The selective and low-damaged etching of p-type GaN or AlGaN layer is inevitable process for AlGaN/GaN high-power transistors. We have investigated an electrochemical etching of p-GaN layer grown on AlGaN/GaN heterostructures, consisting of an anodic oxidation of p-GaN surface and a subsequent dissolution of the resulting oxide. The p-GaN layer was electrochemically etched by following the pattern of the SiO<sub>2</sub> film that acted as an etching mask. Etching depth was linearly controlled by cycle number of triangular waveform at a rate of 25 nm/cycle. The AFM, TEM and μ-AES results showed that the top p-GaN layer was completely removed after 5 cycles applied, and the etching reaction was automatically sopped on the AlGaN surface. <i>I-V</i> and <i>C-V</i> measurements revealed that no significant damages were induced in the AlGaN/GaN heterostructures.
Surface characterization and control technologies were applied to GaN and AlGaN surfaces. It was found
that a unique "air-gap CV" technique is effective in evaluating surface state density on free AlGaN surfaces. A
photoelectrochemical process, utilizing a mixed solution of propylene glycol and tartaric acid, was employed to
form a thin oxide layer on GaN and AlGaN. We observed an enhancement of drain current in the AlGaN/GaN
HEMT having a narrow channel width of 200 nm after the oxidation of the channel walls by the electrochemical
process. To improve the uniformity of the effective electric field in the channel, a multi-mesa-channel (MMC)
AlGaN/GaN HEMT has been proposed and developed. With forming a periodic trench just under the gate region
by an ECR-plasma assisted dry etching, the MMC HEMT has parallel mesa-shaped channels with 2-dimensional
electron gas (2DEG) surrounded by the top- and side-gate electrodes.