HEMT-based read-out of a thickness-mode AlGaN/GaN resonator

A multi-gigahertz AlGaN/GaN resonator is introduced, where the fundamental thickness-mode resonance at 2.1 GHz is excited exhibiting a quality factor (Q) of 105. For the first time, acoustic strain in the vertical direction is excited and sensed through a two-dimensional electron gas (2DEG), induced at the AlGaN/GaN interface. The 2DEG sheet is used as the bottom electrode for piezoelectric actuation, as well as the transistor conduction channel for acoustic sensing. In this design, acoustic resonance is sensed by modulation of the HEMT drain current (I_D); thus, the transistor is biased in the linear region of operation. Here, we study the dependency of the acoustic transconductance on the readout HEMT biasing and show that the read-out HEMT senses the drain current modulation only when the 2DEG channel is not pinched. To use the full potential of the HEMT intrinsic amplification, the transistor needs to be biased in the saturation region, which would require a modified RB-HEMT design.