Analysis of breakdown voltage of field-plate AlGaN/GaN HEMTs as affected by buffer layer's acceptor density

We perform a numerical analysis of field-plate AlGaN/GaN HEMTs with a Fe-doped buffer layer with only a deep acceptor as a deep level and study how its density N _DA and the length of field-plate L _FP affect the breakdown voltage V _br. The calculated characteristics usually show abrupt increases in gate and drain currents due to impact ionization, resulting in breakdown. But, in some cases, V _br is limited by current flow through the buffer, and this current is higher for lower N _DA. Therefore, V _br becomes higher for higher N _DA. V _br takes a maximum value at some L _FP, and the highest average electric field between gate and drain becomes about 3.2 MV cm^-1 when the breakdown occurs.