Polarization-doped n-p-i-p-n GaN-based parallel phototransistor with thick GaN absorption layer for achieving high responsivity

In this report, we propose a polarization-doped n-p-i-p-n GaN-based parallel phototransistor with thick GaN absorption layer. We employ an Al-composition-graded AlxGa1-xN layer for achieving p-type doping feature. We have studied the light propagation in the unintentionally doped GaN (i-GaN) absorption layer with different thicknesses, and the optimized thickness is 2 mu m. As a result, the photo current of 10-2 A cm-2 and the responsivity of 2.12 A W-1 can be obtained at the applied bias of 5 V. In our fabricated device, during the current transport process, the photo-generated carriers are not along the device surface. Therefore, the photoconductive effect will be absent, and hence our device achieves a response speed with a rise time of 43.3 ms and a fall time of 86.4 ms.