AlxSiyNz passivated AlGaN/GaN high electron mobility transistors

The performance of AlGaN/GaN high electron mobility transistors (HEMTs) with an Al_xSi_¿N_z passivation is reported for the first time. Thin films (30 nm) of Al_xSi_¿N_z and Si_¿N_z were used to passivate devices (fabricated side-by-side) and their performance was compared in both small signal and large signal measurement environments. Examination of MIS structures with each dielectric by capacitance-voltage measurements revealed the Al_xSi_¿N_z provides a net negative fixed charge density allowing controlled depletion of the two dimensional electron gas (2DEG) in ungated regions of the channel. This is in contrast to Si_yN_z passivation where the surface depletion of the 2DEG is almost completely removed, which results in the full channel charge existing in the ungated portions of the channel. Reducing the charge in the ungated portions of the channel can be used to reduce the electric field at high drain bias with small increases in source and drain access resistance. Reduction of channel charge using a MIS gate extension (field plate) is now commonly used to increase the device performance at large drain bias. The charged dielectric approach described in this study allows for the elimination of the field plate (and its associated parasitic capacitances) while maintaining state-of-the-art performance at drain biases up to 55 V for a device with a 0.2 micron gate length.