Papers by Author: Mehran Mehregany

Abstract: The Seebeck coefficient of heavily-nitrogen-doped n-type polycrystalline 3C-SiC (n-SiC) and platinum (Pt) thin films has been measured from room temperature up to 300 °C by using a microfabricated test structure. At room temperature, the absolute Seebeck coefficient of the n-SiC is -10 μV/°C. With ambient temperature increase, the absolute Seebeck coefficient of the n-SiC is found to gradually increase, reaching -20 μV/°C at 300 °C.

Abstract: The SiC vacuum field-effect transistor (VacFET) was first reported in 2010 as a diagnostic tool for characterizing the fundamental properties of the inverted SiC semiconductor surface without confounding issues associated with thermal oxidation. In this paper, interface state densities are extracted from measurements of threshold voltage instability on a SiC VacFET and a SiC MOSFET. It is shown that removing the oxide can reduce the interface state density by more than 70%.

Abstract: We introduce the vacuum field-effect transistor (VacFET), the first SiC FET to use a vacuum-sealed cavity in place of the traditional, solid gate dielectric. This device architecture eliminates the need to thermally oxidize the SiC surface, a practice which has been widely reported to inhibit the performance and reliability of SiC MOSFETs. Using a combination of batch-compatible electronics and micromachining processing techniques, a polycrystalline SiC bridge is suspended above a 4H-SiC substrate, and the underlying cavity is sealed under vacuum. The fundamental studies made possible by such a device could shed much-needed light on the basic electronic properties of an inverted SiC surface. In this introductory report, we detail the analytical design and fabrication necessary to manufacture the VacFET, and we also demonstrate proof of the concept using turn-on and output characteristics of the first functional SiC device.

Abstract: We report fabrication of lateral, n-channel, depletion-mode, junction-field-effect-transistor (JFET) monolithic analog integrated circuits (ICs) in 6H-SiC. Ti/TaSi2/Pt forms the contact metalization, Ti/Pt the interconnect metal, and the SiO2/Si3N4/SiO2 interlayer dielectric. The threshold voltage and pinch off current indicate that the actual channel doping and thickness is close to the nominal values specified. The wafer yield for good circuits of a single-stage differential amplifier is 54% out of 46 copies.

Abstract: Fully monolithic, transimpedance and differential voltage amplifiers are reported in this paper based on 6H-SiC, n-channel, depletion-mode JFETs. The single-stage transimpedance amplifier has a low-frequency gain of ~222 kΩ at room temperature, with ~2% gain matching for copies on a 6-mm x 6-mm die. The transimpedance gain is set by an integrated resistor and is ~1.1 MΩ at 450oC. The single-stage, differential voltage amplifier has a typical gain-bandwidth of ~2.8 MHz at 600oC and a typical open-loop voltage gain of ~35.8 dB at 25oC, with less than 1-dB gain variation from 25-600oC.

Abstract: This paper reports fabrication and electrical characterization of 6H-SiC n-channel, depletion-mode, junction-field-effect transistors (JFETs) for use in high-temperature analog integrated circuits for sensing and control in propulsion, power systems, and geothermal exploration. Electrical characteristics of the resulting JFET devices have been measured across the wafer as a function of temperature, from room temperature to 450oC. The results indicate that the JFETs are suitable for high-gain amplifiers in high-temperature sensor signal processing circuits.

Abstract: This paper presents silicon carbide sensor interface circuits and techniques for MEMSbased sensors operating in harsh environments. More specifically, differential amplifiers were constructed using integrated, depletion-mode, n-channel, 6H-SiC JFETs and off-chip passive components. A three-stage voltage amplifier has a differential voltage gain of ~50 dB and a gainbandwidth of ~200 kHz at 450oC, as limited by test parasitics. Such an amplifier could be used to amplify the signals produced by a piezoresistive Wheatstone bridge sensor, for example. Design considerations for 6H-SiC JFET transimpedance amplifiers appropriate for capacitance sensing and for frequency readout from a micromechanical resonator are also presented.

Abstract: Micro-hotplate platforms, including the heating and sensing resistors, are fabricated by surface micromachining techniques from nitrogen-doped, polycrystalline SiC films deposited by low-pressure chemical vapor deposition. The resulting heated elements are operated without the need for metal electrodes. For comparison, platinum is also used as the heating/sensing resistor on top of otherwise similar SiC micro-hotplates. After characterizing the fundamental thermal transient response of the resulting micro-hotplates, accelerated aging tests are carried out by increasing the input power until the heating resistors fail. Material-related kinetic degradation analysis is conducted to estimate the life time of such elements as infrared emitters.

Abstract: A polycrystalline silicon carbide (poly-SiC) surface-micromachined capacitive accelerometer is designed, fabricated and tested. Leveraging the superior thermo-mechanical and chemical resistance properties of SiC, the device is a first step toward cost-effective implementation of a new class of extreme environment accelerometers, for example for high temperature vibration and shock measurements, even thought this initial work is at room temperature. The accelerometer described herein is designed for a range of 5000 g and a bandwidth of 18 kHz, specifications consistent with commercially available piezoelectric devices for high-level mechanical impact measurements. Test results demonstrate the sensor achieving a resolution of 350 mg/√Hz at 1kHz with a sensitivity of 12 μV/g and a bandwidth of 10 kHz at room temperature.