Investigation of Thermally Induced Threshold Voltage Shift in Normally-OFF p-GaN Gate HEMTs

In this article, the thermally induced threshold voltage (
${V}_{\text{TH}}$
) shift was investigated on two types of normally- OFF p-GaN gate high-electron-mobility transistors (HEMTs), featuring either an Ohmic-type or a Schottky-type gate. A positive
${V}_{\text{TH}}$
shift in a Schottky-type device and a negative
${V}_{\text{TH}}$
shift in an Ohmic-type device were observed at elevated temperatures. Temperature-dependent gate current–voltage (
${I}-{V}$
) and the transfer-length-method (TLM) analysis revealed that the bias condition across the p-GaN/AlGaN/GaN heterostructures was changed in both Schottky- and Ohmic-type devices as temperature increased. We propose that: 1) temperature-enhanced hole injection through thermionic emission which lowers the knee voltage of the Ohmic-metal/ p-GaN contact in Ohmic-type devices and 2) thermally enhanced gate current which enlarges the bias voltage across the Schottky-metal/p-GaN junction in Schottky-type devices, resulted in the observed
${V}_{\text{TH}}$
instability characteristics. The photoluminescence measurements were conducted to verify the thermally-enhanced ionization of magnesium dopants in the p-GaN and the corresponding instability issues.