PHOTODETECTOR

PRESENTED BYGIRISH HARMUKHM.Sc. ELECTRONICS

ContentsIntroduction

Basic requirements of a photodetector

Types of photodetector(a)-Photoconductive photodetector(b)-Junction photodetector

Application of photodector

Reference

INTRODUCTION

TYPES OF PHOTODETECTORPhotoconductive Detector

Junction photodetector

Photoconductive Detector

Photoconductive photodetector widely known as light dependent Resistor (LDR). On light incidence the electrical conductivity andHence the resistance of the device changes from several mega ohm (in dark condition) to few kilo ohm (in light condition) and thereby detectsThe presence of light and its intensity.

JUNCTION PHOTODETECTOR

P-n Junction photodiode

PIN Photodiode

There are several types of junction photodetector

Avalanche Photodiode

Schottky photodiode

P-n Junction photodiodeIn general p-n junction diode like current rectification can also Be used for light detection purpose

PhotodetectorsAbsorption Coefficient and Photodiode Materials

][24.1][eVE

mg

g

Absorbed Photon create Electron-Hole Pair.

Cut-off wavelength vs. Energy bandgap

Absorption CoefficientDirect bandgap

semiconductors (GaAs, InAs, InP, GaSb, InGaAs, GaAsSb), the photon absorption does not require assistant from lattice vibrations. The photon is absorbed and the electron is excited directly from the VB to CB without a change in its k-vector (crystal momentum ħk), since photon momentum is very small.

10

E

CB

VB

k–k

Direct Bandgap Eg Photon

Ec

Ev

(a) GaAs (Direct bandgap)

E

k–k

(b) Si (Indirect bandgap)

VB

CB

Ec

Ev

Indirect Bandgap, Eg

Photon

Phonon

(a) Photon absorption in a direct bandgap semiconductor. (b) Photon absorptionin an indirect bandgap semiconductor (VB, valence band; CB, conduction band)

© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

0 momentumphoton VBCB kk

Absorption coefficient α for direct bandgap semiconductors rise sharply with decreasing wavelength from λg (GaAs and InP).

Absorption CoefficientIndirect bandgap

semiconductors (Si and Ge), the photon absorption requires assistant from lattice vibrations (phonon). If K is wave vector of lattice wave, then ħK represents the momentum associated with lattice vibration is a phonon momentum.

11

E

CB

VB

k–k

Direct Bandgap Eg Photon

Ec

Ev

(a) GaAs (Direct bandgap)

E

k–k

(b) Si (Indirect bandgap)

VB

CB

Ec

Ev

Indirect Bandgap, Eg

Photon

Phonon

(a) Photon absorption in a direct bandgap semiconductor. (b) Photon absorptionin an indirect bandgap semiconductor (VB, valence band; CB, conduction band)

© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

Thus the probability of photon absorption is not as high as in a direct transition and the λg is not as sharp as for direct bandgap semiconductors.

PhotodetectorsAbsorption Coefficient and Photodiode Materials

E

CB

VB

k–k

Direct Bandgap Eg

E

k–k

VB

CBIndirect Bandgap

Photon

Phonons

Photon absorption in an indirect bandgap semiconductor

EgEC

EV

EC

EV

Photon absorption in a direct bandgap semiconductor.

Photon

PhotodetectorsQuantum Efficiency and Responsivity

hPeI

photonsincidnetofNumbercollectedandgeberatedEHPofNumber ph

0

External Quantum Efficiency

0(W)(A)

PI

PowerOpticalIncidentntPhotocurre

R ph

Responsivity

che

he

R

Spectral Responsivity

The pin PhotodiodeThe 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

Reverse-biased p-i-n photodiode

PhotodetectorsThe pin Photodiode

pin energy-band diagram

pin photodiode circuit

Avalanche PhotodiodeThe avalnche photodiode is another n+p-I-p+ which provides veryHigh 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.

Avalanche Photodiode

Avalanche Photodiode

h+

n+ p

e–

Avalanche region

E

e-

h+

E c

E v

Impact ionization processes resulting avalanche multiplication

Impact of an energetic electron's kinetic energy excites VB electron to the CV.

SCHOTTKY PHOTODIODESchottky 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 deviceAnd its depletion layer is one sided only (n side).

SCHOTTKY PHOTODIODE

PhototransistorThis 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.

APPLICATION

In Fiber optics coomunication

In camera

Many medical device

Bar code sensor

In security system

ReferenceOptoelectronics and optical fiber sensors by Asit baran maity.PHI pub.