Self-aligned ALD AlO sub(x) T-gate insulator for gate leakage current suppression in SiN sub(x)-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 AlO sub(x) gate insulator and SiN sub(x) 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 AlO sub(x) dielectric and Ni/Au gate metal layers. By keeping the temperature low (100 C) during the atomic-layer deposition (ALD) of AlO sub(x), 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 AlO sub(x) 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) SiN sub(x) passivation of the exposed AlGaN surface access regions of AlO sub(x)-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 sub(G)) as short as 160 nm with f sub(T) = 35 GHz and f sub(max) = 77 GHz small-signal performance. Substantial output conductance and f sub(T) super(. L) sub(G)product roll-off were observed at short gate lengths for both AlO sub(x)-insulated and Schottky gate devices.