Novel Dielectrics for GaN Device Passivation and Improved Reliability

Abstract

Proper surface cleaning processes and the type of passivation film (SiNX, Sc2O3, MgO) used to reduce the current collapse phenomena in the devices are very critical to reduce the inter-device isolation leakage currents in mesa-isolated AlGaN/GaN high electron mobility transistors and to improve reliability. Three different passivation layers (SiNX, MgO, and Sc2O3) were examined for their effectiveness in mitigating surface state-induced current collapse in AlGaN/GaN high electron mobility transistors (HEMTs). The plasma-enhanced chemical vapor deposited SiNX produced ~80–85% recovery of the drain-source current, independent of whether SiH4/NH3 or SiD4/ND3 plasma chemistries were employed. Both the Sc2O3 and MgO produced essentially complete recovery of the current in GaN-cap HEMT structures and ~80–95% recovery in AlGaN-cap structures. The Sc2O3 had superior long-term stability with no change in HEMT behavior over 5 months aging. The use of MOSFETs could allow the use of complementary devices, thus producing less power consumption and simpler circuit design. The same novel oxides employed for alleviating many of the problems encountered in current Schottky-based devices were successfully used as the gate dielectric for MOS-diode and MOSFETs. Both MgO and Sc2O3 were shown to provide low interface state densities (in the 1011 eV−1 cm−2 range) on n- and p-GaN, making them useful for gate dielectrics for metal-oxide-semiconductor (MOS) devices and also as surface passivation layers to mitigate current collapse in GaN/AlGaN high electron mobility transistors (HEMTs). Clear evidence of inversion was demonstrated in gate-controlled MOS p-GaN diodes using both types of oxide. Charge-pumping measurements on diodes undergoing a high temperature implant activation anneal showed a total surface state density of ~3×1012 cm−2. On HEMT structures, both oxides provided effective passivation of surface states and these devices showed improved output power. The MgO/GaN structures were also found to be quite radiation resistant, making them attractive for satellite and terrestrial communication systems requiring a high tolerance to high energy (40 MeV) protons.