MBE-Grown Ultra-shallow AlN/GaN HFET Technology

Due to large polarization effects, two-dimensional electron gas (2DEG) concentrations higher than 1x10 cm can be produced at the AlN/GaN heterojunction with AlN barriers as thin as 2 nm. This ultra-shallow channel together with the wide bandgap of AlN (6.2 eV) makes AlN/GaN heterojunction field effect transistors (HFET) extremely attractive for high frequency (>100 GHz) high power applications. At Notre Dame, these structures have been grown using molecular beam epitaxy and the record transport properties among III-V nitrides are achieved: a sheet resistance of ~ 170 ohm/square for a single heterostructure at room temperature. HFETs have been fabricated with optical lithographically defined gates. At present the device dc characteristics show a maximum drain current of 800 mA/mm and transconductance of 180 mS/mm for 3 μm long gate. This clearly demonstrates its value toward high speed devices. The development as well as challenges of this technology will be discussed here.