p-Type InGaN Cap Layer for Normally Off Operation in AlGaN/GaN Heterojunction Field Effect Transistors

The normally off operation of AlGaN/GaN heterojunction field effect transistor (HFET) devices with a p-type InGaN cap layer under a gate electrode was demonstrated. The threshold gate voltage V Gth was 0.5 V, and the maximum transconductance g m was about 120 mS/mm. The maximum drain current I Dmax was more than 210 mA/mm. RF characteristics were also measured, and it was found that f T is 2.1 GHz and f max is 6.4 GHz when the gate length is 2 µm.