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

Abstract

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.

A Ferroelectric Capacitor (Fecap) Based Unreleased Resonator

“…Table2provides a comparison to other IC-integrated and piezoelectric resonators, including previously reported Resonant Body Transistors in GaN MMIC technology. As evident from the table, this work shows higher keff 2 than the GaN MMIC-integrated resonators primarily due to limits of the material[7][8]. However, this work achieved comparable f•Q products with most of previous PZT resonators[2],[10], but with the added benefit of direct CMOS integration.ACKNOWLEDGEMENTThe authors thank Texas Instruments for in-depth design and layout discussions and SEMs, as well as for fabrication of the devices presented here.…”

An 8.7 GHZ Gan Micromechanical Resonator With an Integrated Algan/Gan Hemt

“…Two approaches are taken to realize a GaN oscillator in GaN/AlGaN MMIC (Fig. 2): (1) Resonant body HEMTs, shown in [3][4][5], which are readily scalable to higher frequencies, but suffer from low acoustic gain insufficient for self-oscillation, and (2) cascade of a resonator and a HEMT (focus of this work). Capacitive feed-through, which becomes a critical issue at higher frequencies, can be removed from the resonator response using a dummy, unreleased resonator in a differential amplifier configuration [6], hence allowing to scale the resonant frequencies deep into the GHz regime where GaN ICs usually operate at.…”

“…Higher frequency resonators are sought for use as local oscillators (LOs) in high-power mm-wave GaN transceivers. Higher frequency resonances have been achieved using resonant body HEMTs [3][4][5], which offer intrinsic filtering and signal amplification. However, the acoustic gain of reported resonant body HEMTs proved to be too small for implementing an LO without the need of external amplifiers.…”

GaN Micromechanical Resonators with Meshed Metal Bottom Electrode

“…Because of such issues, different approaches have been taken to implement GaN-based piezoelectric transducers. The solutions that are sought so far include: (a) sputtering metal on the backside of the resonators, which requires release of the structure from the backside using deep reactive ion etching (DRIE) of the substrate [ 1 , 2 , 3 , 4 ]—DRIE is costly and usually not desired; (b) relying on lateral actuation without any bottom electrode—lateral excitation is not efficient as it relies on the weaker piezoelectric coefficient (d 31 ) or the weaker lateral electric field, and yields lower electromechanical coupling; (c) using a two-dimensional electron gas (2DEG) as the bottom electrode [ 5 , 6 , 7 , 8 , 9 ] that is unique to III-V hetero-structures—the 2DEG is generally 20–30 nm below the surface of the structure due to restriction of lattice-mismatched epitaxial growth, considerably limiting the thickness of the active piezoelectric layer compared to the resonant stack and making it inefficient as the actuator. This work seeks a different solution using embedded bottom electrodes for piezoelectric actuation of GaN resonators.…”

“…However, the location of 2DEG sheet is predetermined by the growth conditions and is commonly very close to the stack surface. For example, in [ 7 ] and [ 9 ], the 2DEG is only 20 nm below the resonator top surface. A 20 nm thick active piezoelectric layer is not an efficient actuator for excitation of a 1–3 μm thick GaN layer.…”