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

Effects 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 voltageV _ D OFFis near the breakdown voltage, the current collapse or the dynamic ON-resistanceR _ D ONin 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 orR _ D ON. The dependence ofR _ D ONonV _ D OFFis also studied. It is shown that whenV _ D OFFis relatively low,R _ D ONincreases withV _ D OFFand 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, whenV _ D OFFis high in the case of hard switching,R _ D ONtends to saturate with increasingV _ D OFFor decreases slightly because of hole’s impact ionization and the hole trapping by the deep donors. Then,R _ D ONbecomes smaller in the hard switching than in the soft switching whenV _ D OFFbecomes high.