J.A. Higgins

Low-noise GaAs FET's prepared by ion implantation

The rapidly improving performance of low-noise GaAs FETs can to a large extent be attributed to advances in the material preparation technology. Ion implantation directly into bulk-grown semi-insulating substrate material represents an optimum approach by providing excellent control and reproducibility over the doping parameters in the active layer. This paper will review development efforts carried out to capitalize on these inherent advantages and discuss the results obtained from transistors fabricated by this method. Device modeling has been used to investigate the effects of profile tailoring and has served as a guide for selecting implant species, doses, and energies. This effort has paralleled the development of the implantation technology, which has addressed the problems of selecting suitable substrate material, deposition of suitable capping material for the post-implantation anneal, the study of doping profiles, and the diffusion during the anneal. The advances made in these areas will be discussed along with the fabrication procedures for the low-noise FETs. Favorable doping profiles were obtained by using Se implants as evidenced by the small variation in the measured transconductance versus gate voltage. The excellent uniformity and reproducibility obtained in the active layer parameters have resulted in tightly distributed transconductances, pinchoff voltages, and S-parameters. Measured parameters from five wafers have given typical standard deviations of less than 10 percent of the mean value. Packaged transistors with a nominal gate length of 1 µm have yielded noise figures of 1.1 dB at 4 GHz with 12- dB associated gain, while 2.5-dB noise figures have been achieved at 15 GHz with 7-dB gain from transistors mounted on a low parasitic carrier. These improved RF results combined with a high level of reproducibility present ion implantation as a very attractive method for fabricating low-noise GaAs FETs.

A numerical approach to modeling the ultrashort-gate MESFET

A simple numerical model compatible to small computers is developed for an ultrashort-gate GaAs MESFET that takes into account the transient electron dynamics leading to velocity overshoot of electrons. It is assumed that because of the velocity overshoot phenomenon, the carriers move with constant high mobility at fields greater than the threshold field necessary for intervalley scattering for a certain time before relaxing to the equilibrated velocity of the low-mobility satellite valley. These time constants are taken from results of Monte Carlo calculations. The model also takes into account nonuniform channel doping as well as the nonabrupt depletion boundary. It is shown that changing the gate length from 1.0 to 0.5 µm does result in improved gain bandwidth although not in perfect proportion to the gate length reduction. This is due to excess charge dipole buildup in saturation as well as capacitance from fringing effects. The effects of profiles on the peak electric fields are also pointed out.

Modeling the influence of carrier profiles on MESFET characteristics

Continuing development of materials methods and the need to improve MESFET device performance calls for accurate modeling of the effect of free-carrier profiles upon MESFET performance. A computer program known as SATVO is described which provides, by means of a numerical solution, saturation characteristics and gate-electrode capacitances for devices of specified geometry fabricated from layers of any profile which may be specified by either Gaussian expressions (as for ion-implanted layers) or by piecewise-linear descriptions. Examples of the use of this program are also provided.

Highly doped GaInAs using diethylberyllium by MOCVD for InP-based heterostructure bipolar transistor applications

Diethylberyllium has been used to produce high hole concentrations in latticematched Galnas grown by metalorganic chemical vapor deposition (MOCVD). Doping concentrations from p = 1 × 1019 up to 7.7 × 1019 cm−3 have been achieved at 500°C. To our knowledge, this is the highest doping level achieved by MOCVD in this material system. Secondary ion mass spectroscopy analysis showed a significant inclusion of oxygen in the as-grown material. Using beryllium-doped Galnas as the base layer, we have demonstrated InP-based heterostructure bipolar transistors. This shows diethylberyllium to be a promising p-type dopant for MOCVD applications.

Progress in broad-band GaAs monolithic amplifiers

Monolithic microwave amplifiers showing gain from 0 to 8 GHz have been fabricated on semi-insulating GaAs. These amplifiers utilized MESFET devices with gate lengths of approximately 0.75 µm and gate widths of 300 µm. The active devices were used to provide both gain and impedance matching. The performance of these amplifiers is promising and indicates that stable, high gain, microminiature gain modules can be made by this monolithic microwave integrated circuit (MMIC) technology.

MOMBE-grown carbon-doped base self-aligned AlGaAs/GaAs heterojunction bipolar transistors for microwave applications

The authors report the first demonstration of completely MOMBE (metal-organic molecular beam epitaxy) grown carbon-doped base self-aligned AlGaAs/GaAs HBTs (heterojunction bipolar transistors) operating at microwave frequencies. Excellent DC and RF characteristics have been achieved. For a 1400-AA-thick base doped at a level of 1*10/sup 19/ cm/sup -3/, cutoff frequency f/sub T/ of 45 GHz and f/sub max/ of 90 GHz were measured from common emitter HBTs. Common base HBTs with total emitter area of 360 mu m/sup 2/ delivered 560 mW output power with 43% of power-added efficiency and 8.9 dB gain at 10 GHz. A high-power MMIC (monolithic microwave integrated circuit) amplifier operating in 8-11 GHz bandwidth has achieved 2 W output power. The device modeling based on measured S-parameters indicates that the performance can be further improved with optimal layer structure and doping profiles.<<ETX>>