Trading Off between Threshold Voltage and Subthreshold Slope in AlGaN/GaN HEMTs with a p-GaN Gate

The measured values of the threshold voltage of AlGaN/GaN high-electron-mobility transistors (HEMTs) with a p-GaN gate are generally more positive than what is expected from a classical HEMT. The transfer characteristics exhibit subthreshold slopes which are higher compared to the standard 60 mV per decade at room temperature. The higher threshold voltage values and subthreshold slopes are related to the specific structure of the p-GaN gate, consisting of two back-to-back diodes. The dominating diode-either the metal to p-GaN Schottky diode or the p-GaN/AlGaN barrier/GaN channel diode-dictates how much gate current flows, determines the subthreshold behavior, and, thus also the threshold voltage. The trade-off between the threshold voltage and the subthreshold slope is discussed revealing the intricate dynamic relation between those two device metrics and the gate leakage current.