Numerical Analysis of Impact Ionization Effects on Hard Switching in AlGaN/GaN HEMTs

of impact ionization on hard switching in AlGaN/GaN HEMTs are studied by 2-D analysis where undoped semi-insulating buffer including deep-donor traps is assumed. When the OFF-state drain voltage V-DOFF is near the breakdown voltage, the current collapse or the dynamic ON-resistance R-DON in the hard switching becomes smaller with impact ionization than those without impact ionization and in the case of soft switching. This is because in the case of hard switching with impact ionization, when the gate voltage becomes on and the drain voltage is high, generated holes by impact ionization flow into the buffer and are trapped by deep donors. Then, the buffer layer's positive space charges increase, resulting in the smaller current collapse or R-DON. The dependence of R-DON on V-DOFF is also studied. It is shown that when V-DOFF is relatively low, R-DON increases with V-DOFF and becomes higher in the hard switching than soft switching. This is because in the hard switching, electron can be injected into the buffer and trapped by deep donors when the gate voltage becomes on, and the buffer layer's negative space charges can increase. However, when V-DOFF is high in the case of hard switching, R-DON tends to saturate with increasing V-DOFF or decreases slightly because of hole's impact ionization and the hole trapping by the deep donors. Then, R-DON becomes smaller in the hard switching than in the soft switching when V-DOFF becomes high.

Automated summary

The impact of ionization on hard switching in AlGaN/GaN HEMTs is studied. It is found that impact ionization can reduce current collapse and dynamic ON-resistance. In hard switching, the generated holes by impact ionization are trapped, resulting in an increase of positive space charges in the buffer layer, which leads to a smaller current collapse. Additionally, R-DON increases with V-DOFF when it is relatively low, and is higher in hard switching.