James C. M. Hwang

Modeling and characterization of dielectric-charging effects in RF MEMS capacitive switches

For the first time, charging and discharging of traps in the dielectric of state-of-the-art RF MEMS capacitive switches were characterized in detail. Densities and time constants of different trap species were extracted under different control voltages. It was found that, while charging and discharging time constants are relatively independent of control voltage, steady-state charge densities increase exponentially with control voltage. A simple charge model was constructed to predict the amount of charge injected into the dielectric and the corresponding shift in actuation voltage. Good agreement was obtained between the model prediction and experimental data.

2D materials advances: From large scale synthesis and controlled heterostructures to improved characterization techniques, defects and applications

Author(s): Lin, Z; McCreary, A; Briggs, N; Subramanian, S; Zhang, K; Sun, Y; Li, X; Borys, NJ; Yuan, H; Fullerton-Shirey, SK; Chernikov, A; Zhao, H; McDonnell, S; Lindenberg, AM; Xiao, K; Le Roy, BJ; Drndic, M; Hwang, JCM; Park, J; Chhowalla, M; Schaak, RE; Javey, A; Hersam, MC; Robinson, J; Terrones, M | Abstract:

Acceleration of Dielectric Charging in RF MEMS Capacitive Switches

To design and validate accelerated life tests of RF MEMS capacitive switches, acceleration factors of charging effects in switch dielectric were quantitatively characterized. From measured charging and discharging transient currents at different temperatures and control voltages, densities and time constants of dielectric traps were extracted. A charging model was constructed to predict the amount of charge injected into the dielectric and the corresponding shift in actuation voltage under different acceleration factors such as temperature, peak voltage, duty factor, and frequency of the control waveform. Agreement was obtained between the model prediction and experimental data. It was found that temperature, peak voltage, and duty factor were critical acceleration factors for dielectric-charging effects whereas frequency had little effect on charging

Initial observation and analysis of dielectric-charging effects on RF MEMS capacitive switches

Capacitance voltage and RF-output characteristics of electrostatically actuated MEMS switches were measured under different control and stress voltages. It was found that positive voltage stress caused negative charging of the dielectric whereas negative voltage stress caused positive charging of the dielectric. This is consistent with the amphoteric nature of traps in the silicon oxynitride dielectric used for the switches. A hypothesis of charge injection in minutes and charge migration in milliseconds was proposed to explain real-time and nonsymmetrical drift of pull-down and hold-down voltages of the switches.

Direct extraction of equivalent circuit parameters for heterojunction bipolar transistors

A new method is presented for the direct extraction of hybrid-T equivalent circuits for heterojunction bipolar transistors. The method differs from previous ones by extracting the equivalent circuit without using test structures or numerical optimization techniques. Instead, all equivalent circuit parameters are calculated analytically from small-signal S-parameters measured under different bias conditions. The analysis includes the distributed nature of the HBT base. The calculated parameters are essentially frequency-independent and they exhibit systematic bias dependence over the typical operating range of the transistor. Thus, the present method ensures unique and physically meaningful parameters for transistor design improvement and large-signal circuit simulation. In addition, the present method is much faster than the numerical optimization method. >

High-Cycle Life Testing of RF MEMS Switches

RF MEMS capacitive switches capable of order-of-magnitude impedance changes have demonstrated operating lifetimes exceeding 100 billion switching cycles without failure. In situ monitoring of switch characteristics demonstrates no significant degradation in performance and quantifies the charging properties of the switch silicon dioxide film. This demonstration leads credence to the mechanical robustness of RF MEMS switches.

Quantum Dashes on InP Substrate for Broadband Emitter Applications

We report on the development of InAs/InGaAlAs quantum-dash-in-well structure on InP substrate for wideband emitter applications. A spectral width as broad as 58 meV observed from both photoluminescence and surface photovoltage spectroscopy on the sample indicating the formation of highly inhomogeneous InAs-dash structure that results from the quasi-continuous interband transition. The two-section superluminescent diodes (SLDs), with integrated photon absorber slab as lasing suppression section, fabricated on the InAs dash-in-well structure exhibits the close-to-Gaussian emission with a bandwidth (full-width at half-maximum) of up to 140 nm at ~ 1.6 mum peak wavelength. The SLD produces a low spectrum ripple of 0.3 dB and an integrated power of ~ 2 mW measured at 20degC under 8 kA/cm2. The oxide stripe laser exhibits wide lasing wavelength coverage of up to 76 nm at ~ 1.64 mum center wavelength and an output optical power of ~ 400 mW from simultaneous multiple confined states lasing at room temperature. This rule changing broadband lasing signature, different from the conventional interband diode laser, is achieved from the quasi-continuous interband transition formed by the inhomogeneous quantum-dash nanostructure.

A transient SPICE model for dielectric-charging effects in RF MEMS capacitive switches

A transient simulation program with integrated circuit emphasis (SPICE) model for dielectric-charging effects in RF microelectromechanical system (MEMS) capacitive switches was developed and implemented in a popular microwave circuit simulator. In this implementation, the dielectric-charging effects are represented by R-C subcircuits with the subcircuit parameters extracted from directly measured charging and discharging currents in the picoampere range. The resulted model was used to simulate the actuation-voltage shift in switches due to repeated operation and dielectric charging. Agreement was obtained between the simulated and measured actuation-voltage shift under various control waveforms. For RF MEMS capacitive switches that fail mainly due to dielectric charging, the present SPICE model can be used to design control waveforms that can either prolong lifetime or accelerate failure

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.

Superposition Model for Dielectric Charging of RF MEMS Capacitive Switches Under Bipolar Control-Voltage Waveforms

Bipolar control-voltage waveforms, under which the control voltage alternates between positive and negative after each cycle, have been proposed to mitigate dielectric charging in electrostatically actuated RF microelectromechanical system capacitive switches. In this study, dielectric charging under bipolar waveforms is modeled and characterized quantitatively. In general, the experimental results agree with predictions based on the superposition of unipolar charging models that are extracted under positive and negative voltages, respectively. The basic assumptions for such a superposition model are examined in detail and validated experimentally. The current analysis indicates that, while bipolar waveforms can reduce charging, it is difficult to fine tune the waveforms to completely eliminate charging.