Hideyuki Hanawa

Numerical Analysis of Breakdown Voltage Enhancement in AlGaN/GaN HEMTs With a High-

2-D analysis of breakdown characteristics in AlGaN/GaN high electron mobility transistors (HEMTs) is performed by considering a deep donor and a deep acceptor in a buffer layer. The dependence of the OFF-state breakdown voltage on the relative permittivity of the passivation layer εr and the thickness of the passivation layer d are studied. It is shown that as εr increases, the OFF-state breakdown voltage increases. This is because the electric field at the drain edge of the gate is weakened as εr increases. This occurs because in the insulator the applied voltage tends to drop uniformly in general, and hence when the insulator is attached to the semiconductor, the voltage drop along the semiconductor becomes smoother at the drain edge of the gate if the εr of the insulator is higher. It is also shown that the OFF-state breakdown voltage increases as d increases because the electric field at the drain edge of the gate is weakened as d increases. It is concluded that AlGaN/GaN HEMTs with a high- k and thick passivation layer should have high breakdown voltages.

k

We analyze breakdown characteristics and current collapse in AlGaN/GaN HEMTs, with the passivation layer’s relative permittivity er as a parameter. It is shown that the off-state breakdown voltage is considerably enhanced by introducing a high-k passivation layer because the electric field at the drain edge of the gate is weakened and the buffer leakage current is reduced. The breakdown voltage in the high-er region increases when the gate voltage is changed from −8 to −10 V, because the buffer leakage current is reduced. It is also shown that drain lag and current collapse in AlGaN/GaN HEMTs could be reduced by introducing a high-k thick passivation layer, because the electric field at the drain edge of the gate is reduced, leading to less electron injection into the buffer layer and weaker buffer trapping effects.

Passivation Layer

Two-dimensional analysis of lag phenomena, current collapse and breakdown voltages in source-field-plate AlGaN/GaN HEMTs is performed by considering a deep donor and a deep acceptor in a buffer layer, and the results are compared with those for the case of gate-field-plate structure. It is shown that the reduction rate of drain-lag is similar between the two structures, but the reduction rates of gate lag and current collapse are smaller for the source-field-plate structure. This is because the electric field at the drain edge of the gate becomes higher in the off state and the trapping effects become more significant. For this reason, an off-state breakdown voltage is a little lower in the source-field-plate structure. It is suggested that there is an optimum thickness of SiN passivation layer to minimize the buffer-related current collapse in both structures.

Analysis of high-k passivation-layer effects on buffer-related breakdown and current collapse in AlGaN/GaN HEMTs

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 V br improves as in a case with an undoped semi-insulating buffer layer. In the Fe-doped case, V br becomes a little higher in the case where the passivation layers 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, V br becomes higher in the region where er is high. This occurs because the leakage current via the buffer layer becomes smaller.

Similarities of lags, current collapse and breakdown characteristics between source and gate field-plate AlGaN/GaN HEMTs

A two-dimensional transient analysis of source-field-plate AlGaN/GaN high-electron-mobility transistors (HEMTs) is performed by considering a deep donor and a deep acceptor in a buffer layer, and the results are compared with those in the case of gate-field-plate structures. It is shown that the reduction rate of drain lag obtained by introducing a field plate is quantitatively similar between source- and gate-field-plate structures. However, the gate-lag rate is rather higher in the source-field-plate structure because the electric field at the drain edge of the gate is higher in the off state, and hence electron injection into the buffer layer and the resulting trapping effects are more significant. Hence, current collapse is slightly larger in the source-field-plate structure. It is also shown that an optimum SiN passivation layer thickness is required to minimize buffer-related current collapse in source-field-plate AlGaN/GaN HEMTs.