I. Introduction
Gallium-Nitride-Based semiconductor materials have recently attracted much attention in ultraviolet (UV) photodetector applications due to their large direct-bandgap energy, high electron saturation velocity, superior radiation hardness, and high-temperature resistance. In order to detect very weak UV signals, photodiodes with high internal gain and low dark current are greatly needed. To realize high internal gain in GaN-based photodiodes, an avalanche multiplication mechanism offering internal photocurrent gain by impact ionization is the best choice. However, due to the large lattice mismatch between GaN and its foreign substrate like sapphire, a heteroepitaxial GaN film often contains high-density dislocations and other structural defects, which would not only cause high dark current but also lead to a premature microplasma breakdown in the active region of GaN-based avalanche photodiodes (APDs) [1]. To minimize the impact of structural defects, past GaN-based APDs reported often have to employ a small active device area of less than 100 in diameter [2], [3]. A major approach to improve epilayer quality is to develop a GaN homoepitaxy technique. Recently, visible-blind GaN p-i-n APDs with high multiplication gain of more than have been fabricated on a bulk GaN substrate. However, the device reported suffers from an enhanced red shift of the absorption edge due to deep impurity band absorption when its depletion region extends into the p-type layer at high reverse bias [4]. Compared with p-i-n-type photodiodes, metal–semiconductor–metal (MSM) photodiodes offer many attractive advantages for practical applications, such as no p-type doping required, low capacitance, and ease of fabrication and integration. Thus, high-performance GaN-based APDs are expected to be achievable in a planar MSM configuration.