photo-detector by girish harmukh
TRANSCRIPT
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PHOTODETECTOR
PRESENTED BYGIRISH HARMUKHM.Sc. ELECTRONICS
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ContentsIntroduction
Basic requirements of a photodetector
Types of photodetector(a)-Photoconductive photodetector(b)-Junction photodetector
Application of photodector
Reference
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INTRODUCTION
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TYPES OF PHOTODETECTORPhotoconductive Detector
Junction photodetector
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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.
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JUNCTION PHOTODETECTOR
P-n Junction photodiode
PIN Photodiode
There are several types of junction photodetector
Avalanche Photodiode
Schottky photodiode
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P-n Junction photodiodeIn general p-n junction diode like current rectification can also Be used for light detection purpose
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PhotodetectorsAbsorption Coefficient and Photodiode Materials
][24.1][eVE
mg
g
Absorbed Photon create Electron-Hole Pair.
Cut-off wavelength vs. Energy bandgap
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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.
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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).
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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.
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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.
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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
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PhotodetectorsQuantum Efficiency and Responsivity
hPeI
photonsincidnetofNumbercollectedandgeberatedEHPofNumber ph
0
External Quantum Efficiency
0(W)(A)
PI
PowerOpticalIncidentntPhotocurre
R ph
Responsivity
che
he
R
Spectral Responsivity
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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.
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Reverse-biased p-i-n photodiode
PhotodetectorsThe pin Photodiode
pin energy-band diagram
pin photodiode circuit
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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.
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Avalanche Photodiode
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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.
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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).
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SCHOTTKY PHOTODIODE
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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.
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APPLICATION
In Fiber optics coomunication
In camera
Many medical device
Bar code sensor
In security system
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ReferenceOptoelectronics and optical fiber sensors by Asit baran maity.PHI pub.
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