Jack R. East

A micromachined high-Q X-band resonator

The authors present a new structure which can be used as a microwave high-Q resonator for the development of narrow-band low-loss filters in a planar environment. The resonator is made of a low-loss micromachined cavity which is easy to integrate with monolithic circuits. Compared to conventional metallic resonators, the performance of this resonator is similar, but the weight and size are significantly reduced.

Numerical modeling of abrupt heterojunctions using a thermionic-field emission boundary condition

We present a numerical model in which the thermionic and tunneling mechanisms across an abrupt heterojunction interface are taken into account on the basis of the one dimensional drift-diffusion formulation. We use an expression of thermionic-field emission current formulated based on the WKB approximation as a boundary condition at the abrupt heterointerface which eventually limits the current transport over the barrier while maintaining the current continuity. The I-V characteristics of three types of GaAs/AlGaAs heterojunctions are analyzed by varying device dimension, doping density, and temperature and compared with those obtained by the thermionic emission model to illustrate the significance of both tunneling and thermionic emission mechanisms. We demonstrate that the role of tunneling in the overall current transport is very important in these abrupt heterojunctions especially at high doping densities and low temperatures. In the case of an MBE-grown AlGaAs triangular heterojunction barrier, the temperature-dependent I-V characteristics are measured and compared with the theoretical results. Good agreement is obtained when the tunneling process is taken into account by employing the thermionic-field emission boundary condition presented here.

Power and stability limitations of resonant tunneling diodes

Stability criteria for resonant tunneling diodes are investigated. Details of how extrinsic elements, such as series inductance and parallel capacitance, affect the stability are presented. A GaAs/AlAs/InGaAs/AlAs/GaAs double-barrier diode is investigated, showing the effect of different modes of low-frequency oscillation and the extrinsic circuit required for stabilization. The effect of device stabilization on high-frequency power generation is described. The main conclusions of the paper are: (1) stable resonant tunneling diode operation is difficult to obtain, and (2) the circuit and device conditions required for stable operation greatly reduce the amount of power that can be produced by these devices. >

Evidence for field enhanced electron capture by EL2 centers in semi-insulating GaAs and the effect on GaAs radiation detectors

The performance of Schottky contact semiconductor radiation detectors fabricated from semi‐insulating GaAs is highly sensitive to charged impurities and defects in the material. The observed behavior of semi‐insulating GaAs Schottky barrier alpha particle detectors does not match well with models that treat the semi‐insulating material as either perfectly intrinsic or as material with deep donors (EL2) of constant capture cross section compensated with shallow acceptors. We propose an explanation for the discrepancy based on enhanced capture of electrons by EL2 centers at high electric fields and the resulting formation of a quasineutral region in the GaAs. Presented is a simple model including field enhanced electron capture which shows good agreement with experimental alpha particle pulse height measurements.

Nonlinear amplifier effects in communications systems

This paper introduces a figure-of-merit to investigate tradeoffs between amplifiers and modulation waveforms in complex digital communications systems. Class-AB amplifiers are investigated with a variety of modulation schemes to better understand the relations between amplifier efficiency, amplifier distortion, signal in-band and adjacent channel interference, and power consumption. The goal is to better understand the tradeoffs needed to design low-energy communications systems.

High-efficiency class-A power amplifiers with a dual-bias-control scheme

A new scheme for power amplifiers is proposed, which can provide both high efficiency and linearity. The proposed amplifier operates in a virtual class-A mode under dual-bias control to maximize the power-added efficiency along with its inherent class-A linearity. The dynamic dual-bias control involves controlling both bias current and voltage of the amplifier with a varying envelope of input RF signals. The efficiency of the proposed amplifier is theoretically evaluated and compared with that of other conventional amplifier schemes. Based on theoretical analyses, several promising schemes for dual analog and digital bias control are proposed and discussed.

Present status of undoped semi-insulating LEC bulk GaAs as a radiation spectrometer

Abstract Bulk GaAs has undergone extensive research by several groups in order to ascertain its usefulness as a room temperature radiation spectrometer. The results of an experimental program studying the properties of detectors fabricated from bulk GaAs are summarized in this paper. Electric field models of the active region are compared with measured results. Limitations of bulk LEC GaAs as a material for radiation spectrometers are discussed.

Quasi-two-dimensional modeling of GaAs MESFET's

A numerical simulation of GaAs MESFET structures is presented. The approach taken in this paper combines an analytical solution with a full simulation. Poissons equation, the current continuity equation, and an electron-temperature equation are formulated in terms of a geometry factor that defines the shape of the conducting channel in the MESFET. The transport equations are then solved in one dimension and the channel geometry factor is found analytically. This method was found to be considerably faster than full two-dimensional simulations. The model has been compared to full two-dimensional drift-diffusion and energy-momentum results to determine its validity.

Fully micromachined finite-ground coplanar line-to-waveguide transitions for W-band applications

A fully micromachined finite-ground coplanar (FGC) line-to-waveguide transition for W-band applications has been designed, fabricated, and tested. The transition utilizes a printed E-plane probe, inserted into the broad sidewall of a micromachined waveguide. This type of transition plays an important role in many applications where coupling between the popular FGC line and a waveguide is required. Excellent performance across the entire W-band of such a transition is presented in this paper. The investigated waveguide, micromachined in silicon using the deep reactive ion etching technique, demonstrates its potential as an alternative to costly conventional waveguides at high frequencies. A similar transition with a micromachined waveguide formed via bulk micromachining using a wet etchant is also demonstrated. The free-standing probe utilized in this second transition proves the potential of such transitions to be applicable well into the submillimeter and terahertz range.

Large-signal numerical and analytical HBT models

Several large-signal heterojunction bipolar transistor (HBT) models are investigated to determine their usefulness at millimeter-wave frequencies. The most detailed model involves numerically solving moments of the Boltzmann transport equation. A description of the numerical model is given along with several simulated results. The numerical model is then used to evaluate two analytical HBT models, the conventional Gummel-Poon model and a modified Ebers-Moll model. It is found that the commonly used Gummel-Poon model exhibits poor agreement with numerical and experimental data at millimeter-wave frequencies due to neglect of transit-time delays. Improved agreement between measured and modeled data results b including transit-time effects in an Ebers-Moll model. The simple model has direct application to millimeter-wave power amplifier and oscillator design. Several measured results are presented to help verify the simple model. >