Understanding material and process limits for high breakdown voltage aluminum gallium nitride/gallium nitride HEMTs

The breakdown voltage in AlGaN/GaN HEMTs is known to be triggered by the gate leakage caused by the electric field crowding at the drain-side edge of the gate. The effect of gate leakage on breakdown is mitigated by relieving the peak electric field at the drain-side edge of the gate and by decreasing the tunnelling probability with the use of gate dielectrics.; Multiple field-plates were used to split the single electric field peak into several smaller peaks without compromising the frequency response too much. As predicted by the device simulations, this increased the breakdown voltage of the fabricated devices to 900 V with two field-plates. A technique yielding an integrated field-plate self-aligned with the gate (trench gate technology) was developed, in which the SiNx passivation was deposited before the gates, followed by trenches being etched in the passivation and gate metal deposited in the trenches. The profile of the etched trench wall could be controlled to shape the electric field profile. Zirconium oxide produced by ozone oxidation shows promise as a high-k gate dielectric for GaN. Increasing the Fe-doping in the buffer was shown to reduce the buffer leakage and enhance the breakdown voltage. Furthermore, alloyed ohmic contacts were identified as a source of buffer leakage.; Beyond 400 V, the parasitic breakdown of air was identified to be limiting the device breakdown voltage. Devices made with trench gate technology, when immersed in Fluoroinert liquid, had a breakdown voltage of more than a kilo-volt (up to 1900 V was measured). Switching measurements were done on the diced devices wirebonded to a compact switching test setup. A 5.5 mm wide device switched 2.4 A at 150 V with a turn-off time of less than 20 ns. The measured switching speed is still limited by the gate drive speed. The on-resistance is limited by the wirebond resistance and the increase in access resistance of the device due to some amount of dispersion. This study has helped to give definitive direction in developing AlGaN/GaN HEMTs for power applications.