kilo ohm (in light condition) and thereby detects
The presence of light and its intensity.
7. JUNCTION PHOTODETECTOR
P-n Junction photodiode
PIN Photodiode
There are several types of junction photodetector
Avalanche Photodiode
Schottky photodiode
8. P-n Junction photodiode
In general p-n junction diode like current rectification can also
Be used for light detection purpose
9. Photodetectors
Absorption Coefficient and Photodiode Materials
][
24.1
][
eVE
m
g
g =µλ
Absorbed Photon create Electron-Hole Pair.
Cut-off wavelength
vs. Energy bandgap
12. Photodetectors
Absorption Coefficient and Photodiode Materials
E
CB
VB
k–k
Direct Bandgap Eg
E
k–k
VB
CB
Indirect Bandgap
Photon
Phonons
Photon absorption in
an indirect bandgap semiconductor
EgEC
EV
EC
EV
Photon absorption in
a direct bandgap semiconductor.
Photon
13. Photodetectors
Quantum Efficiency and Responsivity
ν
η
hP
eI
photonsincidnetofNumber
collectedandgeberatedEHPofNumber ph
0
==
External Quantum Efficiency
0(W)
(A)
P
I
PowerOpticalIncident
ntPhotocurre
R
ph
==
Responsivity
ch
e
h
e
R
λ
η
ν
η == Spectral Responsivity
14. The pin Photodiode
The pn junction photodiode has
two drawbacks:
Depletion layer capacitance is not
sufficiently small to allow
photodetection at high modulation
frequencies (RC time constant
limitation).
Narrow SCL (at most a few microns)
long wavelengths incident photons
are absorbed outside SCL low QE
The pin photodiode can
significantly reduce these
problems.
14
16. Avalanche Photodiode
The avalnche photodiode is another n+p-I-p+ which provides very
High gain 200 and can detect even very low level of light.due to theas
Reasons,this detector is widely used in dedicated optical communication.
The thickness of I-region is very large as compared with the other region
Of device,so that the incoming light quickly passes through n+ and p
Layers and spend major time in this region creating large electron and
hole pairs.this is known as absorption region of the device.
19. SCHOTTKY PHOTODIODE
Schottky photodiode is one of the simple metal semiconductor
Junction .the advantage of this type of photodiode isits high
Speed which arises from the facts it is a majority carrier device
And its depletion layer is one sided only (n side).
21. Phototransistor
This is a special class of photodetector.like avalanche photodiode,photo-
-transistor which provides high gain,the device on receiving a weak
Signal amplifies internally and then passes on its output terminal.