Richard L. Waters

Micromachined Fabry–Perot interferometer for motion detection

The monolithic integration of a Fabry–Perot interferometer and a (100) silicon photodiode is reported for use as a highly sensitive transduction method in the detection of minute displacements of a proof mass attached to a spring. The combination results in a compact device with active transistor-like amplification and minimal parasitic elements. The transducer is fabricated using standard surface micromachining techniques. The finesse of the optical cavity, incident optical power, and geometry of the mirror and support structure control the sensitivity of the transducer. A transduction of more than 2285 A/m, percent change in transmission with displacement of 3%/nm, small-signal voltage amplification of 460 V/V, output resistance of 100 MΩ and transconductance of 1 mA/V have been obtained thus far for a single device without amplification.

Temperature-dependent tunneling through thermally grown SiO2 on n-type 4H– and 6H–SiC

The temperature dependence of field emission through thermally grown silicon dioxide (SiO2) on n-type 4H and 6H silicon carbide (SiC) substrates is reported. Room-temperature SiO2/SiC barrier heights, ΦB, of 1.92 and 2.12 V are extracted for the 4H– and 6H–SiC samples, respectively, using a Fowler–Nordheim analysis. Barrier heights of 2.2 and 2.4 V along with a linear temperature-dependent barrier height lowering, ΔΦB/ΔT, of 2.4 and 2.0 mV/K for 4H– and 6H–SiC are extracted using an alternative analytical expression for tunneling from semiconducting substrates derived previously. In both analyses, the temperature-dependent flatband voltage, using the measured room-temperature value, was included.

ON FIELD EMISSION FROM A SEMICONDUCTING SUBSTRATE

A theoretical examination of field emission from the conduction band of a semiconducting substrate is reported. The analysis includes a comparison with Fowler–Nordheim theory, and it is concluded that the formalism of the Fowler–Nordheim theory is incorrect when applied to carriers originating from a semiconducting substrate. The use of a Fowler–Nordheim analysis results in an error in the extraction of the barrier height that is dependent upon the applied electric field across the oxide, conduction band offset, and temperature. A lower limit of the error was calculated to be between 5% and 15%. An analytical expression is developed to describe the field emission of electrons from the conduction band of a semiconductor.

Fowler–Nordheim tunneling of holes through thermally grown SiO2 on p+ 6H–SiC

Fowler–Nordheim tunneling of holes through thermally grown silicon dioxide on 6H–silicon carbide is reported. Oxides of 5.2, 10, and 14.2 nm thickness were grown on the p+ face of a p+n SiC junction. The p+n junction served to separate the electron and hole tunneling currents. Hole tunneling was found to be the dominant current mechanism through the oxide. Fowler–Nordheim analysis, using a parabolic E–K relationship, was performed to extract a barrier height–effective mass product, ΦB3/2(mox/m0)1/2, for electrons and holes of 2.88%±4.9% and 2.38%±3.8% (V3/2) respectively. An estimate for the effective mass of holes within the oxide was made using both the parabolic and Franz dispersion relations.

Micromechanical optoelectronic switch and amplifier (MIMOSA)

We report for the first time the monolithic integration of a micromechanical modulator and a p-n photodiode on a silicon substrate yielding a versatile optoelectronic device. Because both devices are monolithically integrated on a silicon substrate, the combination is compact with minimal parasitic elements. We demonstrate that such a device combination fields a transistor-like element with positive and negative small-signal voltage amplification. The maximum small-signal voltage gain achieved is 500 while the modulated current of the device exhibits a maximum ON-OFF ratio of 3:1. In addition, while the theoretical current gain of the device is infinite, a 10-pA noise level limited the measured dc current gain to 10/sup 6/.

GaN/SiC HBTs and related issues

Abstract This paper reviews recent progress in the field of GaN/SiC heterojunction bipolar transistors. Key issues are identified and discussed. These include the comparison of a double-mesa and a selectively grown emitter structure, the evaluation of the GaN/SiC heterojunction and the problems faced when operating the devices in a common-emitter mode. A circuit simulation is presented indicating that common-emitter operation can be obtained for devices with low leakage current and modest gain.

Electro-optical ultra sensitive accelerometer

We present a novel optical transducer concept in the initial stages of development that promises to be inexpensive, small, lightweight, highly sensitive and durable. The successful development of this sensor will result in an optical accelerometer with resolution under 1 /spl mu/g (1g=9.8 m/s/sup 2/), which is two to three orders of magnitude more sensitive than current state-of-the-art MEMS-based accelerometers. This accelerometer is also expected to have a wide dynamic range with a resolution under 1 /spl mu/g at 100 Hz and improved low frequency response over existing MEMS technologies. This will yield much improved velocity and acceleration aiding to GPS tracking loops under high dynamic conditions, permitting continued low bandwidth tracking, a concomitant mitigation of external noise, and an increased jamming immunity. Also, the successful development of this accelerometer may enable the use of Distributed Tactical Navigation Tools (DISTANT) IMUs where distributed ultra-sensitive accelerometers may replace one or more expensive gyroscopes in an integrated IMU system.

Micro-mechanical optoelectronic switch and amplifier (MIMOSA)

We report for the first time the monolithic integration of a micro-mechanical modulator and a p-n photodiode on a silicon substrate. Such a structure can be used as a wavelength tunable detector and as a versatile optoelectronic device. In addition, we also demonstrate that such a device combination yields a transistor-like element with positive and negative small-signal voltage gain.

Factors influencing the noise floor and stability of a time domain switched inertial device

A new paradigm in inertial sensing is presented based on measurement of accurate time intervals using physical proximity switches deposed on a resonating beam structure. This paper discusses the time domain sensing concept and noise sources that can influence the performance of the sensor. The ultimate noise floor (-178 dB/√Hz)is limited by the accuracy of a time-to-digital convertor.

Intelligent polynomial curve fitting for time-domain triggered inertial devices

A method of blended-polynomial curve fitting is employed to improve performance of time domain triggered inertial devices. In such devices, a spring-mass system is perturbed to oscillate sinusoidally. This sinusoidal motion of the mass (the carrier) is perturbed by the time varying motion caused by inertial accelerations (the signal). When the mass passes several predefined locations, a trigger accurately measures the time of crossing. For each half-oscillation period, the overall motion of the mass is mapped by fitting a high order polynomial function to the triggering data. An analytic approximation of the motion of the mass is then obtained for all time by smoothly blending neighboring and overlapping polynomial fits together. If the carrier frequency is known, the inertial acceleration signal can be isolated from the overall polynomial fit by intelligently choosing the times to out-sample data. A 10 second real-time simulation is performed: having up to ±10 g inertial accelerations resulting in roughly 1 km of displacement. The average error magnitude is shown to be less than 1 μg, and the error in the navigational position estimate is 36.94 μm per 1 km of travel, or roughly 3 parts in 108 using a timing uncertainty of 10 ps.