Normally off AlGaN/GaN high electron mobility transistors with p-InGaN cap layer

Normally off AlGaN/GaN HEMTs with a p-In0.23Ga0.77N cap layer were successfully fabricated. The key idea in this approach is to employ the polarization-induced field in the InGaN cap and negative charge in the p-InGaN cap; this raises the conduction band at the AlGaN/GaN interface and leads to normally off operation. A threshold voltage of 1.2 V and maximum transconductance of 146 mS/mm were obtained. The gate leakage current of the InGaN-capped AlGaN/GaN HEMTs was investigated in detail. The V-shaped defects originating from the AlGaN surface have been shown to be responsible for the gate leakage current. The insertion of a thin GaN layer between the p-InGaN and AlGaN layers was shown to be effective in suppressing the formation of V-shaped defects and decreasing the gate leakage current.