Metal-Semiconductor-Metal Ultraviolet Avalanche Photodiodes Fabricated on Bulk GaN Substrate

We report the first demonstration of a GaN-based planar metal-semiconductor-metal (MSM) ultraviolet (UV) avalanche photodiode (APD). The MSM APD with semitransparent interdigitated Schottky electrodes is fabricated on a low-defect-density GaN homoepitaxial layer grown on a bulk GaN substrate by metal-organic chemical vapor deposition. The dislocation density of the GaN homoepilayer characterized by a cathodoluminescence mapping technique is similar to 5 x 10(6) cm(-2). The photodiode exhibits a low dark current density of similar to 1.4 x 10-9 A/cm(2) and a high UV-to-visible rejection ratio up to five orders of magnitude under 20-V bias. At high bias, a room-temperature avalanche gain of more than 1100 is achieved under 365-nm UV illumination. The breakdown voltage of the APD shows a positive temperature coefficient of 0.15 V/K, confirming that the high-voltage gain is dominated by the avalanche breakdown mechanism.