Back-illuminated GaN metal-semiconductor-metal UV photodetector with high internal gain

Back illuminated metal-semiconductor-metal photodetectors based on GaN grown on (0001) sapphire were fabricated and characterized. The photodetector with a thin GaN layer of 0.3 μm exhibited low dark current with a saturation value of 2.06 pA. A visible rejection of more than four orders of magnitude was observed in the spectral responsivity under 5 μW/cm² irradiance at 5 V bias. The responsivity was 16.5 A/W at a wavelength of 350 nm, which is 55 times that of the top illumination case. The results imply the presence of high internal gain under back illumination, due to the high density of charge generation.