We analyze off-state breakdown characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) with a Fe-doped buffer layer where a deep acceptor located above the midgap is included. It is shown that by introducing a high-k passivation layer, the breakdown voltage Vbr improves as in a case with an undoped semi-insulating buffer layer. In the Fe-doped case, Vbr becomes a little higher in the case where the passivation layer's relative permittivity er is rather higher when the energy levels determining the Fermi level are set equal in the two buffers. It is also shown that when the energy level of deep acceptor is deeper, Vbr becomes higher in the region where er is high. This occurs because the leakage current via the buffer layer becomes smaller.
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