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.
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