Todd R. Weatherford

Single event induced charge transport modeling of GaAs MESFETs

Two-dimensional computer simulations of charge collection phenomena in GaAs MESFETs have been performed for alpha and laser ionization. In both cases, more charge is collected than is created by the ionizing event. The simulations indicate that a bipolar transport mechanism (t 40 ps) are responsible for this enhanced charge collection. The first mechanism is a bipolar type effect that injects charge into the bulk of the device and is collected at the drain due to the electric field. The second is a back channel turn-on mechanism that is associated with a positive hole density located beneath the channel and exists on a much longer time scale. These results show that electrons supplied by the source implant are responsible for charge collected at the drain in excess of any collected deposited charge. >

Laser confirmation of SEU experiments in GaAs MESFET combinational logic (for space application)

Majority vote and self-scrubbing circuitry were utilized to harden the registers of a GaAs logic circuit to single event upsets (SEUs). Ion beam testing of the hardened part at low scrub frequencies showed fewer system upsets than that of an unhardened part. At high frequencies, the upset rate increased with frequency, indicating a clock-dependent SEU sensitive node. A pulsed laser was used to identify an input node in the self-scrubbing circuitry as being the source of the upsets at high frequencies. Because the node was only sensitive for a short duration prior to the rising clock edge, more clock-edge-dependent SEUs could occur at higher frequencies. The relative SEU thresholds of the registers and the input node measured with the laser showed excellent agreement with ion data. All single-point SEU failure nodes were identified and their upset thresholds measured. With this information it was possible to redesign the circuit to reduce its sensitivity to SEU at high scrub frequencies. >

SIGNIFICANT REDUCTION IN THE SOFT ERROR SUSCEPTIBILITY OF GAAS FIELD-EFFECT TRANSISTORS WITH A LOW-TEMPERATURE GROWN GAAS BUFFER LAYER

The use of a low temperature grown GaAs buffer layer beneath the channel of a metal‐semicon‐ ductor field‐effect transistor is shown via computer simulation to reduce ion‐induced charge collection by two or more orders of magnitude. This reduction in collected charge is expected to reduce the heavy ion soft error rate by four to seven orders of magnitude in GaAs integrated circuits, and could have significant implications for the applicability of GaAs technology in space‐based systems.

Pulsed laser-induced charge collection in GaAs MESFETs

Pulsed picosecond lasers with variable wavelength were used to investigate the details of charge collection in GaAs MESFETs. In short gate-length devices, charge collection at the drain may be much larger than at the gate and greater than the charge produced by the laser pulses. The results show that a pulsed laser is very useful in studies of charge collection. Two particularly useful features are the absence of the radiation damage which accompanies ion measurements and the ability to observe visually the point at which charge is being produced in the device. >

Historical perspective on radiation effects in III-V devices

A historical review of radiation effects on III-V semiconductor devices is presented. The discussion ranges from examining early material and device studies to present-day understanding of III-V radiation effects. The purpose of this paper is to provide present researchers with a summary of discoveries and lessons learned from previous failures and successes.

Modeling single-event effects in a complex digital device

A methodology to quantify the impact of SEEs on complex digital devices has been developed. This methodology is based on the SEE State-Transition Model and was validated by radiation testing of a complex digital device.

Charge collection in GaAs MESFETs fabricated in semi-insulating substrates

Charge-collection in GaAs MESFETs fabricated in semi-insulating substrates is investigated. Current transients are measured at short times (/spl sim/few picoseconds) after either an alpha-particle strike or a laser pulse. In addition, the total charge is obtained by integrating the collected current. Measurements show the existence of three mechanisms for charge collection: (1) the drift of holes and electrons to the gate and drain electrodes, respectively, (2) bipolar-gain, and (3) channel-modulation. The charge collected by drift of holes or electrons gives rise to an instrument limited response (within 20 ps) after a laser pulse. The bipolar-gain mechanism peaks in approximately /spl sim/200 ps and is responsible for most of the collected charge. The channel-modulation mechanism is responsible for charge collection at longer times. These results are different than previous results for MESFETs fabricated on top of a buried p-layer, where most of the charge was found to be collected by the channel-modulation mechanism. Our results indicate that in order to harden GaAs transistors to single event upset, one must use techniques that reduce the effects of the bipolar-gain and channel-modulation mechanisms.

Proton Radiation-Induced Void Formation in Ni/Au-Gated AlGaN/GaN HEMTs

AlGaN/GaN high-electron mobility transistors (HEMTs) were exposed to 2-MeV protons irradiation, at room temperature, up to a fluence of 6 × 1014 H+/cm2. Aside from degradation resulting from radiation-induced charge trapping, transmission electron microscopy and electrical measurements reveal a radiation-induced defect located at the edges of the Ni/Au Schottky gate in the proton-irradiated devices. At the edges of the Ni/Au gate, the Ni of the Ni/Au gate diffused up into the Au layer and migrated into the AlGaN barrier, leaving voids in the Ni layer at the gate edges after irradiation. These radiation-induced voids are caused by diffusion of Ni through vacancy exchange, known as the Kirkendall effect, resulting in reduced gate area and degrading the HEMT performance.

Effects of low-temperature buffer-layer thickness and growth temperature on the SEE sensitivity of GaAs HIGFET circuits

Heavy-ion Single Event Effects (SEE) test results reveal the roles of growth temperature and buffer layer thickness in the use of a low-temperature grown GaAs (LT GaAs) buffer layer for suppressing SEE sensitivity in GaAs HIGFET circuits.

Degradation mechanisms of 2 MeV proton irradiated AlGaN/GaN HEMTs

Proton-induced damage in AlGaN/GaN HEMTs was investigated using energy-dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM), and simulated using a Monte Carlo technique. The results were correlated to electrical degradation using Hall measurements. It was determined by EDS that the interface between GaN and AlGaN in the irradiated HEMT was broadened by 2.2 nm, as estimated by the width of the Al EDS signal compared to the as-grown interface. The simulation results show a similar Al broadening effect. The extent of interfacial roughening was examined using high resolution TEM. At a 2 MeV proton fluence of 6 × 1014 H+/cm2, the electrical effects associated with the Al broadening and surface roughening include a degradation of the ON-resistance and a decrease in the electron mobility and 2DEG sheet carrier density by 28.9% and 12.1%, respectively.