Current gain is demonstrated in III-N hot electron transistors (HETs) for the first time using base current controlled common emitter characteristics. The emitter and collector barriers (ØBE and ØBC) are implemented using AlN and In 0.1 Ga 0.9 N layers as polarization-dipoles, respectively. The entire structure is grown by plasma-assisted molecular beam epitaxy. Current gain is observed when the base thickness is reduced from 13 to 7 nm. Ohmic contacts to the base 2-D electron gas (2DEG) are achieved using a BCl 3 /SF 6 etch to remove the emitter and selectively stop on the AlN. Subsequent metallization results in a tunnel contact from the metal to the base 2DEG across the thin AlN layer. This dual purpose served by the AlN layer is shown to be critical for achieving scaled base and current gain in III-N HETs.

Transistor operation by common emitter (CE) current modulation is shown for the first time in III-N hot electron transistors (HETs). The emitter and collector barriers (φ BE and φ BC ) are implemented using Al 0.45 Ga 0.55 N and In 0.1 Ga 0.9 N layers as polarization dipoles, respectively. CE modulation is achieved by increasing the E-B barrier height beyond the B-C barrier height by increasing the Al 0.45 Ga 0.55 N thickness (t). Similar CE performance is seen in the identical HET structures grown on both bulk GaN and sapphire. A maximum α of ~0.3 is achieved using a GaN base thickness of 10 nm. The InGaN dipole used as the collector barrier is shown to be instrumental in enabling ohmic base contacts, low base sheet resistance, and low collector leakage, simultaneously.

In conclusion, we show that injection energy and base thickness are the critical parameters for achieving gain in III-N HETs. It is important to make sure that high leakage currents and large base resistance do not result in inaccurate extraction of gain and transfer ratio. A better understanding of the design space is required to achieve CE current gain and move towards our goal of building a high-frequency III-N HET.