Avalanche Photodiode. Instructor: Prof. Dietmar Knipp Presentation by Peter Egyinam. 4/19/2005 Photonics and Optical communicaton

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1 Avalanche Photodiode Instructor: Prof. Dietmar Knipp Presentation by Peter Egyinam 1

2 Outline Background of Photodiodes General Purpose of Photodiodes Basic operation of p-n, p-i-n and avalanche photodiodes Motivation for APD production Structure of APDs Operation and Performance of APDs Factors for achievement of high internal gain General drawbacks Types of APDs References 2

3 Background of Photodiodes basically a p-n junction operated under reverse bias conditions (voltage) for high-frequency operation, the depletion region must be kept thin to reduce the transit time to increase quantum efficiency, the depletion layer must be sufficiently thick to allow a large fraction of the incident light to be absorbed hence trade-off between response speed and quantum efficiency 3

4 General purpose of photodiodes Purpose: to convert received optical signal into an electrical signal which is then amplified for further processing hence its performance and compatibility in a communication system is highly essential Requirements: high sensitivity at the operating wavelengths large electrical response to the received optical signal small size low cost high reliability etc 4

5 Basic operation of p-n, p-i-n and avalanche photodiodes Reverse bias carrier drift p-n p-i-n avalanche 5

6 Motivation for APD production An avalanche photodiode is operated under reverse bias voltage which is sufficient to enable avalanche multiplication to take place. This multiplication results in internal current gain, and the device can respond to light modulated at frequencies as high as microwave frequencies. Benefits: has an extremely high electric field region approx Vcm -1. this leads to a high internal gain as compared to p-i-n and p-n photodiodes. provides an increase in sensitivity of between 5 and 15dB 6

7 Structure 7

8 Operation and Performance Avalanching takes place under large reverse biases. In diagram (a) there is a small reverse bias, so no avalanching occurs. In diagram (b), a larger reverse bias gives rise to more electron-hole pairs through the avalanching process. 8

9 Factors for achievement of high internal gain defect-free materials microplasmas excessive leakage at the junction edges 9

10 General drawbacks of APDs fabrication difficulties due to their more complex structure and hence increased cost the random nature of the gain mechanism which gives an additional noise contribution the often high bias voltages required (50-400v) which are wavelength dependent the variation of the gain (multiplication factor) with temperature as shown in fig on next slide for a silicon RAPD, thus temperature compensation is necessary to stabilize the operation of the device 10

11 Current gain vrs. Reverse bias at 825nm for Silicon RAPD 11

12 Types of APDs Silicon APD Germanium APD III-V alloy APD 12

13 Summary APDs have a distinct advantage over photodiodes without internal gain for the detection of the very low light levels often encountered in optical fiber communications. But due to this, APDs typically suffer from high noise because of the impact ionization process. 13

14 References avalanche/ John M. Senior, Optical Fiber Communications: Principles and Practice, second edition, Prentice Hall, 1992 Photonics and Optical Communications, Prof. D.Knipp, International University Bremen, Spring 05 14

Optical Receivers Theory and Operation

Optical Receivers Theory and Operation Photo Detectors Optical receivers convert optical signal (light) to electrical signal (current/voltage) Hence referred O/E Converter Photodetector is the fundamental