Observation of two-dimensional hole gas distribution and transport in GaN composite p-channel at cryogenic temperatures

The density of two-dimensional hole gas (2DHG) in gallium nitride (GaN)-based heterojunctions with strong polarization effects is generally considered to be temperature-independent. In this work, we observed that the hole transport in the GaN 2D/3D composite p-channel, which is preferred for GaN complementary devices and circuit integration, exhibits unconventional temperature dependence. At cryogenic temperatures, three distinct regions can be clearly observed. For T > 200 K, the measured hole density decreases exponentially with decreasing temperature due to the freeze-out of the bulk p-GaN. For 70 < T < 200 K, the hole transport is dominated by the 2DHG, and the hole sheet density shows weak temperature dependence. For T < 70 K, a spurious increase in the hole density occurs due to the increased significance of the defect-band conduction. This work reveals that the 2DHG density continues to decrease as a result of the variation of the Fermi level in the bulk p-GaN, which is restricted by compensation doping. Increasing the 2DHG density with higher modulation doping concentration in the barrier layer can weaken the temperature dependence of the 2DHG density and preserve the functionality of devices made thereon at cryogenic temperatures.