Pradeep Pai

Micro-Plasma Field Effect Transistor Operating With DC Plasma

This letter presents the smallest microplasma field effect transistor (MOPFET) reported to date. The MOPFET has a gaseous (atmospheric pressure He) channel and operates in the sub-Paschen breakdown regime, where the channel breakdown voltage depends (nearly) linearly on the channel length. The gate field effect is explained by noting that the channel ionization depends on the primary electron density that is controlled by both VDS and VG; negative VG increased the channel electron density lowering the channel breakdown voltage (VDS-B), whereas positive VG attracted the channel electrons and reduced their density for ionization in the channel increasing the VDS-B. A simple empirical model using Townsend breakdown criteria is developed to include the effect of the gate electric field in VDS-B.

Precision curved micro hemispherical resonator shells fabricated by poached-egg micro-molding

This paper presents a new technique for the fabrication of high resolution non-planar precision microshells for hemispherical resonating gyros via a poached-egg micromolding (PEM) method. In PEM, precision ball lenses are used as starting molds. Hemispherical shells are formed on the lenses using five major steps: (1) isotropic coating with sacrificial and shell layers, (2) stencil transfer of coated lenses to substrate, (3) shell top removal by anisotropic etching, (4) sacrificial layer etching and (5) release of the ball lens molds. Using PEM, we fabricated posted sputtered ultra-low-expansion glass hemispherical shells of 1 mm diameter and 1.2 μm thickness. The microshells have better than 120 ppm uniformity in thickness and less than ±0.125 μm deviation from a perfect sphere. The measured microshell resonant frequency at 100 mTorr was 17.32 kHz and the Q was ~ 20,000.

MEMS-based hemispherical resonator gyroscopes

This paper introduces a fabrication technique that uses planar MEMS micromachining processes to produce hemispherical resonating shells for gyroscopes. The hemispheres exhibit a quality factor in excess of 20,000 with resonant frequencies in the range of 20 kHz for the 4-node wineglass mode. The fabrication process enables production of almost perfect hemispheres (less than 1% asphericity near the pedestal) with an average surface roughness of 5nm. The high degree of sphericity contains the relative frequency mismatch Δf/f between the two degenerate modes to 0.02%. Simplicity of the fabrication process and the successful testing of the drive/sense mechanism in the resonator make it a good candidate for use as gyroscopes.

Fabrication and testing of hemispherical MEMS wineglass resonators

This work focuses on understanding the behavior of 3D hemispherical shells operating in wineglass resonance mode through finite element modeling (FEM). Fabrication of the hemispherical shells was done using micromachining technique. The quality factor of the device was in excess of 10,000 when operated in 50mT vacuum. The shell showed better than 95% sphericity and had an rms surface roughness of ~5nm. The separation in the degenerate frequencies of 4-node wineglass resonance was 5 Hz at a resonant frequency of 22 kHz. Modeling of the device behavior relates the frequency mismatch to the asymmetry in the shell and quantifies it.

Non-intrusive electric power sensors for smart grid

An electric power sensor that measures near-field voltage and current waveforms through the insulation layer on a power cord is presented. To measure the line current, we examined Hall, giant magneto-resistive (GMR) and inductive sensors and found that for sensing 60 Hz current through its magnetic field, the inductive probe resulted in the best performance. To measure the voltage waveform, we developed a near-field electric dipole antenna that consisted of two strips of copper approximately 3 mm long. The voltage and current sensors were then calibrated and uncertainties due to the placement of the sensors over power cords were determined. A method was developed to enable the power sensor to perform auto calibration to estimate miss-alignments between the sensors and the wires in the cord. Power measurement accuracy of better than 5% was achieved.

SUB 3-micron gap microplasma fet with 50 V turn-on voltage

This work reports the smallest microplasma field-effect transistor reported till date that operates with a low turn-on voltage of ~50V dc; a more than 3x reduction in the turn-on voltage compared to earlier reported work. Our previous work used plasma from an external source to operate the transistor while in the present work we use rf or dc voltage to directly generate plasma in the transistor channel and use dc gate field effect to control the device conduction. The reduction in turn-on voltage is achieved by a small plasma cavity of 1.5μm gap width.

Microplasma Logic Gates

This paper demonstrates for the first time the use of microplasma to perform AND, OR, and XOR logic operations. The logic circuits are implemented similar to diode-resistor logic. Output voltages are in the range of 5-40 V and can be modified by varying the load resistor. The active area of the microplasma device is just 50 μm × 5 μm. The small interelectrode gap kept breakdown voltage to ~250 V when operated in He gas at atmospheric pressure. The use of plasma as a source of free carriers is also demonstrated, with free carriers available up to 500-μm away from plasma.

Magnetically coupled resonators for rate integrating gyroscopes

This work demonstrates for the first time the use of magnetic field to couple mechanical resonators that can be separated by long distances (8mm in this work) compared to coulombic/electrostatic coupling (~ 1 nm - 1 μm depending on the surface charge density or E-field). The resonators were composed of electroplated copper cantilevers with proof-mass resulting in 1 kHz resonant frequency when uncoupled. The coupling magnetic field was produced by 1 mT rare earth magnets mounted on the resonators. Electrostatic actuation and sensing was used to excite and sense the collective motion of the coupled resonators. The radial motion of the resonators in the presence of an in-plane rotation gives rise to the Coriolis force. Thus, the rotation changes the rate of energy exchange between the coupled resonators enabling rate integrating gyros.

Plasma interconnects and circuits for logic gates and computer sub-circuits

Logic gates using plasma-linked microplasma devices are demonstrated in this work. The space charge around a microplasma was used to lower the breakdown voltage of nearby device by 20–40 V. This mechanism was used to establish electrical connection between neighboring microplasma devices without the use of metallization traces. The decay lengths of the space charge were in the range of 178–400 μm depending on the type of gas used. He and Ar gases were used at atmospheric pressure to evaluate the effect of gas species on the space charge decay length. Plasmas can be used to connect devices in 3 dimensions, and their decay constant can be adjusted using pressure, boundary conditions, and gaseous species. Using plasma interconnects, universal gates including OR, AND, NOT, and XOR and computer sub-circuits such as 1 bit adders were designed and characterized.

Microplasma Field Effect Transistors

Micro plasma devices (MPD) with power gains are of interest in applications involving operations in the presence of ionizing radiations, in propulsion, in control, amplification of high power electromagnetic waves, and in metamaterials for energy management. Here, we review and discuss MPDs with an emphasis on new architectures that have evolved during the past seven years. Devices with programmable impact ionization rates and programmable boundaries are developed to control the plasma ignition voltage and current to achieve power gain. Plasma devices with 1–10 μm gaps are shown to operate in the sub-Paschen regime in atmospheric pressures where ion-assisted field emission results in a breakdown voltage that linearly depends on the gap distance in contrast to the exponential dependence dictated by the Paschen curve. Small gap devices offer higher operation frequencies at low operation voltages with applications in metamaterial skins for energy management and in harsh environment inside nuclear reactors and in space. In addition to analog plasma devices, logic gates, digital circuits, and distributed amplifiers are also discussed.