Strained GaN quantum-well FETs on single crystal bulk AlN substrates

We report the first realization of molecular beam epitaxy (MBE) grown strained GaN quantum well field-effect transistors on single-crystal bulk AlN substrates. The fabricated double heterostructure FETs exhibit a two-dimensional electron gas (2DEG) density in the excess of 2 × 1013/cm2. The ohmic contacts to the 2DEG channel were formed by the n+ GaN MBE regrowth process, with a contact resistance of 0.13 Ω · mm. The Raman spectroscopy using the quantum well as an optical marker reveals the strain in the quantum well and strain relaxation in the regrown GaN contacts. A 65-nm-long rectangular-gate device showed a record high DC drain current drive of 2.0 A/mm and peak extrinsic transconductance of 250 mS/mm. Small-signal RF performance of the device achieved the current gain cutoff frequency fT∼120 GHz. The DC and RF performances demonstrate that bulk AlN substrates offer an attractive alternative platform for strained quantum well nitride transistors for the future high-voltage and high-power microwave ap...