Azadeh Ansari

Gallium Nitride as an Electromechanical Material

Gallium nitride (GaN) is a wide bandgap semiconductor material and is the most popular material after silicon in the semiconductor industry. The prime movers behind this trend are LEDs, microwave, and more recently, power electronics. New areas of research also include spintronics and nanoribbon transistors, which leverage some of the unique properties of GaN. GaN has electron mobility comparable with silicon, but with a bandgap that is three times larger, making it an excellent candidate for high-power applications and high-temperature operation. The ability to form thin-AlGaN/GaN heterostructures, which exhibit the 2-D electron gas phenomenon leads to high-electron mobility transistors, which exhibit high Johnsons figure of merit. Another interesting direction for GaN research, which is largely unexplored, is GaN-based micromechanical devices or GaN microelectromechanical systems (MEMS). To fully unlock the potential of GaN and realize new advanced all-GaN integrated circuits, it is essential to cointegrate passive devices (such as resonators and filters), sensors (such as temperature and gas sensors), and other more than Moore functional devices with GaN active electronics. Therefore, there is a growing interest in the use of GaN as a mechanical material. This paper reviews the electromechanical, thermal, acoustic, and piezoelectric properties of GaN, and describes the working principle of some of the reported high-performance GaN-based microelectromechanical components. It also provides an outlook for possible research directions in GaN MEMS.

Gallium nitride-on-silicon micromechanical overtone resonators and filters

In this paper, for the first time, we report on high-performance GaN-on-silicon micromechanical resonators and filters. A GaN-on-silicon resonator is reported which exhibits a quality factor of 1850 at 802.5 MHz, resulting in an f×Q value twice the highest reported for GaN-based resonators to date. The effective coupling coefficient for the GaN resonator is extracted to be 1.7%, which is among the best reported in the literature.

A Thickness-Mode AlGaN/GaN Resonant Body High Electron Mobility Transistor

A multigigahertz AlGaN/GaN resonant body transistor (RBT) is reported, wherein the mechanical resonance and electrical signal modulation are achieved simultaneously. A 175-Å-thick AlGaN layer is used as the piezoelectric transduction layer, and the 2-D electron gas present at the AlGaN/GaN interface is employed as the bottom electrode as well as the transistor conducting channel. The carrier concentration of the channel is modulated when the device undergoes acoustic strain. A quality factor of 250 and acoustic transconductance of 25 μS is achieved at resonance frequency of 4.23 GHz, marking the highest frequency and highest transconductance reported to date for GaN-based RBTs. The frequency×Q of this device is among the best reported for GaN-based resonators.

Monolithic integration of GaN-based micromechanical resonators and HEMTs for timing applications

A platform for intimate integration of high-frequency gallium nitride (GaN) micromechanical resonators and AlGaN/GaN high electron mobility transistors (HEMTs) is reported. For the first time, cascade of a two-port GaN bulk acoustic resonator and AlGaN/GaN HEMT was co-fabricated on a silicon substrate. A high quality factor (Q) of 7413 is reported for a GaN contour-mode resonator at the resonance frequency of 119.8 MHz. More than 30 dB of signal tuning was achieved by using integrated HEMT for signal readout and amplification at the resonator output.

A high-Q AlGaN/GaN phonon trap with integrated hemt read-out

This paper presents novel phonon traps, the main building blocks of self-sustained all-GaN oscillators, implemented on an AlGaN/GaN electro-acoustic platform. The geometry of acoustic cavities has been engineered to efficiently trap the energy in the central region of devices, where the interdigitated excitation/sense electrodes and HEMT read-outs are located. Such an energy trapping method obviates the need for narrow tethers. This facilitates routing of multiple lines connected to the source, gate, and drain of the read-out HEMTs as well as allowing co-integration of different interdigitated transducer (IDT) sets for efficient piezoelectric lateral field excitation. Furthermore, elimination of narrow tethers improves the power handling capability of the GaN resonators, making them suitable for high-power applications. Two types of AlGaN/GaN-based devices are discussed in this work, both operating in the ninth-order width-extensional mode that is excited using Schottky IDTs. In the first device, another set of Schottky IDTs it used for sensing, while the second device utilizes a HEMT to read-out the signal. An unloaded quality factor (Q) of ~13,000 has been measured at room temperature and atmospheric pressure for a device operating at ~740 MHz, resulting in a frequency × Q value of 0.96×1013, which is the highest value reported to date for GaN-based resonators and is very close to the intrinsic limits for AlN and GaN, set by phonon-phonon interactions.

A Temperature-Compensated Gallium Nitride Micromechanical Resonator

A GaN bulk acoustic wave resonator is presented in this letter, showing fundamental thickness-mode resonance at 2.18 GHz, with a quality factor (Q) of 655 and a coupling coefficient (k_t²) of 1%. The resonator is integrated with an AlGaN/GaN high electron mobility transistor (HEMT); the integrated resonator/HEMT structure is coated with a silicon dioxide (SiO₂) passivation layer. It is shown that a 400-nm-thick SiO₂ layer reduces the temperature coefficient of frequency (TCF) of the GaN-based resonator by 50%, while improving Q and k_t² of the fundamental thickness-mode resonance. The effect of SiO₂ passivation layer is studied on k_t², Q, and TCF of the device. Furthermore, the effects of temperature and input RF power on the resonator performance are characterized.

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 (ID); 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.

Observation of acoustoelectric effect in micromachined lamb wave delay lines with AlGaN/GaN heterostructure

We report on the first time observation of acoustoelectric (AE) effect from the interaction of acoustic Lamb waves and two-dimensional electron gas (2DEG) in an AlGaN/GaN heterostructure. Micro-fabricated Lamb wave delay lines are used to launch and guide travelling acoustic waves through the 2DEG region, resulting in a DC current flow between two ohmic contacts positioned on the delay line. The Lamb wave delay line shows much better acoustic transmission efficiency than the conventional surface acoustic wave (SAW) counterpart. The dependence of AE current on RF power and frequency is also verified.

Frequency-tunable current-assisted AlGaN/GaN acoustic resonators

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.

Lamb wave dispersion in gallium nitride micromechanical resonators

In this paper, we explore the phase velocity dispersion of Lamb wave in gallium nitride (GaN) thin film. For this study, ultra-high-frequency (UHF) GaN micromechanical resonators with various electrode pitch distances are designed as test vehicles. Fabricated by a process compatible with commercial monolithic microwave integrated circuit (MMIC) technology, our GaN resonators demonstrate promising performance (e.g. high quality factor of 1200 at 1GHz) and can be monolithically integrated with active (i.e. HEMTs) devices in an all-GaN platform. The zero-order symmetric (S0) and antisymmetric (A0) Lamb wave modes are observed in our measurements. The experimentally extracted phase velocity dispersion relations of GaN for both S0 and A0 modes are in good agreement with the theoretical data. Out results indicate that S0 mode is a better choice than A0 mode for high frequency GaN Lamb wave resonators because of its high phase velocity and low dispersion characteristics.