AlGaN/GaN polarization-doped field-effect transistor for microwave power applications

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 Å. 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 μm gate length devices had a cutoff frequency, $fτ=19 GHz,$ and maximum oscillation frequency, $fmax=46 GHz.$ 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.