AlGaN/GaN High Electron Mobility Transistors(HEMTs) Passivation
 
Influence of MgO and Sc2O3 Passivation on AlGaN/GaN HEMTs

One frequently reported problem for the AlGaN/GaN HEMTs is that the rf power obtained is still much lower than expected from the dc characteristics. This problem is manifested by a collapse in drain current or frequency dispersions in transconductance and output resistance, leading to severely reduced output power and power-added efficiency. Several mechanisms have been identified, including the presence of surface states between the gate and drain which deplete the channel in this region with time constant long enough to disrupt modulation of the channel charge during large signal operation or of trap states in the buffer layer. Several studies have shown that the use of SiNX passivation layers can be effective in reducing the effects of surface states. One drawback of typical plasma enhanced chemical vapor deposited SiNX is the high hydrogen content which could migrate into the GaN or the gate metallization. We have proposed two alternative candidates for HEMT passivation are MgO and Sc2O3, which also can be used as the gate dielectrics. These materials have larger bandgaps (8eV for MgO, 6.3eV for Sc2O3) than the previously reported Gd2O3(5.3eV) and smaller lattice mismatches to GaN (6.5% for MgO, 9.2% for Sc2O3 versus 20% for Gd2O3). These oxides prevent much of the gate lag response found in unpassivated devices.

Figure Caption: Gate lag measurements on unpassivated AlGaN/GaN HEMTs(Left) and Sc2O3 passivated AlGaN/GaN HEMTs(Right).
For Sc2O3-passivated HEMTs, the IDS increases upon deposition of the oxide and there is essentially complete mitigation of the degradation in drain-source current. More importantly, these is no significant change in these characteristics after 5 months aging. This preliminary data shows that with Sc2O3 passivation looks very promising.

Figure Caption: Gate lag measurements before and after Sc2O3 passivation and following 5 months aging
There is a strong time dependence to the measured isolation leakage current in different surface treatment before passivation. UV ozone exposure before passivation significantly reduces the isolation leakage current. The lowest isolation currents were obtained with Sc2O3 and these were roughly a factor of 6 lower than with the PECVD SiNX.
Figure Caption: (Left)Isolation current measured at 40V for mesa-isolated HEMTs, as a function of different SiNX pre-deposition treatments. (Right) Isolation current measured at 40V for mesa-isolated HEMTs, as a function of the film used for surface passivation.