Three-dimensional metal–semiconductor–metal bipolar ultraviolet phototransistor based on GaN p-i-n epilayer

Abstract

GaN-based ultraviolet (UV) detectors have a considerable application potential in many fields. In this Letter, we report an alternative strategy to realize a high-optical-gain bipolar UV phototransistor based solely on a GaN p-i-n epilayer. The device consists of two tightly adjacent vertical p-i-n structures with a common n-type layer as a floating base. The collector and emitter electrodes are deposited on the two p-type mesas, forming a three-dimensional metal–semiconductor–metal (MSM) like photodetector. As a result, a peak responsivity of 11.7 A/W at a wavelength of 358 nm at 5 V is realized, corresponding to an optical gain of 40 with the assumption of 100% internal quantum efficiency. Different from traditional GaN-based n-p-i-n phototransistors, the optical gain of this detector originates from the accumulated electrons in the n-type floating base upon illumination, which can lower the barrier height between the base and emitter, leading to hole emission from the emitter. Although the structure of this phototransistor is similar to a planar back-to-back Schottky-type MSM photodetector, the response speed is much faster because the gain mainly results from carrier emission rather than MS interface defects.