GaAs Transferred Electron Devices Research Articles

Effect of backgating on the field distribution in planar thin-film GaAs structures

The distribution of the electric field in planar film–substrate GaAs structures under backgating is considered. It is shown that backgating can make the film exhibit a long-length region of a low-gradient electric field exceeding the threshold of N-type negative differential mobility, the magnitude of negative differential mobility in this region being high enough. At values of the film doping density and film thickness typical of GaAs transferred-electron devices, this region can be as long as several tens of micrometers. The underlying physical mechanism is discussed.

Batch Fabrication of Millimeter-Wave Silicon Impatts

Large number of millimeter wave Impatt diodes have been fabricated out of reasonably large size silicon wafer with very good uniformity in electrical characteristics using the technique reported here. Modification in processing adopted in the present work led to very high yield and improved thermal impedance during packaging of Impatts. These improvements are due to better surface finish in integral heatsink pads and controlled thinning. The present process has been tried on silicon SDR and DDR Impatts over a frequency range of 10 to 100 GHz. Other devices like silicon Baritts and GaAs transferred electron devices have also been batch fabricated successfully using the same technique.

On the design of doping concentration profiles for GaAs transferred-electron device layers

The doping concentration profile of the GaAs region has a significant effect on the d.c.-microwave conversion efficiency of a transferred-electron device operating as a source of microwave power. Transferred-electron device layers are commonly grown so that the doping concentration profile of the active region is predominantly uniform. However, for higher device efficiency the doping concentration should be graded, gradually increasing from the cathode to the anode. Also the operating temperatures across the active region should be minimized by ensuring that the length of the highly-doped GaAs region between the active region and the heat sink is as short as possible. In this paper, the design of the active region doping concentration profile for high-efficiency GaAs transferred-electron devices is discussed. The measured power output of a microwave oscillator, using devices with uniform and graded doping concentration profiles, is given. The d.c.-microwave conversion efficiency of the devices with graded doping concentration profiles is shown to be significantly superior. Furthermore, to calculate accurately the operating temperatures across the active region, novel analytic expressions for the temperature profile are given as a function of the geometry and material composition of the device and the d.c. input power.

Thermal analysis for improved performance of transferred-electron oscillators

Novel analytical solutions to the heat-flow equation are presented for GaAs transferred-electron devices to be operated in microwave oscillators. Uniform and linearly varying power dissipation density in the active region is considered. The results show that, to reduce operating temperatures, devices should be operated in the heat-sink anode configuration rather than the conventional top-cap anode configuration. This will ensure improved microwave power output from transferred-electron oscillators.

An accurate calculation of the d.c. electrical and thermal characteristics of GaAs transferred-electron devices

A physical model to simulate the d.c. behaviour of GaAs transferred-electron devices has been developed. An accurate calculation of the electrical and thermal characteristics is made possible by solving the semiconductor device equations self-consistently with the heat flow equation. A novel heat flow analysis has been developed to model non-uniform heat generation/absorption across the whole GaAs region including high-low junctions, substrates, buffer and contact regions. Since a thermal current term is included in the current density equation, commonly consisting of drift and diffusion terms only, the analysis is referred to as a drift-diffusion-thermal model. The model is shown to be useful for the thermal design of transferred-electron devices for operation in CW transferred-electron oscillators. In addition to the design of the heat sink configuration, the doping concentration profile of the GaAs region can also be designed to reduce operating temperatures, thereby ensuring improved d.c. microwave conversion efficiency and lifetime.

High efficiency and output power from second- and third-harmonic millimeter-wave InP-TED oscillators at frequencies above 170 GHz

InP TED (transferred electron device) oscillators have been experimentally investigated for frequencies between 170 and 279 GHz. It has been found that output powers of more than 7 and 0.2 mW are possible at 180 and 272 GHz using second- and third-harmonic mode operation, respectively. Conversion efficiencies of more than 13% and 0.3% between fundamental and second harmonic and fundamental and third harmonic, respectively, have been found. The conversion efficiencies are comparable to GaAs TEDs. The output powers, conversion efficiencies, and tuning ranges (more than 22%) are the biggest reported for InP TEDs at these frequencies. The output power at third harmonic was sufficient for supplying a superconducting mixer with local oscillator power.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Efficient transferred electron device simulation method for microwave and millimeter wave cad applications

A set of hydrodynamic transport equations, which govern a physical model for GaAs transferred electron devices, are formulated using phenomenological transport parameters derived from large scale Monte Carlo particle simulations. A fast and efficient numerical solution method is developed, allowing inexpensive and systematic investigations of a wide variety of TED samples under many operating conditions. The method not only permits insight into important semiconductor transport processes, but also permits the identification of candidate device samples for microwave and millimeter wave circuit applications.

Triggering pulse durations in transferred electron devices

Pairs of 1.0 ps optical pulses, each focused onto a different region of a planar GaAs transferred electron device, were separated by a variable delay time τ to initiate and then inhibit dipole domain formation. The minimum pulse durations which could trigger domains were thus observed; the minimum value of τ which generated domains in the device studied was 7 ± 3 ps.

GaAs transferred electron devices and field effect transistors in monolithic integrated circuits

Analog-to-digital conversion is being extended to data rates of up to a few giga-samples per second, by advancements in materials, device and process technology. Specific improvements in the material are the result of improved substrate qualification procedures. The primary process improvements include improved annealing and capping for repeatability in ion implantation and selective implantation for reduced contact resistance in ohmic regions. Further, selective ion implant and isolation implants have shown promise for planar processing. TRW has combined the technological developments to fabricate high frequency mesa and planar circuits using transferred electron devices (TED’s) and metal–semiconductor field effect transistors (MESFET’s) as the principal circuit elements.

Transferred-electron logic gates

A two-dimensional modeling technique is used to simulate GaAs transferred-electron devices operated as a logic gate (the TELD) and as a threshold gate. The simple logic gate has a good transfer characteristic but is shown sensitive to bias variations and operates with monostable output. For an input logic swing of 0.6 V and a fanout of 2, a propagation delay of 26 ps and gain of 1.25 is predicted. A bi-stable threshold gate shows a turn-on time of about 80 ps. An FET-triggered two-terminal transferred-electron device is calculated to have propagation delay of 27 ps with a gain of -1.2. Subsequent similar stages would require a noninverted output obtainable from a capacitive electrode on the TED. However, it is shown that additional anode load resistance is required to obtain a significant positive pulse output from such capacitive electrodes. The bias power requirement is estimated to be similar to the simple TELD gate.

Frequency limitation of GaAs transferred-electron devices: influence of operating d.c. and r.f. field values

The possibility of making use of the transferred-electron effect for power generation in the whole millimetre-wave range is shown for long and uniform-field GaAs samples. A great improvement in the upper frequency limit under large-signal operation is reported.

Contact degradation of GaAs transferred electron devices

The migration of contact material into GaAs devices may degrade the device performance (noise, power output) 1) and can shorten the mean time to failure of operational adevices. In order to understand contact degradation the interdiffusion of gold/germanium contacts on gallium arsenide has been studied using Rutherford backscattering, ion microprobe ∗ (SIMS) and microbeam 2) techniques. Both the depth profiling and imaging properties of the ion microprobe have been used. The microbeam was used for focused Rutherford backscattering giving a lateral resolution ∼ 10 μm. When used in conjuction with angle lapped samples it allows analysis to be conducted to greater depths and hence one may use thicker contact films. The use of these techniques has enabled both the lateral and depth distributions of gold, germanium, gallium and arsenic to be obtained. These distributions have revealed gold and germanium spike formation beneath the contact. The information obtained provides insight into the mechanisms involved in the formation of ohmic contacts to gallium arsenide. The degradation of such contacts as a result of thermal and electrical effects is discussed.

Uniaxial stress investigations of the three-level conduction band structure of GaAs

Measurements are reported of low-field resistivity and Gunn effect threshold field as a function of (100) and (111) uniaxial stress for a number of epitaxial and bulk GaAs samples. The results provide the first direct evidence that the L1c minima are situated lower in energy than the X1c minima and are therefore involved in the operation of GaAs transferred-electron devices. Intervalley separations are obtained at 300K, but these are dependent on density-of-states values. Further information on band structure parameters such as intervalley coupling constants should now be obtainable by fitting the high-field data to Monte-Carlo calculations currently in progress. Deformation potentials for the satellite minima have been determined.

Contact-degradation studies on GaAs transferred-electron devices using a focused backscattering technique

Focused backscattering microanalysis has been used to overcome some of the problems associated with conventional backscattering. The small beam size enables the study of (a) small areas typical of real microwave devices, (b) lateral variations of the contact interdiffusion and (c) deep profiles when used in conjunction with angle lapped samples.

Radiation-induced degradation of ohmic contacts

Ohmic contacts can be severely degraded by the introduction of small amounts of radiation damage at the metal-semiconductor interface. This can lead to failure of GaAs transferred-electron devices; a safe limit for proton bombardment of ohmic contacts is proposed.

Bistable switching in supercritical n+-n-n+GaAs transferred electron devices

Bistable switching in supercritically doped n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -n-n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> GaAs transferred electron devices (TED's) is investigated experimentally and interpreted in computer simulations, for which details of the computer program are given. Three switching modes all leading to stable anode domains are discussed, namely: 1) cathode-triggered traveling domain; 2) cathode-triggered accumulation layer; 3) anode-triggered domain. Relative current drops up to 40 percent, and switching times down to 60 ps are obtained in low-duty-cycle pulsed experiments with threshold currents around 400 mA. Optimum device parameters are shown to be as follows: 1) doping in the 3-4 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> range; 2) length around 6 µm; 3) doping gradients below 20 percent; 4) high-quality interfaces.

Stabilizing effects for the supercritical GaAs n+nn+-transferred electron amplifier

In this paper effects of importance for the stabilization of supercritical n + nn + GaAs transferred electron devices are considered. By small-signal impedance calculations and measurements it is shown that doping- as well as temperature gradients of correct polarity reduce the device negative resistance and enhance stability. It is also found that an increasing doping density reduces the negative resistance. Finally it is demonstrated that relaxation effects have a profound influence on the impedance, and that such effects have to be included in a small-signal analysis in order to give reasonable agreement with measurements.

A broad-band negative conductance GaAs TED

Small-signal negative conductances from 5 to 17 GHz in both voltage polarities are reported for CW-operated 10-µm supercritically doped n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> nn <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> GaAs transferred electron devices. The conceptually different behavior in the two polarities for the frequency dependence of the negative conductance is interpreated by means of computer simulations, and the stabilizing mechanisms involved are discussed.

Computer simulation of transferred electron devices using the displaced Maxwellian approach

Computer simulations of GaAs transferred electron devices have been performed by solving the time- and space-dependent Boltzmann equation and assuming displaced Maxwellian distributions for the two conduction band valleys. Since this model includes particle, momentum, and energy relaxation (as distinct from models using instantaneous velocity field dependence) and retains the spatial dependence (as distinct from uniform field models) it is particularly well suited for studying cases that are characterized by a strong x-dependence of electron temperature. Short devices as well as long ones operating at very high frequencies have been studied extensively, since these devices were thought and, in fact, have been found to be strongly affected by nonuniform energy relaxation due to contacts, domains, and other types of space charge. The results have been compared with experimental findings and with theoretical results computed by others.

A high current drop GaAs bistable switch

A current drop of 41.5 percent and a switching time of 110 ps is reported for a 10-µm supercritically doped n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -n-n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> GaAs transferred electron device operated as a bistable switch into a 25-Ω resistive load. An investigation to check that no microwave oscillations occur is presented.