Investigation of Single-Event Transients in AlGaN/GaN MIS-Gate HEMTs Using a Focused X-Ray Beam

Focused, pulsed X-rays are used to generate singleevent transients (SETs) in metal-insulator-semiconductor (MIS)-gate AlGaN/GaN high-electron-mobility transistors (HEMTs) in order to investigate the mechanisms responsible for SETs. Unlike Schottky-gate GaN HEMTs, where current flows between gate and drain when the device is biased "OFF," for the MIS-gate HEMTs, current flows between the source and drain. The presence of the SiNx insulating layer under the gate effectively blocks electrons on the gate from moving into the semiconductor. Short-duration SETs at the gate are caused by the collection of holes injected by the X-rays. SETs collected at the source and drain have an initial short-duration component, followed by a long-duration tail that can last for hundreds of nanoseconds. The long tails are due to positive charges (holes) that are less mobile than the electrons and so take longer to exit the HEMT, and to holes that become trapped at defects in the material. The holes lower the potential barrier between the source and the region under the gate, allowing for the injection of electrons that flow from the source to the drain. The durations of the SET tails are determined by the lifetimes of the trapped holes that are found to vary with X-ray photon energy and electrical bias.