Voltage-Stress- and Radiation-Induced Gate Leakage During Cryogenic Operation of AlGaN/GaN HEMTs

GaN-based (band gap E_G = 3.4 eV) electronics are leading candidates for high frequency RF power devices to meet requirements of improved size, weight, and power (SWaP) metrics, fast and energy-efficient communication, and data processing. They are also promising candidates for future defense and space applications in cryogenic conditions, such as space exploration of Mars orbiters, landers and rovers, Europa oceanic exploratory instrumentation, and outer planetary exploration and deep space probes.
Gate leakage is one of the most important factors affecting the reliability of GaN-based HEMTs, leading to decreasing power efficiency, reduced gain in RF applications, and reductions in breakdown voltage and switching frequency. Threshold voltage (V_TH) instabilities are also of concern for high frequency device applications. These can reduce switching speeds and lead to unpredictable lag in device operation. The bias and temperature dependence of gate leakage have been reported for AlGaN/GaN HEMTs. GaN-based HEMTs can exhibit surface and barrier (bulk) related leakage. Gate leakage current in AlGaN/GaN HEMTs also can be associated with variable range hopping of carriers through the AlGaN barrier layer and/or charge trapping at or near the interface of the passivation layer (typically Si₃N₄) and the barrier layer. Gate leakage has been observed to increase or decrease as a result of proton irradiation, mostly with all terminals grounded or (inappropriately, due to uncontrolled fields and enhanced charging) with pins floating.
We have evaluated the V_TH hysteresis and gate leakage of commercial AlGaN/GaN HEMTs as functions of temperature, bias stress, and/or 1.8-MeV proton irradiation. We demonstrate fundamentally different gate leakage mechanisms in GaN-based HEMTs. In as-processed and OFF-bias irradiated devices, gate leakage currents increase monotonically with increasing temperature, with maximum currents less than 0.1 nA, consistent with Mott hopping in AlGaN. In contrast, devices subjected to ON-bias stress and/or irradiation can exhibit gate leakage currents of 1 mA or greater at 80 K. These large gate currents at low temperature and negative gate bias are most likely due to positive charge generation, transport, and trapping and/or reaction processes. In particular, hole transport and dehydrogenation reactions with ON-H impurity centers in AlGaN can lead to efficient trap-assisted tunneling in stressed devices at cryogenic temperatures. These results provide insights into instabilities that affect the performance, reliability, and radiation response of GaN-based HEMTs in cryogenic applications. At low temperatures and large negative values of gate bias, the enhanced trap-assisted tunneling through the AlGaN barrier layer of devices previously subjected to ON-bias stress and/or ON-bias proton irradiation most likely occurs via pathways that include ON⁻ centers. Trapped positive charge in passivation layers of stressed devices may facilitate electron tunneling through the AlGaN layer of previously stressed or irradiated devices by lowering barriers for charge injection. These results provide insight into instabilities and leakage currents that affect the reliability and radiation response of GaN-based HEMTs in cryogenic applications.