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The gain and photoresponse characteristics have been numerically studied for back-illuminated separate absorption and multiplication (SAM) GaN avalanche photodiodes (APDs). The parameters of fundamental models are calibrated by simultaneously comparing the simulated dark and light current characteristics with the experimental results. Effects of environmental temperatures and device dimensions on gain characteristics have been investigated, and a method to achieve the optimum thickness of charge layer is obtained. The dependence of gain characteristics and breakdown voltage on the doping concentration of the charge layer is also studied in detail to get the optimal charge layer. The bias-dependent spectral responsivity and quantum efficiency are then presented to study the photoresponse mechanisms inside SAM GaN APDs. It is found the responsivity peak red-shifts at first due to the Franz-Keldysh effect and then blue-shifts due to the reach-through effect of the absorption layer. Finally, a new SAM GaN/AlGaN heterojunction APD structure is proposed for optimizing SAM GaN APDs.

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