A combination of atomic force microscopy （AFM） and scanning electron microscopy （SEM） is used to characterize dislocation etch pits in Si-doped GaN epilayer etched by molten KOH. Three types of etch pits with different shapes and specific positions in the surface have been observed,and a model of the etching mechanism is proposed to explain their origins. The pure screw dislocation is easily etched along the steps that the dislocation terminates. Consequently a small Ga-polar plane is formed to prevent further vertical etching,resulting in an etch pit shaped like an inverted truncated hexagonal pyramid at the terminal chiasma of two surface steps. However, the pure edge dislocation is easily etched along the dislocation line,inducing an etch pit of inverted hexagonal pyramid aligned with the surface step. The polarity is found to play an important role in the etching process of GaN.
Using depletion approximation theory and introducing acceptor defects which can characterize radiation induced deep-level defects in AlGaN/GaN heterostructures,we set up a radiation damage model of AlGaN/GaN high electron mobility transistor （HEMT） to separately simulate the effects of several main radiation damage mechanisms and the complete radiation damage effect simultaneously considering the degradation in mobility. Our calculated results,consistent with the experimental results,indicate that thin AlGaN barrier layer,high Al content and high doping concentration are favourable for restraining the shifts of threshold voltage in the AlGaN/GaN HEMT;when the acceptor concentration induced is less than 10^14cm-3,the shifts in threshold voltage are not obvious;only when the acceptor concentration induced is higher than 10^16cm-3,will the shifts of threshold voltage remarkably increase;the increase of threshold voltage,resulting from radiation induced acceptor,mainly contributes to the degradation in drain saturation current of the current-voltage （Ⅰ-Ⅴ） characteristic,but has no effect on the transconductance in the saturation area.
