Melvin P. Shaw

The Physics of Instabilities in Solid State Electron Devices

Introduction. Stability. Tunnel Diodes. The Avalanche Diode. The Gunn Diode. Superconducting Junctions. SNDC Multilayer Semiconductor Structures. Thermal and Electrothermal Instabilities. Electrothermal Switching in Thin Chalcogenide Films. Index.

The eureka process: A structure for the creative experience in science and engineering

Abstract: There is little in the literature that focuses on both the thoughts and feelings of scientists and engineers when they are in the throes of scientific creation or invention. This investigation was performed to address the question: What is the experience of modeling natural phenomena? Special emphasis was placed on mathematical modeling. A formal heuristic procedure was used to gather data from 12 scientists and engineers. The interviews were tape recorded, transcribed, and analyzed for general themes and points of view. The data, represented primarily in the form of a structural flow diagram, emphasize both the cognitive and emotional features of the process. The major features that emerged include the existence of creative feedback loops, unipolar‐positive phases of both illumination and acceptance, and unipolar negative phases of blockages to the attainment of the positive poles. Implications and applications emerged in the areas of creativity, validation, education, and motivation. It was co...

On the form and stability of electric-field profiles within a negative differential mobility semiconductor

Using a fixed cathode boundary field model we examine analytically the form and stability of inhomogeneous prethreshold electric-field profiles in long bulk negative differential mobility (NDM) semiconductors. We show that the electric-field profiles and their associated current-voltage relationships depend in a detailed way on the characteristics of the bulk material (e.g., the velocity-electric-field relation) and on the boundary conditions imposed on the bulk. We investigate situations leading to the nucleation of a high-field domain at the cathode and find that for its occurrence: 1) the electric field at the cathode boundary must lie within the NDM region; and 2) space-charge neutrality must be approximated within the vicinity of the cathode. When these conditions are satisfied the current instability at threshold is accompanied by bulk electric-field values that are below the threshold field for NDM. The stability conclusions are drawn from a small-signal analysis that avoids many of the approximations of earlier studies. Supplementary numerical computations are included to demonstrate that the subsequent time development of the current instability depends in a detailed way on: 1) conditions at the cathode boundary; and 2) the external circuit. It is shown that the recycling Gunn effect cathode-to-anode transit-time mode in a resistive circuit is not a necessary consequence of the bulk having a region of NDM.

Study of the electronic structure of amorphous silicon using reverse‐recovery techniques

We have carried out a series of reverse‐recovery experiments on pin diodes of amorphous‐Si:F:H of thicknesses up to 3.5 μm under pulsed high‐level injection conditions. No evidence for charge storage was obtained. Our results indicate that the room‐temperature band mobility for electrons moving near the mobility edge in a‐Si:F:H is larger than 100 cm2/Vs. In addition, our data suggest that either an intrinsic or induced gap exists in the density of localized states below the conduction‐band mobility edge, in surprising contrast with conclusions deduced from a variety of recent experiments.

Transport and the electronic structure of hydrogenated amorphous silicon

Abstract The puzzles of the magnitude of the free-electron mobility and the sign of the effective correlation energy of the dangling-bond defect in hydrogenated amorphous silicon are examined. It is suggested that the existence of a negative effective correlation energy in the less-strained regions of the material can resolve the apparently contradictory results for the magnitude of the band mobility. The internal fields arising from the presence of positively and negatively charged dangling bonds can be neutralized under high-level double-injection conditions, effectively increasing the mobility by more than an order of magnitude. We present experimental results that are consistent with the hypothesis.

An experimental study of the influence of boundary conditions on the Gunn effect

The electrical characteristics of long inhomogeneous bulk negative differential mobility (NDM) semiconductor elements (n-GaAs and n-InP) exhibiting various modes of current instabilities at low microwave frequencies have been experimentally observed. Measurements were made of the time-dependent sample current and voltage and of the prethreshold electric-field distribution within the sample. The measurements were made under conditions in which the configuration of the circuit, the properties of the boundary of the NDM element, the sample geometry, the temperature, and the magnetic field were varied. We have obtained excellent agreement between the experimental results and a model in which the sample is assumed to have an assigned value for the electric field at the cathode boundary. Different modes of instability may be accurately modeled by choosing an appropriate value for this field. Three regions of behavior may be identified which correspond to three regions for the cathode boundary field. These regions are 1) cathode boundary field Ecless than the threshold electric field Epfor the onset of NDM; the samples are likely to operate as bulk oscillators, 2)E_{c} > E_{p}but less than the electric field Evcorresponding to the onset of the saturated electron-drift velocity; the samples operate in the Gunn domain mode with a peak-to-valley ratio determined by Ec, and 3)E_{c} > E_{v}the samples yield saturating current versus voltage curves and only weak oscillations are possible.

Analysis of Bulk Negative Differential Mobility Element in a Circuit Containing Reactive Elements

The time and space evolution of the electric field in an inhomogeneous bulk negative differential mobility (NDM) element with a high n0l product, and the response of the surrounding lumped‐element circuit, is theoretically investigated. We consider a circuit in which an NDM element of low‐field resistance R0 and intrinsic capacitance C0 is in parallel with a parasitic capacitor Cp and in series with a load resistor, inductor L, and a battery. Exact numerical and approximate analytic solutions are obtained and discussed in terms of a damping parameter A [=(LC)1/2/R0C, where C=C0+Cp] and the ratio C0/C. Various oscillatory modes can all occur in the same NDM element for appropriate values of the circuit parameters. The dominant mode of circuit‐controlled oscillation in a series L‐parallel C circuit is the quenched multiple‐dipole relaxation mode. LSA relaxation oscillations occur only for a narrow range of circuit parameters and LSA sinusoidal oscillations are excluded because of domain domination, even for...

Observation of two modes of current transport through phosphorus‐doped amorphous hydrogenated silicon Schottky barriers

The influence of phosphorus impurities in the active layer of amorphous hydrogenated silicon Schottky barriers is investigated by experimentally studying the current‐voltage characteristics of the structure and the physical and electronic properties of the material. With increasing phosphorus concentration excess diode currents develop. Numerical analysis shows that (1) these currents are due to hopping within an impurity band produced by the impurities and (2) a two‐channel conduction mechanism is in quantitative agreement with the data.

Preswitching and postswitching phenomena in amorphous semiconducting films

Low‐duty‐cycle pulsed dc switching experiments have been performed on a variety of thin‐film Te‐based semiconducting glasses. No premonitory effects are observed in the current‐time profile of the nonswitching off state. If, however, the threshold voltage is exceeded, a continuous current increase with time throughout the normal delay‐time regime (preswitching off state) is always observed prior to switching. In a given device or film this current rise is interrupted, independent of overvoltage, by the rapid switching transition at about the same relative current increment above the current background extrapolated to zero time (leakage current). In the switched or on (filamentary conduction) state the current level, as limited by the series load resistor, can condition the response of the device in the subsequent preswitched off state. For loads in excess of ∼1 kΩ, there is no effect on the preswitching off state characteristics whereas, for appreciably smaller loads, there is a continuous increase in del...

Amorphous—Crystalline heterojunction transistors

The characteristics of a new hybrid amorphous-crystalline three-terminal device are described in detail. The device uses a threshold-switching-type chalcogenide glass as the emitter, with a p-type crystalline-silicon base and an n-type silicon collector. When the amorphous emitter is in the off state, the small-signal current gain is significantly smaller than unity. However, once the voltage across the glass is sufficient to produce the on state, the device operates as a transistor with gains well in excess of unity. (Small-signal gains of up to 15 have already been observed in unoptimized configurations.) Under certain conditions, the high-gain on state can be preserved after the switching pulse is removed, suggesting memory-type applications. Previous results on amorphous-crystalline heterojunctions are used to construct a band model for the transistor. This model is used to analyze the steady-state and pulsed-mode characteristics of the device. Recent studies of the on-state current density and carrier concentration are used to calculate the expected gain as a function of current and base doping concentration, with results in good agreement with the experimental data. The behavior of the devices provides another confirmation of the electronic nature of threshold switching in chalcogenide glasses.