Frequency-tunable current-assisted AlGaN/GaN acoustic resonators

Abstract

This work reports on frequency tunable AlGaN/GaN acoustic resonators that utilize piezoelectric actuation based on depletion-mediated strain in the AlGaN layer and piezo-resistive readout utilizing the two-dimensional electron gas (2-DEG) induced at the AlGaN/GaN interface. The effects of the DC current flowing through (I) forward-biased Schottky inter-digitated electrodes in Class I resonators, and (II) drain/source Ohmic contacts of an integrated AlGaN/GaN HEMT in Class II resonators are studied. The readout electrodes in Class I resonators are Ni/Au Schottky contacts, whereas in Class II resonators, Ti/Al/Ti/Au metal stack is deposited and annealed to form Ohmic contacts. In both classes of devices, wide-range frequency tuning is achieved by flowing DC current through the contacts, causing large elastic modulus change due to Joule heating of the device. Frequency tuning allows for compensation of effects of fabrication variations as well as environmental changes. The 9th-order width-extensional resonance mode at 730 MHz of Class I resonators is tuned by more than 500 ppm at 25 mW of input DC power, while maintaining a quality factor (Q) of ~4,500 with no performance degradation over the tuning range. The same mode of Class II resonators at ~719 MHz shows Q amplification from 1,710 at VDS= 4 V to 13,851 at VDS= 9 V, with more than 2,500 ppm of frequency tuning. Resonant devices with such large frequency tuning are perfect candidates as in-situ temperature sensors, where the resonance frequency shift is an indicator of the temperature rise in the channel of the suspended HEMT.