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![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
](https://image.slidesharecdn.com/hemlataharmukh-160828082124/85/Photo-detector-by-GIRISH-HARMUKH-9-320.jpg)
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Photo-detector by GIRISH HARMUKH
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- 2. Contents Introduction Basic requirements ofa photodetector Types of photodetector (a)-Photoconductive photodetector (b)-Junction photodetector Application of photodector Reference
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- 5. TYPES OF PHOTODETECTOR PhotoconductiveDetector Junction photodetector
- 6. Photoconductive Detector Photoconductive photodetectorwidely 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 Junctionphotodiode PIN Photodiode There are several types of junction photodetector Avalanche Photodiode Schottky photodiode
- 8. P-n Junction photodiode Ingeneral p-n junction diode like current rectification can also Be used for light detection purpose
- 9. Photodetectors Absorption Coefficient andPhotodiode Materials ][ 24.1 ][ eVE m g g =µλ Absorbed Photon create Electron-Hole Pair. Cut-off wavelength vs. Energy bandgap
- 10. Absorption Coefficient Direct bandgapsemiconductors (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) (a) Photon absorption in a direct band in an indirect bandgap semiconductor © 1999 S.O. Kasap, Optoelectronics (Prentic 0momentumphotonVBCB ≈=− kk Absorption coefficient α for direct bandgap semiconductors rise sharply with decreasing wavelength from λg (GaAs and InP).
- 11. Absorption Coefficient Indirect 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 absorption in 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.
- 12. Photodetectors Absorption Coefficient andPhotodiode 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 andResponsivity 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 Thepn 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
- 15. Reverse-biased p-i-nphotodiode Photodetectors The pin Photodiode pin energy-band diagram pin photodiode circuit
- 16. Avalanche Photodiode The avalnchephotodiode 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.
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- 18. Avalanche Photodiode h+ πn+ p e– Avalanche region E e- h+ Ec Ev Impact ionization processes resulting avalanche multiplication Impact of an energetic electron's kinetic energy excites VB electron to the CV.
- 19. SCHOTTKY PHOTODIODE Schottky photodiodeis 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).
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- 21. Phototransistor This is aspecial 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.
- 22. APPLICATION In Fiber opticscoomunication In camera Many medical device Bar code sensor In security system
- 23. Reference Optoelectronics and opticalfiber sensors by Asit baran maity.PHI pub.