High-performance GaN/AlGaN-based ultraviolet photodetectors

The detection of light in the UV portion of the electromagnetic spectrum is critical to a number of applications. Until very recently, the primary means of light detection in the UV was with either silicon photodiodes or photomultiplier tubes, both of which have serious drawbacks. With the advent of optoelectronic devices fabricated in the ternary alloy of AlGaN, the possibility exists to produce high-performance solid-state photodetector arrays sensitive to the visible-blind and solar-blind regions of the spectrum. In this paper, we discuss recent advances in the area of UV photodetectors fabricated on GaN and AlGaN. Various device structures are presented, and their peculiar characteristics discussed in terms of responsivity, dark current, gain, temporal response, and frequency response. Models describing the current transport mechanisms and the quantum efficiencies of these photodiodes are discussed. Special emphasis is given to novel device structures that improve on the temporal, spectral, and electrical characteristics of AlGaN-based photodiodes. Specifically, results for a transparent recessed-window p-i- n device, and a semi-transparent electrode device structure are described. Finally, the results of a separate absorption, charge, and multiplication avalanche photodetector are presented. This device structure resulted in a stable gain of > 10 at a reverse bias of approximately 40 V.