Analysis of Impact Ionization Effects on Current Collapse of AlGaN/GaN HEMTs

Computer-aided analysis of impact ionization effects on turn-on characteristics or current collapse of AlGaN/GaN HEMTs is performed. Here, an intrinsic semi-insulating buffer is adopted in which deep donors are assumed to compensate deep acceptors, then the ionized deep donor usually plays a role as an electron trap. Calculated turn-on characteristics show that when impact ionization is not included, the drain current begins to increase relatively slowly because electrons are emitted from the deep donors, showing a large current collapse. On the other hand, when impact ionization is included and an ON-state drain voltage
${V}_{\text {D} {{ \text {ON}}}}$
is high, generated holes between the gate and drain flow toward the buffer and are captured by neutral deep donors, particularly at the source side. The hole capturing time becomes relatively short when
${V}_{\text {D} {{ \text {ON}}}}$
is high and the hole density is high. Because of these increases in positive space charges in the buffer, the drain current increases relatively fast before the electron emission under the gate starts. Therefore, the current collapse becomes weaker when the impact ionization is considered and
${V}_{\text {D} {{ \text {ON}}}}$
is higher. The situation may be similar with a different type of buffer such as an Fe-doped semi-insulating buffer in which the Fe-originated level (deep acceptor) is above the midgap and it usually plays a role as an electron trap.