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 sub-micron tri-layer photoresist pattern to lift-off sequentially-deposited AlOx dielectric and Ni/Au gate metal layers. By keeping the temperature low (100 degrees 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 (L-G) as short as 160 nm with f(T) = 35 GHz and f(max) = 77 GHz small-signal performance. Substantial output conductance and f(T) . L-G product roll-off were observed at short gate lengths for both AlOx-insulated and Schottky gate devices. Published by Elsevier Ltd.