H. T. Weaver

Anisotropic thermionic emission of electrons contained in GaAs/AlAs floating gate device structures

Thermionic emission rates are reported for electrons in GaAs epilayers, where electrostatic barriers within the layers and GaAs/AlAs heterojunctions form different sides of the potential well. Attempt times at the two barrier types were observed to differ by factors above 107 due to differing constraints on momentum conservation during emission. These emissions represent escape mechanisms for charge stored in a closed geometry, III‐V compound floating gate transistor, with potential application as dynamic random access or nonvolatile memories.

Aging behavior of radiation‐damaged titanium tritide from 3He NMR

Transient nuclear magnetic resonance (NMR) measurements on 3He, generated by β decay and contained in aging titanium tritide (TiH1.8), have been carried out. The experiments were conducted over a period of about three years during which the rate of helium evolution from the sample abruptly increases. Qualitative analysis of the temperature dependence of the 3He NMR correlates the onset of rapid increase in release rate with the precipitation of large bubbles within the solid. Further consideration suggests these bubbles are preliminary stages for release, since only a small fraction (<10%) of the 3He in TiH1.8 is ever observed in this configuration. This is the first data which directly infers 3He bubbles at the time of release, although blisters have been observed in earlier experiments.

Radiation response of optically-triggered GaAs thyristors

Gallium arsenide optically triggered thyristors that exhibit tolerance to high X-ray dose rates have been fabricated. These two-terminal epitaxial devices feature breakover voltages of 18 V to 38 V with no radiation. They trigger at less than 2 V with only tenths of a milliwatt of laser light, but they do not trigger at 2*10/sup 9/ rad(Si)/s with a bias level as much as 40% to 60% of the zero-radiation breakover voltage. When these devices are bombarded with neutrons, the reduced carrier lifetimes result in a decreased sensitivity to triggering by light and ionizing radiation. The thyristors show some decrease in optical and X-ray sensitivity upon neutron exposure in excess of 10/sup 13/ n/cm/sup 2/. At 10/sup 14/ n/cm/sup 2/, there are major reductions in photon sensitivity, and some instability is observed. >

Comparison of solar cell performance to calculations using different energy band‐gap narrowing models

Three different energy band‐gap narrowing models were used for exact numerical calculations of silicon solar cell performance. The results are compared to data from high‐efficiency cells. Two of the gap narrowing models are derived empirically from other types of devices, and the third is theoretical. The applicability of the models to these cells is demonstrated.

Long switching delay mechanisms for optically triggered GaAs thyristors

Long switching delays following optical triggering of GaAs thyristors are shown to result from near cancellation of carrier lifetime and current regeneration effects. Numerical simulations and analytical models account for the long delays, which are measured to be more than an order of magnitude larger than any lifetime or transit time in the device. Since the thyristor is in a low current state during the delay, this switching phenomena represents a form of dynamic information storage.

Method for determining emitter recombination in Si solar cells using open‐circuit voltage decay

A method for determining the emitter recombination current and thereby partitioning the total carrier recombination current into the different sections of a silicon solar cell is demonstrated. The method is destructive, requiring that a set of measurements is made on a cell before and after mechanically roughening the back of the cell. The data include short‐circuit current (Jsc), open‐circuit voltage (Voc), and asymptotic decay rates for Jsc and Voc. High open‐circuit voltage (650 mV) silicon cells are studied using the technique. It is shown that the emitter recombination current is large (1.5×10−13 A/cm2), which is consistent with a band‐gap narrowing mechanism. Furthermore, the high voltages are shown to result from unusually long base lifetimes (>300 μs) in 0.3‐Ω cm silicon.

Optically-triggered GaAs thyristor switches: Integrated structures for environmental hardening

Opticallytriggered thyristor switches often operate in adverse environments such as high temperature and high dose-rate transient radiation which can result in lowered operating voltage and premature triggering. These effects can be reduced by connecting or monolithically integrating a reverse-biased compensating photodiode or phototransistor into the gate of the optically-triggered thyristor. We have demonstrated the effectiveness of this hardening concept in silicon thyristors packaged with photodiodes and in gallium arsenide optically-triggered thyristors monolithically integrated with compensating phototransistors.

Investigation of charge storage in InAs/AlAsSb quantum well capacitors

We report the electrical chracterization of an InAs/AlAsSb quantum well heterostructure capacitor fabricated on material grown by molecular beam epitaxy. Hysteresis associated with electron storage was observed in the capacitance‐voltage data from which we derived exponential charge decay constants of 50 s at 77 K.

Simulation of the transient radiation response for GaAs thyristors

Numerical simulations of the transient response of gallium arsenide (GaAs) thyristors to bursts of ionizing radiation are reported. The device simulator BAMBI 2.0, with incorporated GaAs transport models, is used. BAMBI is a 2-D time-dependent transport code using accurate models, such as doping- and electric-field-dependent carrier mobilities and avalanche multiplication. Furthermore, the physical parameters can be very easily modified to isolate the effect of any particular parameter in the device response. This feature is used to study the effect of carrier lifetime on the switching behavior of the irradiated GaAs thyristors by comparing simulation results with the experimental data. The structures simulated are 1-D structures, but the full 2-D simulation is used. Good agreement exists between experimental and simulation results, including the effects of carrier lifetime on the radiation response and the phenomena of delayed switching. >

THE NATURE OF 3He CONFINEMENT IN TRANSITION METAL HYDRIDES

ABSTRACT Although helium is insoluble in metals, large amounts of 3He are known to remain trapped in metal tritides after being introduced into the lattice by radioactive decay of tritium. The mode of confinement is found concentration (age) dependent in that at critical helium-to-metal ratios, abrupt changes in the rate of gas evolution from the hydride occur. This situation presents both practical and fundamental problems relating to the nature of metal hydrides and their stability, even with rather severe internal damage. A large collection of data relating to helium evolution from transition metal tritides, which were collected over a long period of time, now exists. From these data, a general physical picture of tritide aging can be inferred. More recent studies of 3He and 3H nuclear magnetic resonance (NMR) in the hydride systems TiHx and ScHx have been carried out. Results from this work provide a somewhat detailed microscopic picture of hydride behavior with large amounts of helium trapped in the lattice.