The avalanche photodiode is similar to the P-N or PIN photodiode except that it is operated at high reverse-bias voltages so that impact ionization occurs,EHPs are created in the space charge region by photon absorption.Also additional EHPs are created y photon generated electrons and holes through impact ionization.The avalanche photodiode now has a current gain introduced by the avalanche multiplication factors. The current gain-bandwidth product of an avalanche photodiode can be larger than 100 GHz,so the devices can respond to light modulated at microwave frequencies.
The structure of an avalanche photodiode is shown in fig.
It has four regions- N+ and P+ are heavily doped semiconductors and I region is very lightly doped.
For detection of low-levels optical siganls,it is often desirable to operate the photodiode in the avalanche region of its characteristic.In the avalanche photodiode junction must be uniform,and the guard ring is usually employed to ensure against edge breakdown.with proper design a silicon avalanche photodiode can have high sensitivity to low-level optical signals,and the response time is in the neighborhood of 1 ns.These devices are particularly useful in fiber optic communication systems.
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