Mohammad Javad Asadi

For millimeter-wave power applications, GaN high-electron mobility transistors (HEMTs) are often grown epitaxially on a high-purity semi-insulating c-axis 4H-SiC substrate. For these anisotropic hexagonal materials, the design and modeling of microstrip and coplanar interconnects require detailed knowledge of both the ordinary permittivity ε⊥ and the extraordinary permittivity εǁ perpendicular and parallel, respectively, to the c-axis. However, conventional dielectric characterization techniques make it difficult to measure εǁ alone or to separate εǁ from ε⊥. As a result, there is little data for εǁ, especially at millimeter-wave frequencies. This work demonstrates techniques for characterizing εǁ of 4H SiC using substrate-integrated waveguides (SIWs) or SIW resonators. The measured εǁ on seven SIWs and eleven resonators from 110 to 170 GHz is within ±1% of 10.2. Because the SIWs and resonators can be fabricated on the same SiC substrate together with HEMTs and other devices, they can be conveniently measured on-wafer for precise material-device correlation. Such permittivity characterization techniques can be extended to other frequencies, materials, and orientations.

Using epitaxial aluminum nitride (AlN) developed for ultraviolet photonics and high-speed electronics, we demonstrate suspended AlN thin-film bulk acoustic resonators (FBARs) at 9.2 GHz in the X-band (8–12 GHz) of the microwave spectrum. The resonators show a Qmax≈614 and a figure of merit f⋅Q≈5.6 THz. The material stack of these epi-AlN FBARs allows monolithic integration with AlN/GaN/AlN quantum well high-electron-mobility-transistors to a unique RF front end and also enable integration with epitaxial nitride superconductors for microwave filters for quantum computing.

A CMOS closed-loop control circuit was designed, fabricated, and tested for order-of-magnitude improvement of the robustness of a MEMS varactor against ambient temperature change. The control is based on real-time sensing of the capacitance of the MEMS varactor and fine-tuning its bias voltage according to the difference between the sensed and targeted capacitances. Simulation shows that this difference can be sensed with femtofarad sensitivity and a linear output over a piccofarad dynamic range. Measurement shows that, in a hybrid integration of the CMOS control circuit and the MEMS varactor, the varactor capacitance can be kept within ±2% of the targeted value despite a temperature change from 25°C to 80 °C. When the MEMS varactor is used as the tuning element of a substrate-integrated-waveguide (SIW) resonator, the control circuit can maintain the resonance frequency within ±0.08% over the temperature variation.

A closed-loop capacitance sensing and control mixed-signal CMOS circuit with a dedicated sensor electrode and a proportional-integral controller was designed to have a sensitivity of 1 fF, a settling time of 200 μ s, and a voltage capacity of 150 V. In a hybrid integration with a MEMS varactor, the control circuit could switch the varactor capacitance from 270 to 365 fF in 15 ms and hold it within ±2% between 25 °C and 80 °C. Furthermore, using the varactor as the tuning element of a substrate-integrated-waveguide (SIW) resonator, the control circuit could tune its resonance frequency from 33.2 to 33.9 GHz while keeping it within ±0.08% over a temperature range of 25 °C-90 °C. The control circuit occupied an area of 1.7 mm² and consumed 6 mW. The present closed-loop-controlled MEMS-tunable resonators could be used in a quartz SIW multi-pole filter to realize tunable, small, light, low-cost, robust, and reliable millimeter-wave filters that are critical to 5G and other future-generation communication and radar systems.

To make MEMS varactors more robust and reliable, a closed-loop control circuit was designed to sense the varactor capacitance and tune the MEMS bias in real time to adjust the capacitance to the desired value within 1 fF and in 200 μs. The designed circuit was successfully implemented in a standard RF CMOS technology despite the challenge for handling high voltage at high speed and for suppressing the substrate-coupled noise. The fabricated chip was tested to function according to the design simulation and to be capable of switching 150 V repeatedly.

Previously, the design of our novel low-dispersion phase-shifter unit cell with two asynchronous MEMS switches was limited to 90°, so that four unit cells (90°, 90°, 90° and 45°) with a total of eight MEMS switches were required for a 3-bit phase shifter. This not only increased the size and loss of the phase shifter, but also decreased its yield and reliability. Recently, we hypothesized that by using two synchronized MEMS switches, the phase shift of a unit cell could be extended to 180° so that only three unit cells (180°, 90° and 45°) with a total of six MEMS switches would be required for a 3-bit phase shifter. This paper confirms the hypothesis with measured characteristics of a fabricated 180° phase-shifter unit cell. The measured characteristics compare well with both equivalent-circuit model prediction and three-dimensional finite-element electromagnetic simulation, provided the worse-than-expected loss from individual MEMS switches is taken into account. The results provide the proof of the design principle, so that with improved MEMS fabrication process such as thicker metallization, the switch loss can be reduced and low-dispersion phase shifters can be realized with compact size, low loss, low cost, and high reliability.

This paper reports a low-dispersion metamaterial-based 3-bit phase shifter which occupies an area of approximately 5 mm 2 and uses only six microelectromechanical systems (MEMS) switches. The phase shifter is based on a coplanar slow-wave structure with defected ground and comprises three unit cells of 180°, 90° and 45° phase shifts, respectively. Each unit cell uses two single-pole-single-throw MEMS capacitive switches in series and parallel configurations, respectively, to switch between right-handed (low-pass) and left-handed (high-pass) states for the specified phase shift. Three-dimensional finite-element electromagnetic simulation was used to help optimize the compact layout. The worst-case performance across the band of 24-28 GHz was simulated to have less than 9° root-mean-square phase error, less than 1.7 dB insertion loss, and greater than 13 dB return loss.