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 β is around 90 calculated from the Gummel plot of a 20×20 μm
²
HBT device. Common-emitter I-V family curves exhibit offset voltage (V
_offset
) <; 0.5 V and knee voltage (V
_knee
) <; 6.5 V. A high current density JC 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
_CEO
) exceeds 98 V comparable with direct-growth GaN-based HBTs.