David B. Rensch

Iterative Diffraction Calculations of Transverse Mode Distributions in Confocal Unstable Laser Resonators

A theoretical investigation has been undertaken to study the transverse modes of two-dimensional positive branch, confocal unstable resonators. Mode amplitude and phase information is obtained from a numerical-iterative type calculation that uses the Fresnel integral for propagating the cavity radiation back and forth between resonator mirrors. Near- and far-field distributions for empty cavity resonators are presented for various resonator Fresnel numbers and magnifications, along with results of resonator mode stability and diffraction losses when cavity perturbations such as mirror misalignment and/or a uniformly saturable gain medium are included. In addition, the diffractive calculations are compared with results obtained from geometric models.

39.5-GHz static frequency divider implemented in AlInAs/GaInAs HBT technology

A static divide-by-four frequency divider operating at 39.5-GHz input frequency is reported. Graded emitter-base junction AlInAs/GaInAs heterojunction bipolar transistor (HBT) technology lattice-matched to InP substrates has been used to implement the divider. The graded junction HBTs feature unity gain cutoff frequency and maximum frequency of oscillation of 130 GHz and 91 GHz, respectively. The devices have a very low turn-on voltage of about 0.7 V at collector current density of 5*10/sup 4/ A/cm/sup 2/. The divider operated at a power supply voltage of -3 V and consumes a total DC power of 425 mW, corresponding to 77 mW per flip-flop.<<ETX>>

Self-aligned AlInAs-GaInAs heterojunction bipolar transistors and circuits

AlInAs-GaInAs heterojunction bipolar transistors (HBTs) and static flip-flop frequency dividers have been fabricated. An f/sub t/ and an f/sub max/ of 49 and 62 GHz, respectively, have been achieved in a device with a 2*5- mu m/sup 2/ emitter. Current-mode logic (CML) was used to implement static divide-by-two and divide-by-four circuits. The divide-by-two circuit operated at 15 GHz with 82-mW power dissipation for the single flip-flop. The divide-by-four circuit operated at 14.5 GHz with a total chip power dissipation of 444 mW.<<ETX>>

Three-Dimensional Unstable Resonator Calculations with Laser Medium

A numerical procedure that uses an explicit finite difference method to solve the wave equation is described. This technique results in a propagation algorithm that can accurately propagate an arbitrary electric field through a uniform medium or a medium that is nonuniform, transversely flowing, saturable, and contains index inhomogeneities. By using the propagation algorithm to propagate an arbitrary field back and forth between two resonator mirrors, the three-dimensional transverse mode and the output beam characteristics for a laser resonator can be determined. The advantage of the finite difference method is that unlike integral techniques the computational accuracy and efficiency improve as the resonator Fresnel number increases. The computational techniques are explained, and results for several specific empty cavity confocal unstable resonators are presented and compared to results obtained using an established calculation technique. The application of the finite difference method to inhomogeneous laser media is described, and computational results for an existing CO(2) gas dynamic laser are presented and compared to measured data. The medium kinetics and shock wave models used in the calculations are described.

High‐temperature stable W/GaAs interface and application to metal–semiconductor field‐effect transistors and digital circuits

The thermal stability of the physical, chemical, and electrical properties of W thin films sputter deposited on GaAs were investigated. A variety of characterization methods, including thin film stress analysis, Auger analysis, Rutherford backscattering spectrometry (RBS) analysis, and Schottky barrier measurements showed that the W/GaAs interface remains stable after high‐temperature furnace annealing at 900 °C for 15 min or rapid‐lamp annealing at 1000 °C for 11 s. Some refractory metal compounds were also investigated, including, WSi, WNx, and TaSix. Pure W films produced the best Schottky diode characteristics. The average Schottky barrier height was 0.70±0.009 V across a 2‐in wafer after furnace annealing at 800 °C/15 min. Pure W self‐aligned gate (SAG) metal‐semiconductor field‐effect transistors (MESFET) and digital circuits were also fabricated. Transconductances as high as 300 mS/mm (Lg =1.0 μm) were measured for enhancement mode SAG MESFET’s. Circuits were fabricated with SAG MESFET enhancement‐...

The effects of base dopant diffusion on DC and RF characteristics of InGaAs/InAlAs heterojunction bipolar transistors

The effects of base p-dopant diffusion at junction interfaces of InGaAs/InAlAs HBTs with thin base thicknesses and high base dopings are reported. It is shown that HBTs with compositionally graded emitter-based (E-B) junctions are very tolerant to base dopant outdiffusion into the E-B graded region. The RF performance is nearly unaffected by the diffusion, and the DC current gain and E-B junction breakdown voltages are improved with finite Be diffusion into the E-B graded region.<<ETX>>

33-GHz monolithic cascode AlInAs/GaInAs heterojunction bipolar transistor feedback amplifier

Microwave cascode feedback amplifiers with 8.6-dB gain and DC to 33-GHz bandwidth were developed. The amplifiers utilize AlIn-As/GaInAs heterojunction bipolar transistors having f/sub max/=70 GHz and f/sub tau /=90 GHz. Because of the significant collector-base feedback time constant the cascode configuration provides a large improvement in amplifier bandwidth, but a low-impedance bias node must be provided for the common-base transistor. An active bias network was thus used which eliminates the need for on-wafer Si/sub 3/N/sub 4/ bypass capacitors. >

High-performance microwave power AlInAs/GaInAs/InP double heterojunction bipolar transistors with compositionally graded base-collector junction

We report, for the first time, on the use of compositional grading at the base-collector (B-C) junction of AlInAs/GaInAs/InP double heterojunction bipolar transistors (HBT) to achieve high-performance devices for microwave power and analog circuit applications. To overcome the conduction band potential barrier at the B-C junction, we have inserted an n-GaInAs spacer and a novel 100 nm compositional grade between the GaInAs base and the wide bandgap InP Collector. A base-collector breakdown voltage of 32 V, collector-emitter breakdown voltage of 26 V, and a f/sub max/ of 93 GHz have been achieved. Preliminary power measurements at 10 GHz on 240 /spl mu/m/sup 2/ emitter DHBTs resulted in over 5 W/mm output power at a collector bias of 10 V.<<ETX>>

Performance of the focused-ion-striped transistor (FIST)&#8212;A new MESFET structure produced by focused-ion-beam implantation

We report the fabrication and performance of the focused-ion-striped transistor (FIST), which is a GaAs MESFET structure having a channel with stripes of high conductance, going from the source to the drain, separated by regions of semi-insulating material. Calculations show that this structure produces a depletion layer that wraps around the conducting channel stripes and this should result in improved transconductance and output resistance. Experimental results are reported for devices having 1-µm gates and the FIST channels produced by focused-ion-beam implants of silicon with a width of 0.2 µm and a spacing that is varied from 0.2 to 0.5 µm. These verify the basic performance characteristics of the FIST including an increase in stripe transconductance, a two-fold increase in output resistance, and larger values of fTfor small values of Idsnear pinchoff.

Improved high frequency performance of AlInAs/GaInAs HBTs through use of low temperature GaInAs

GaInAs grown at lower than normal substrate temperatures was used to reduce the amount of beryllium out-diffusion from the heavily doped bases of AlInAs/GaInAs Npn HBTs. A combined 20-nm-thick spacer structure of p-doped and undoped GaInAs grown at 300 degrees C prevented excessive amounts of beryllium from diffusing into the AlInAs emitter. This allowed base beryllium doping concentrations up to 10/sup 20/ cm/sup -3/ to be achieved, thereby reducing base resistance and increasing f/sub max/ to 70 GHz. A fifteen-stage ring oscillator utilizing these HBTs demonstrated a gate delay of 15.8 ps. The reduced outdiffusion was confirmed by secondary ion mass spectrometry (SIMS) elemental profiles in addition to electrical measurements.<<ETX>>