Development of enhancement mode AlGaN/GaN MOS-HEMTs using localized gate-foot oxidation

A new method for realising enhancement mode AlGaN/GaN devices using a localized gate-foot oxidation is described. Thermal oxidation of the AlGaN barrier layer converts the top surface/part of this layer into aluminium and gallium oxides, which serve as a good gate dielectric and improve the gate leakage current by several orders of magnitude compared to a Schottky gate. The oxidation process leaves a thinner AlGaN barrier which can result in normally off operation. Without special precaution, however, the oxidation of the AlGaN barrier is not uniform from the top but occurs at higher rates at the defect/dislocation sites. This makes it impossible to control the barrier thickness and so rendering the barrier useless. To avoid the problem of non-uniform oxidation, a thin layer of Aluminum is first deposited on the barrier layer and oxidized to form Al₂O₃ on top. This additional oxide layer seems to ensure uniform oxidation of the AlGaN barrier layer underneath on subsequent further oxidation. Preliminary results of the fabricated 2μm × 100μm AlGaN/GaN MOS-HEMTs with a partially oxidized barrier layer showed a maximum drain current of more than 700 m A/mm at high gate bias of 5V with very less current compression.