Kouhei Morizuka

Donor Levels in Si-Doped AlGaAs Grown by MBE

Donor levels of MBE-grown Si-doped AlxGa1-xAs have been characterized by a combination of the C-V method and capacitance and current transient spectroscopy. Although most electrons are supplied by so-called DX centers in the AlAs mole fraction (x) range of 0.3~0.7 in this material, it is found that a small amount of shallow donors are still present. The concentrations of the DX center and the shallow donor are determined in detail as a function of AlAs mole fraction and Si doping level. The activation energy obtained by the Hall effect measurement is discussed in association with these data.

Emitter—Base bandgap grading effects on GaAlAs/GaAs heterojunction bipolar transistor characteristics

Emitter-base bandgap grading effects on GaAlAs/GaAs heterojunction bipolar transistor characteristics are theoretically investigated using an accurate one-dimensional numerical model including aluminum mole-fraction-dependent velocity versus field characteristics and donor energy level. The bandgap grading is shown to influence not only the electron injection but also the carrier recombination and the hole injection, resulting in a significant common-emitter current gain dependence on the graded layer thickness. The cutoff frequency dependence on the graded layer thickness is also described. Detailed discussion is given for the underlying physical mechanism that determines the device performance.

LPMOCVD growth of C doped GaAs layers and AlGaAs/GaAs heterojunction bipolar transistors

Low-pressure MOCVD growth of GaAs using trimethylgallium (TMG) and arsine was investigated under conditions of low growth temperatures and low V/III ratios. The growth rate was limited not by the supply rate of TMG but by surface kinetics. Incorporation of carbon atoms was enhanced by decreasing the growth temperature and V/III ratio. GaAs layers with hole concentrations as high as 6×1019 cm-3 were obtained with a V/III ratio near unity. AlGaAs/GaAs heterojunction bipolar transistors with carbon doped GaAs bases were fabricated using this novel growth technique. The diffusion of carbon atoms was so small that a spacer layer, in the base, as narrow as 10Awas found to be sufficiently effective to establish the emitter-base junction.

Transit-time reduction in AlGaAs/GaAs HBTs utilizing velocity overshoot in the p-type collector region

The effect of electron-velocity overshoot in a p-type GaAs collector on the transit-time reduction of AlGaAs/GaAs HBTs (heterojunction bipolar transistors) is investigated. A cutoff frequency improvement of about 30% over the conventional n-type GaAs collector was obtained in p-type collector HBTs for the same collector depletion-layer width. A significant increase in electron velocity in the p-type GaAs collector layer was confirmed by a simple analysis.<<ETX>>

AlGaAs/GaAs HBTs fabricated by a self-alignment technology using polyimide for electrode separation

A self-aligned HBT (heterojunction bipolar transistor) technology using polymide for insulating the emitter contact from the base contact is described. A 1- mu m emitter-width HBT with maximum oscillation frequency of 86 GHz was successfully fabricated. This processing was also applied to fabricate frequency-divider ICs. An operating frequency of 18 GHz was obtained with good reproducibility.<<ETX>>

AlGaAs/GaAs HBT receiver ICs for a 10 Gbps optical communication system

Key component ICs for an optical communication system such as an equalizing amplifier a rectifier, and a limiting amplifier were fabricated with AlGaAs/GaAs HBTs. The equalizing amplifier with an amplitude detection circuit had an 11 GHz bandwidth, 30-dB maximum voltage gain and over 30-dB controllable gain width. The rectifier could extract 10-GHz signals, and the limiting amplifier had a 36-dB voltage gain at 10 GHz and an over 20-dB dynamic range with an under 10 degrees phase-shift deviation. These optical receiver ICs have a performance sufficient for 10-Gbit/s optical communication system.<<ETX>>

Electron space-charge effects on high-frequency performance of AlGaAs/GaAs HBTs under high-current-density operation

The high-frequency characteristics of AlGaAs/GaAs heterojunction bipolar transistors (HBTs) under high-current-density biasing condition are investigated in conjunction with the electron space charge in the collector depletion layer. A significant increase in cutoff frequency f/sub t/ and maximum oscillation frequency f/sub max/ at the early stage of the base push-out was observed in HBTs with a lightly doped n-type collector structure, and is attributed to the collector depletion layer widening and the enhancement of the velocity overshoot effect caused by the increasing electron density.<<ETX>>

15 GBPS MUX/DMUX implemented with AlGaAs/GaAs HBTs

AlGaAs/GaAs HBTs (heterojunction bipolar transistors) with improved high-speed performance obtained by a self-alignment technology and a new layer design have been used to construct ultra-high-speed MUX (multiplexer) and DMUX (demultiplexer) ICs. Quantitative error-free operation of these ICs was confirmed up to 10 Gb/s. Although only a few test patterns were available, the maximum operation speeds of the MUX and the DMUX were found to be around 15 Gb/s and 19 Gb/s, respectively. These results demonstrate the promising potential of HBTs for coming ultra-high-speed digital systems.<<ETX>>

Precise extraction of emitter resistance from an improved floating collector measurement

A distributed transistor model is developed which comprehensively explains the non-linear characteristics of floating collector measurements. The non-linearity is attributed to the fact that the intrinsic collector current is not zero when the collector terminal is opened. Based on this understanding, a new procedure to extract emitter resistance is proposed. Combining the measurements of forward and reverse current gains and intrinsic base sheet resistance, the ambiguity in the conventional floating collector measurement is diminished and hence the accuracy is significantly improved. >

Ultrahigh-speed heterojunction bipolar transistor multiplexer/demultiplexer ICs

High-speed 2-b monolithic integrated multiplexer (MUX) and demultiplexer (DMUX) circuits have been developed using self-aligned AlGaAs/GaAs heterojunction bipolar transistors (HBTs) with improved high-speed performance. Both ICs were designed using emitter-coupled logic. The 2:1 MUX was composed of a D-type flip-flop (D-FF) merging a selector gate and a T-type flip-flop (T-FF). The 1:2 DMUX consisted of two D-FFs driven at a clock of half the rate of the input data. Error-free operation with a pseudorandom pattern was confirmed up to 10 Gb/s. The rise and fall times of the output signals of both ICs were 40 and 25 ps, respectively. HBT frequency dividers were used as inputs for both ICs in order to find the maximum operation speed. Although only a few test patterns were available, the maximum operation speeds of the MUX and DMUX were found to be around 15 and 19 Gb/s, respectively. >