Subrata Halder

Robustness of RF MEMS Capacitive Switches With Molybdenum Membranes

This paper compares the characteristics of an RF microelectromechanical systems (MEMS) capacitive switch with a molybdenum membrane versus that of a switch with similar construction but with an aluminum membrane. In comparison, the molybdenum switch exhibits a significantly reduced sensitivity to ambient temperature change so that its pull-in voltage varies by less than 0.035 V/°C. In addition, large-signal RF performance of the switches was compared under both continuous wave and pulse conditions. The results show that under large RF signals, the self-biasing effect is exacerbated by the self-heating effect and the self-heating effect is in turn amplified by nonuniform current and temperature distributions on the membrane. Measurements of both molybdenum and aluminum switches demonstrate a hot-switched power-handling capacity of approximately 600 mW. Since aluminum has been used as a membrane material for over a decade while molybdenum is new, the above results indicate that molybdenum is a promising membrane material for RF MEMS capacitive switches.

Temperature-Dependent RF Large-Signal Model of GaN-Based MOSHFETs

A temperature-dependent RF large-signal model is constructed by modifying the Verilog-A code of the Angelov model for unique characteristics of GaN MOSHFETs. Different from the previously reported EEHEMT-based model, the present electro-thermal model can fit the temperature effects on threshold shift and transconductance degradation, the drain current in the linear region, and the gate capacitance near the cutoff region. As the result, the power, gain, efficiency, linearity, drain current, and gate current of both Class-A and Class-AB amplifiers are accurately simulated over a wide range of input powers, matching impedances, and ambient temperatures.

Compact RF Model for Transient Characteristics of MEMS Capacitive Switches

A compact model is proposed to facilitate the design and simulation of the control waveform of RF microelectromechanical systems (MEMS) capacitive switches with electrostatically actuated membranes. Following conventional approaches, the pull-in motion of a membrane is simulated by an L-R-C network. However, the present model deviates from conventional approaches by adding another capacitor and a diode to simulate the gradual contact of the membrane with the stationary electrode. After contact, variable mass, spring constant, and damping factor are used to simulate the release process of the membrane. By smoothly bridging the model between pull-in, contact, and release processes, the model can efficiently simulate static and transient S-parameters of the switches up to 50 GHz. The model can be readily installed in popular computer-aided circuit design environments to analyze in the time domain the behavior of the switches and the operation of MEMS-based circuits.

Safe Operating Area of GaAs HBTs Based on Sub-Nanosecond Pulse Characteristics

Using a novel sub-nanosecond pulse current-voltage measurement technique, InGaP/GaAs heterojunction bipolar transistors (HBTs) were shown to survive strong impact ionization and to have a much larger safe operating area (SOA) than previously measured or predicted. As the result, an empirical model for impact ionization with both voltage and current dependence was extracted and added to a commercially available HBT model. The modified model could predict the HBT characteristics across the enlarged SOA, as well as the performance of ultra-wideband pulse generators and the ruggedness of continuous wave Class-C power amplifiers.

Dielectric Charging in Electrostatically Actuated MEMS Ohmic Switches

MEMS switches having separate signal and actuation electrodes with different air gaps are fabricated using a copper-based CMOS interconnect manufacturing process. By using a control voltage high enough to establish metal-metal contact between the signal electrodes while avoiding contact between the dielectric-covered actuation electrodes, dielectric charging appears to be tolerable. By simultaneously measuring the conductance across the signal electrodes and the capacitance across the actuation electrodes, the conductance-force characteristic can be readily monitored and analyzed. For the present switches, the effect of polarization charge appears to be negligible, and dielectric charging is significant only after dielectric contact is made and space charge is injected.

Order-of-magnitude improvement in microwave power capacity of InGaP/GaAs HBT under isothermal pulsed operation

Order-of-magnitude improvement in power capacity was achieved in single-finger heterojunction bipolar transistors with sub-mus pulses and less than 1% duty cycle, thereby approaching isothermal operation. At 6 GHz, the output power capacity was 14 mw/mum2, which was much higher than the common benchmark of 1 mw/mum2. The improvement in power capacity was found to be due to increased breakdown voltage as well as increased peak current under pulsed operation. The increased peak current was in turn due to a much higher Kirk threshold under isothermal operation

Intelligent Bipolar Control of MEMS Capacitive Switches

Closed-loop control of microelectromechanical systems (MEMS) capacitive switches was demonstrated by using an intelligent CMOS circuit. The control was based on fine tuning the bias magnitude of the switches according to the difference between sensed and targeted capacitances. Innovative designs were used to allow the CMOS circuit to sense low capacitance and to handle high voltage. The CMOS die of 3 × 1.5 mm2 was dominated by input/output and voltage regulation/protection circuits; the actual capacitance sense/control circuit was smaller than 0.1 mm . The entire circuit consumed 0.7 mW of power during active sense/control, which could be significantly reduced with less frequent sense/control and advanced CMOS technology. With a maximum actuation voltage of ±40 V and a target capacitance of 0.5 pF, a control accuracy of ±2.5% was demonstrated, which could be improved to ±1% with reduced parasitics through monolithic integration. Intelligence was programmed to alternate the bias sign when its magnitude required to maintain the targeted capacitance drifted significantly due to charging of the switch dielectric. Such intelligent control could also be used to compensate for process variation, material creep, ambient temperature change, and RF power loading, which would make MEMS capacitive switches not only more reliable, but also more robust.

Compact RF large-signal model for MEMS capacitive switches

Last year, we reported on a SPICE-based compact RF small-signal electromechanical model for electrostatically actuated MEMS capacitive shunt switches with movable membranes. We now report on the enhancement of the model to include electrothermal and thermomechanical effects so that the model is applicable under large-signal RF conditions. Specifically, a thermal subcircuit is added to account for the temperature rise in the switch membrane as a function of the dissipated RF power. In turn, the temperature rise is used to evaluate the decrease in the membrane spring constant. These enhancements allow the present multiphysics model to simulate the coupled self-biasing and self-heating effects under RF large signals and to predict the power-handling capacity of MEMS capacitive switches. Additionally, the model has been coded in Verilog, making it portable between different circuit simulation environments.

Tunable pulse generator for ultra-wideband applications

For the first time, an ultra-wideband pulse generator is fabricated in GaAs HBT IC technology. The generator includes delay and differential circuits to convert a TTL input into an impulse signal and a Class-C amplifier to increase the pulse amplitude while compressing the pulse width. By adjusting the bias of the Class-C amplifier, the pulse amplitude can be varied linearly between 3.5 V and 11.5 V while maintaining the pulse width at 0.3±0.1 ns. Alternatively, the pulse width can be varied linearly between 0.25 ns and 0.65 ns, while maintaining the pulse amplitude at 10±1 V. Finally, the amplified impulse signal can be shaped into a monocycle signal by an L-C derivative circuit. These results compare well with that of pulse generators fabricated in GaAs HEMT, Si CMOS or Si discrete technologies.

Intrinsic response of quantum dash lasers under optical modulation

The intrinsic response of quantum dash lasers are measured using a pulse optical modulation technique. Our results show fast well-to-dash carrier capture and intra-dash carrier relaxation with K-factors independent on the pump wavelengths.