Self-aligned ALD AlOx T-gate insulator for gate leakage current suppression in SiNx-passivated AlGaN/GaN HEMTs

A proof-of-concept metal–insulator–semiconductor (MIS) AlGaN/GaN high-electron mobility transistor (HEMT) that uses a self-aligned 10 nm AlOx gate insulator and SiNx passivation in the device access regions was investigated. Self-alignment of the gate insulator to metal was achieved by utilizing a submicron tri-layer photoresist pattern to lift-off sequentially-deposited AlOx dielectric and Ni/Au gate metal layers. By keeping the temperature low (100 °C) during the atomic-layer deposition (ALD) of AlOx, reflow of the photoresist pattern was prevented, which maintained the integrity of its re-entrant profile. After lift-off, the resulting transistor gate had a T-shaped profile with AlOx directly under the gate metal only. In a split wafer comparison, this experimental structure reduced reverse-bias gate leakage current after passivation by one to two orders of magnitude over Schottky gate devices. Plasma-enhanced chemical vapor deposition (PECVD) SiNx passivation of the exposed AlGaN surface access regions of AlOx-insulated gate devices was found to produce pulsed I–V improvements that are similar to those observed in passivated Schottky gate devices. This fabrication technique has been successfully used to demonstrate insulated gate devices with gate lengths (LG) as short as 160 nm with fT = 35 GHz and fmax = 77 GHz small-signal performance. Substantial output conductance and fT · LG product roll-off were observed at short gate lengths for both AlOx-insulated and Schottky gate devices.