AlGaN/GaN Heterojunction Bipolar Transistor With Selective-Area Grown Emitter and Improved Base Contact

We report AlGaN/GaN n-p-n heterojunction bipolar transistors (HBTs) on sapphire substrates with selective-area grown AlGaN emitters by metal-organic chemical vapor deposition. Metal-semiconductor contact on the external p-GaN base layer shows ohmic characteristics thanks to the dry-etching-free process of base-emitter junction. As a result, the maximum current gain \beta is around 90 calculated from the Gummel plot of a {20}\times {20}\,\,\mu \text{m}^{{2}} HBT device. Common-emitter {I} - {V} family curves exhibit offset voltage ({V} _{\text {offset}}) < 0.5 V and knee voltage ({V} _{\text {knee}}) < 6.5 V. A high current density {J}_{\text C} of 8 kA/cm 2 and power density of 75 kW/cm 2 were obtained. These values are the highest in the reported for GaN-based HBTs on sapphire substrates. The open-base breakdown voltage ( {BV}_{\text {CEO}} ) exceeds 98 V comparable with direct-growth GaN-based HBTs.