Improved performance of ultraviolet AlGaN/GaN npn HPTs by a thin lightly-doped n-AlGaN insertion layer

We reported the improved performance of ultraviolet two-terminal Al0.1GaN/GaN npn heterojunction phototransistors with a 10-nm-thick low-doped n-type Al0.1GaN insertion layer between emitter and base. Optical current gain at 2 V bias was increased from 6.6 × 103 to 9.8 × 104 by inserting the thin undoped layer. Spectral response measurements showed a high ultraviolet to visible (350 nm/400 nm) rejection ratio of 6.7 × 104 under 2 V bias, while that of the control sample without the insertion layer is 2.4 × 103. Simulation analysis reveals that the conduction band notch at the interface of the base-emitter (B-E) heterojunction is lowered by the insertion layer, leading to a weakened electric field and a narrowed space-charge region at the interface. This effect can reduce the recombination in the B-E heterojunction and contribute to the improved gain performance of the phototransistor.