Steady state and transient behavior of currents in AlGaN∕GaN planar Schottky diodes and mechanism of current collapse

Abstract

To get information on the current collapse mechanism under gate bias stress in AlGaN∕GaN heterostructure field effect transistors, this article investigates the steady state and transient behavior of current in a planar Schottky metal contact formed on a standard AlGaN∕GaN high electron mobility transistor wafer. Schottky contacts with reduced reverse leakage currents were prepared by using an oxygen gettering process, and their electrical characteristics were studied by current-voltage, capacitance-voltage, and current-time measurements at various temperatures. When the voltage pulse was applied into the pinch-off region, an excess current flew, showing very slow nonexponential transients covering six orders of magnitude of time from millisecconds to thousand seconds. Turn-off transients were also slow and highly nonexponential. The current transients are shown to be due to charging and discharging of the virtual gate capacitor with the electrons in and out of surface trapping states where the rate limiting process is the hopping process of electrons. The process can be described by a dispersive hopping transport, leading to charging behavior with a stretched exponential dependence on time and discharging behavior with power law dependence on time. Theoretical formulas showed good agreements with experiments, and provide ample description of the transients processes directly related to the current collapse.