Pnp AlGaN/InGaN/GaN double heterojunction bipolar transistors with low-base-resistance (< 100 Ω/square)

We investigated the current-voltage characteristics of pnp AlGaN/InGaN/GaN double heterojunction bipolar transistors grown by metalorganic vapor phase epitaxy. The InGaN base layer was heavily doped with Si in the range from 4 × 10¹⁹ o 2 × 10²⁰ cm^-3. Base sheet resistance was less than 100Ω/square for the base width of 30 nm and Si-doping concentration of 2 × 10cm^-3. This is the lowest resistance ever reported for nitride-based heterojunction bipolar transistors. In this device, the current gain was 2.7 at the collector current of 23 mA. We observed no significant degradation in the crystal quality of InGaN in separate Hall-effect measurements of Si-doped InGaN and in the device operation, even though the Si-doping concentration was above 10²⁰ cm^-3. The capability to decrease the base resistance will be advantageous for high-frequency operation.