The effect of charge layer separating absorption and multiplication on the performance of GaN avalanche photodiodes

The photo-response characteristics of a back-illuminated avalanche photodiode are studied theoretically by using a charge layer to separate the absorption and multiplication regions. The results show that the average electric field at the breakdown voltage is approximately 3.0 MV/cm, close to the reported value, in good agreement with that of the experiments. Multiplication gain has been calculated as a function of charge layer thickness and doping concentration. A maximum optimal gain of 7×104 is obtained with the doping concentration of 2×1018 cm−3. The temperature dependence of avalanche voltage shows a large positive coefficient of 0.15 V/K, confirming that the avalanche multiplication is the dominant gain mechanism in the photodiodes.