Enhancement of Breakdown Voltage in AlGaN/GaN HEMTs: Field Plate Plus High-k Passivation Layer and High Acceptor Density in Buffer Layer

We make a 2-D analysis of breakdown characteristics of field-plate AlGaN/GaN HEMTs with a high- ${k}$ passivation layer, and the results are compared with those having a normal SiN passivation layer. As a result, it is found that the breakdown voltage is enhanced particularly in the cases with relatively short field plates because the reduction in the electric field at the drain edge of gate effectively improves the breakdown voltage in the case with the high- ${k}$ passivation layer. In the case with the moderate-length field plate, the enhancement of breakdown voltage due to the high- ${k}$ passivation layer occurs because the electric field profiles between the field-plate edge and the drain become more uniform. It is also studied how the breakdown voltage depends on a deep-acceptor density in the Fe-doped semi-insulating buffer layer when a high- ${k}$ passivation layer is used. It is shown that the breakdown voltage increases with increasing the relative permittivity of the passivation layer $\varepsilon -{\text{r}}$ and with increasing the deep-acceptor density N_DA. When $\varepsilon -{\text{r}} = 60$ and $N-{\mathrm {DA}} = 2$ - $3 \times 10^{17}$ cm^-3 at the gate length of $0.3~\mu \text{m}$ , the breakdown voltage becomes about 500 V at a gate-to-drain distance of $1.5~\mu \text{m}$ , which corresponds to an average electric field of about 3.3 MV/cm between the gate and the drain.