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/cm2 and power density of 75 kW/cm2 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. [ABSTRACT FROM AUTHOR]