UC Santa Barbara

We discuss an AlGaN/GaN metal-semiconductor field-effect transistor (MESFET) structure grown without any impurity doping in the channel. A high-mobility polarization-induced bulk channel charge was created by grading the channel region linearly from GaN to Al0.3Ga0.7N over 1000 Angstrom. This polarization-doped FET (PolFET) was fabricated and tested under dc and rf conditions. Current density of 850 mA/mm and transconductance of 93 mS/mm was observed under dc conditions. The 0.7 mum gate length devices had a cutoff frequency, f(tau)=19 GHz, and maximum oscillation frequency, f(max)=46 GHz. We demonstrate that the PolFETs perform better than comparable MESFETs with impurity-doped channels, and are suitable for high power microwave applications. An important advantage of these devices over AlGaN/GaN high electron mobility transistors is that the transconductance versus gate voltage profile can be tailored by compositional grading for better large-signal linearity. (C) 2004 American Institute of Physics.