High-Gain Persistent Nonlinear Conductivity in High-Voltage Gallium Nitride Photoconductive Switches

Wide-bandgap GaN optically-controlled switches have the potential for driving down the cost and size and improving the efficiency and capabilities of high voltage pulsed-power applications. Key to these applications is the understanding of the high-field photo-conductive properties of this material to determine if it will operate in a high-gain and/or sub-bandgap triggering mode, such as has been observed in GaAs. Photoconductive semiconductor switches (PCSS) were fabricated and tested from GaN wafers from Kyma Technologies and Ammono. With relatively low voltages applied the switch, linear photoconductive currents are measured in response to exposure to laser pulses of sub-bandgap wavelength (532 nm). Above a threshold voltage applied to the PCSS, persistent conductivity is measured that lasts well beyond the duration of the laser pulse and discharges the charged transmission line in the system. The sustaining field for this switching mode is approximately 3kV/cm. Another known distinguishing characteristic of high-gain switching is the formation of filamentary current channels that can be imaged due to the emission of recombination radiation from the plasma within the filaments. These filaments have been also observed in the GaN switches. High-gain switching has been initiated in GaN devices with as little as 2.5 μJ laser energy. This persistent photoconductivity is a distinguishing characteristic of what is known as "lock-on" effect most notably known in GaAs-based photoconductive switches, and is the basis for highly-efficient photoconductive switching requiring relatively little laser energy.

I. Introduction

There is a vast need for fast high voltage switches to be used in power electronics and pulsed power applications. Wide bandgap (WBG) semiconductor materials have multiple advantages over typical semiconductors that have helped significantly advance high voltage switching over the last decade [1]. Table 1 shows the superior material properties of SiC and GaN to Si or GaAs for high voltage switching, namely their wide bandgap and breakdown field. GaN, specifically, has the largest bandgap, which results in less thermally generated carriers, a high breakdown field, which results in smaller device widths for similar breakdown voltages in Si or GaAs devices, and a mid-range thermal conductivity, which allows for elevated temperature operation of GaN PCSS. These parameters lead towards smaller systems with smaller devices and lesser thermal management requirements.