ieg4030-part3 - IEG 4030 Optical Communications Part III...

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Prof. Lian K Chen Part 3 - Photodioe and Receiver 1 IEG 4030 Optical Communications Part III. Photodiode and Receiver Professor Lian K. Chen Department of Information Engineering The Chinese University of Hong Kong [email protected]
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Prof. Lian K Chen Part 3 - Photodioe and Receiver 2 Outline Photo-diode – pn, p-i-n, APD Quantum Efficiency and Responsivity Noises in Photodiode – Shot Noise, Thermal Noise, Gain Noise Receiver Performance – Signal-to-Noise Ratio – Bit-Error-Rate Receiver sensitivity •O p t i c a l R e c e i v e r – high impedance, low impedance, and transimpedance receiver front- end Ref: [Keiser Ch6 and Ch7.1, 7.2, 7.4, 7.5 ][Agrawal Ch4]
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Prof. Lian K Chen Part 3 - Photodioe and Receiver 3 Performance requirement • high sensitivity (at 1.3 and 1.55 μ m for telecommunication) • high conversion efficiency ( P Æ I ) • fast response (multi-GHz) • high fidelity (linearity, dynamic range) • low noise (low dark current, leakage current) • temperature stability •c o s t
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Prof. Lian K Chen Part 3 - Photodioe and Receiver 4 • p-n diode • P-I-N diode • avalanche photo-diode (APD) • schottky-barrier diode (Metal-Semiconductor-Metal) • photo conductor • photo transistor • photonmultiplier used for optical communication Types of common Photo-detectors
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Prof. Lian K Chen Part 3 - Photodioe and Receiver 5 P-N junction operates in reversed-bias voltage When an incident photon has energy > bandgap energy , an electron-hole pair ( photocarrier ) can be generated. The carriers are separated by the electric field in the depletion region and collect by the reverse-bias junction. Æ photo-current I photo is generated. P-N Diode V+ Ec Ev h ν E g Depletion region n p E barrier potential - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + -
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Prof. Lian K Chen Part 3 - Photodioe and Receiver 6 Region (1) : depletion region, electrons & holes swept by E (electric field) -- drift current (fast) Region (2) : hole and electrons diffuse randomly towards depletion region - - diffusion current (slow) Region (3) : far away from depletion region (useless) Î Diode response is fastest if electron/hole pairs generate mainly in depletion region. (3) (2) (1) (2) (3) P N P-N Diode
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Prof. Lian K Chen Part 3 - Photodioe and Receiver 7 Transit time Usually receivers are operated in strong reverse-biased region, thus – create strong electric field E in the depletion region Æ increase drift velocity v . – increase the width, w , of depletion region Æ increase the photon absorption. Longer width Æ smaller junction capacitance: C= ε A/w ε : the permittivity of the semiconductor A: layer area Transit time t r =w/v where v is the drift velocity, a function of E. trade-off : E increases Æ v and w , then t r ↑↓ ?
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Prof. Lian K Chen Part 3 - Photodioe and Receiver 8 Response Time Response time of photodiode is determined by ± carrier transit time (fast) ± carrier diffusion time (slow) ± RC time constant ± Avalanche build up time (only for APD) Bandwidth (due to transit time): Carrier drift velocity is a function of 1) materials (GaAs, InGaAsP,. ..... ) 2) Electric field E
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This note was uploaded on 05/18/2010 for the course INFORMATIO IEG 4030 taught by Professor Liank.che during the Spring '10 term at CUHK.

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ieg4030-part3 - IEG 4030 Optical Communications Part III...

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