  We report the first realization of molecular beam epitaxy 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 excess of 2x10^13/cm2. Ohmic contacts to the 2DEG channel
were formed by n+ GaN MBE regrowth process, with a contact resistance of 0.13
Ohm-mm. 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 current gain cutoff
frequency fT~120 GHz. The DC and RF performance demonstrate that bulk AlN
substrates offer an attractive alternative platform for strained quantum well
nitride transistors for future high-voltage and high-power microwave
applications.
