Dependence of dark current and photoresponse characteristics on polarization charge density for GaN-based avalanche photodiodes

GaN/AlGaN avalanche photodiodes (APD) are fabricated based on GaN/AlGaN materials of epitaxial growth. Dark current and photoresponse characteristics are shown experimentally and theoretically. The effects of polarization charge density (PCD) on the dark current of GaN/AlGaN APDs are investigated in detail. It is demonstrated that an increased PCD reduces the electric field intensity in the GaN i-region, and results in an increase in breakdown voltage and a decrease in dark current above 65V reverse bias. However, it is indicated that increasing the PCD elevates the electric field intensity at the GaN/AlGaN interface, and thus induces an enhancement of the band-to-band tunnelling and trap-assisted tunnelling processes, and eventually an evident rise in the dark current below 50V reverse bias. Additionally, the effects of PCD on the optical responsivity are studied in detail. Results show that positive polarization charge has little effect on the photoresponse of the device. However, the negative polarization charge significantly affects the photoresponse. A metal-oxide-semiconductor inversion layer model has been proposed to explain the effect of negative polarization charge on photoresponse.