R. Jennifer Hwu

A new source formulation for FDTD simulation of high‐frequency integrated circuits with and without ground planes

A newoltage source formulation for FDTD is reported in this article. By stimulating theoltage source right in the plane of the strip line, this new source formulation can be used in the simulation of a wideariety of high-frequency integrated circuits, including those without ground planes. Compared with existing source formulations, the new source formulation is also relatiely simple for most circuit simula- tions, because, since its relation to the actual sourceoltage is fairly straightforward. In addition to numerical calculations to show thealida- tion, we present in this article the applications of this method in adanced high-frequency integrated circuits such as quasioptical diode array and coplanar waeguide nonlinear transmission lines. Q 1997 John Wiley & Sons, Inc. Microwave Opt Technol Lett 14: 321)324, 1997.

Characterizations of crystal-grid filter in quasi-optical circuits using 3-D anisotropic FDTD formulation and Floquet boundary condition

An analysis of anisotropic crystal grid-plate used in quasi-optical circuits is presented in this paper. The 3-D anisotropy FDTD formulation using permeability tensor is directly implemented with necessary treatments on the local fields specified by the relationship between the electric displacement and field, as well as on the air-anisotropy interface. Floquet boundary condition using one unit cell and a pulse excitation are employed for analysis of the large grid-anisotropy plate. The simulated results of field rotation, transmittance, reflectance, as well as the effects of the grid on the anisotropy plate are obtained and show excellent agreement with experiments.

High‐temperature, radiation‐hard electronic technology

We report on two novel high temperature and potentially highly neutron and gamma radiation resistant electronc technologies that are suitable for nuclear and space applications. The operational effects on these technologies from gamma radiation doses up to 10 megarads and 1‐MeV equivalent neutron fluences up to 1014 neutrons/cm2 are examined using a calibrated (to appropriate ASTM standards) irradiation chamber in the University of Utah TRIGA Nuclear Reactor. The first high temperature, harsh environment technology is based on microminiature vacuum (MTV) devices. The second high temperature technology is gallium arsenide (GaAs) metal semiconductor field effect transistor (MESFET)‐based devices and circuits that can operate at temperatures up to 350 °C. This MESFET‐based technology also allows a wide range of control with respect to the MESFET’s enhanced resistance to breakdown at elevated temperatures. The MESFET‐based technology has general applicability and works equally well with both enhancement and d...

Nonlinear FDTD analysis of back-to-back varactor diode frequency tripler

A quasi-optical diode-grid frequency tripler array was analyzed by the Finite-Difference Time-Domain (FDTD) method in this paper. A nonlinear algorithm was successfully developed in order to deal with the nonlinear characteristic of a varactor. The results obtained for a test case showed excellent agreement between the FDTD and a direct mathematical method. The detailed analysis of a frequency tripler array employing back-to-back AlGaAs/GaAs heterojunction varactor was given.

FDTD simulations of quasi-optical MESFET oscillator arrays

A method for the FDTD analysis of quasi-optical MESFET arrays is presented in this paper. To analyze active devices such as MESFETs, this method creates a set of field-state central finite difference equations; and all the field and state variables are solved simultaneously at the same FDTD time step. It will be shown in this paper that this formulation is simple and straightforward. The analysis of quasi-optical MESFET oscillator arrays have been performed to show the application of this method. Excellent agreements have been obtained between the results from this simulation method, from commercial softwares, and from experimental measurements to show the validity of this method.

High-power and high-temperature FET technology

The performance of heterojunction metal-semiconductor-field- effect-transistors (MESFETs) and junction-field-effect- transistors (JFETs) fabricated with different buffers is presented. For the JFET, carbon was chosen as the p-type dopant because of its relative low diffusivity compared to other doping elements. The viability of heterojunction MESFET and JFET devices operating at 400 degrees C have been demonstrated. Two key factors contributing to the reduction of drain leakage currents were the use of a high resistivity, undoped AlAs buffer layers and the gate contacting layers: n-type AlGaAs for the MESFET and p-type AlGaAs for the JFET. A two LT-AL0.3Ga0.7As layer scheme were used for the first time specifically for use in high temperature applications. Even at 400 degrees, C the gate leakage current density for a gate length of 2 micrometers was 9 by 10-7 A/micrometers at Vds equals 3V Vgs equals -7. The high resistance of LT-AlGaAs materials after annealing was responsible for such low gate leakage currents. The p- HEMTs became leaky at high temperature because of the parallel conduction and buffer design. The gate diode performed better when contacted to the undoped AlGaAs layer. DC and high-temperature performance of GaN-based MESFETs and MODFETs were compared. Al0.3Ga0.7N/GaN MODFETs with a gate-length of 2micrometers exhibited high transconductance was 47mS/mm, and dropped by 12 percent of its initial value to 41.4 mS/mm at 350 degrees C. The decrease in transconductance with temperature can be explained by the temperature dependence of the electron mobility. The large conduction band discontinuity in this material system may play an important role in terms of better electron confinement thus resulting in less degradation in transconductance.

Development of three-dimensional monolithic microwave integrated circuit components

FDTD applications in 3D monolithic microwave integrated circuit (MMIC) design are introduced in this paper. The source implementation for active device modeling, device grid-characterization, a novel spiral component simulations are presented. Our research in this area have shown that FDTD is a very effective tool for the design of 3D MMICs.

Material growth investigation and high performance of InGaAs/InGaAsP/AlGaAs quantum well diode lasers (λ=0.98 μm)

In this work, we conduct a theoretical analysis of the design, fabrication, and performance measurement of high power and high brightness strained quantum well lasers emitting at 0.98 (mu) m. The material system of interest consists of an Al-free InGaAs/InGaAsP active region and AlGaAs cladding layers. The laser material is grown by metal-organic chemical vapor deposition and demonstrates high quality with low threshold current density, high internal quantum efficiency, and extremely low internal loss. High performance broad-area multi-mode and ridge- waveguide single mode laser devices are fabricated. For 100 (mu) m-wide stripe lasers having a cavity length of 800 (mu) m, a high slope efficiency of 1.08 W/A, a low vertical beam divergence of 340, a high output power of over 4.45 W, and a very high characteristic temperature coefficient of 250 K were achieved. Lifetime tests performed at 1.2-1.3 W (12-13 mW/(mu) m) demonstrates reliable performance. For 4 (mu) m-wide ridge waveguide single mode laser devices, a maximum output power of 394 mW and fundamental mode power up to 200 mW with slope efficiency of 0.91 mW/(mu) m are obtained.

High-power 0.98-μm broad-waveguide lasers using novel material system of InGaAs/InGaAsP/AlGaAs

We describe the theoretical design and experimental fabrication of high power 0.98 micrometers strained quantum well lasers employing broad waveguide structure and novel hybrid material system of Al-free InGaAs/InGaAsP active region and AlGaAs cladding layers. The use of AlGaAs cladding, instead of InGaP, provides advantages in flexibility of laser structure design, simple epitaxial growth, improvement of surface morphology and laser performance. In addition to the theoretical study of the new structure, we successfully demonstrate the design by obtaining high performance laser devices. The as-grown InGaAs/InGaAsP/AlGaAs laser material exhibits very high quality with low threshold current density of 200 A/cm2, high internal quantum efficiency of approximately 93 percent, and low internal loss of 1.2 cm-1. For 100 micrometers -wide stripe lasers with cavity length of 800 micrometers , a high slope efficiency of 1.03 W/A, low vertical beam divergence of 36 degrees, high output power of 3.65 W, and very high characteristic temperature coefficient of 250 K are achieved.

Contacts to GaAs, InP, and GaP for high-temperature and high-power applications

There is a significant interest in the area of improving high temperature stable contacts to III-V semiconductors. Two attractive materials that offer promise in this area are dysprosium phosphide (DyP) and dysprosium arsenide (DyAs). This paper reports the electrical characterization of MBE- grown DyP and DyAs on GaAs, GaP, and InP substrates. The characterization methods include Hall and I-V measurements. DyP is lattice matched to GaAs, with a room temperature mismatch of less than 0.01% and is stable in air with no sign of oxidation, even after months of ambient exposure. DyP forms Schottky contacts to n-GaAs, n- and p-GaP, and p- InP with barrier heights of 0.81, 0.9, 0.8 and 0.74 eV, respectively. DyP on n-InP and p-GaAs is found to have ohmic behavior with the specific contact resistance of 1 X 10-4 and 2.9 X 10-5 (Omega) (DOT)cm2, respectively. DyAs also forms Schottky contacts to n-GaAs, p-InP and forms ohmic contacts to n-InP.