Stationary Domains Research Articles

Impact ionization phenomena initiated by the acoustoelectric effect in n-GaAs

Acoustic flux amplified from thermal background in semidegenerate n-GaAs at liquid-air temperature is accompanied by electric fields that initiate impact ionization. When the acoustic flux is concentrated in a narrow domain the interplay between the impact and the acoustoelectric effect brings about current oscillations of frequencies in the MHz range that increase monotonically with the net gain factor of the acoustic flux measured prior to the onset of impact. When wide stationary acoustic domains are generated in such samples, the impact causes an increase of the saturation current typical of the acoustoelectric effect. For initially applied fields higher than the threshold for impact, current modulation is observed when the acoustic flux is still in the embryonic stage and its incubation time is increased. The threshold field for impact in epitaxial layers is about 1500 V/cm, while in bulk samples it is much higher. At high power levels these instabilities may bring about the formation of current filaments that burn channels across the samples. The experimental results and a semiquantitative analysis of the instabilities are presented in detail in this work.

Bias dependence of GaAs and InP MESFET parameters

A comparative analysis of the bias dependence of critical RF parameters in GaAs and InP metal-semiconductor field-effect transistors (MESFET's) led to the following conclusions. 1) The drain-gate feedback capacitance in GaAs MESFET's is lower than in InP MESFET's, because of a stronger tendency in GaAs to form stationary Gunn domains at the typical drain bias levels employed. 2) The drain-source output resistance in InP MESFET's is lower than in GaAs MESFET's mainly for high drain current units, a fact which is linked to a substrate related softer pinch-off behavior in InP. 3) The current-gain cutoff frequency fT, in the current saturation range of the GaAs MESFET decreases strongly with drain bias as a result of the formation of the stationary Gunn domain. In the InP MESFET, this effect is weaker. At the optimum bias, fT is only 10-20 percent higher in InP MESFET's than in GaAs ones.

Stationary Gunn domains created by anode doping gradient current-density reduction

The conditions for the production of stationary anode Gunn-effect domains have been clarified by both experimental and computational results. The stationary domains have been observed in both sandwich and planar device structures and it is proposed that this effect is due to the balancing of drift and diffusion currents at the doping gradient of the anode contact.

Influence of external perturbations on the behaviour of stationary electrical domains in gold-doped n-type Ge

The response of a domain-containing sample of gold-doped n-type Ge to a weak sinusoidal or rectangular electrical perturbation, a rectangular optical pulse (extrinsic excitation), or a constant transverse magnetic field are studied. It is shown the equivalent circuit consists of the voltage-dependent active resistance Z1(U) and capacity C(U) put in series. A possible explanation of the measured dependence Z1(U) is proposed. The influence of a constant transverse magnetic field on the domain velocity and on the critical values of the electrical field and the current for the instability are investigated. It is shown that the domain is tilted in the magnetic field. The tilting angle is equal to the Hall angle in weak electrical field outside the domain. [Russian Text Ignored]

Parametric amplification of subharmonic acoustic waves in stationary acoustoelectric domains in GaAs

Abstract The frequency spectrum of acoustic flux in stationary acoustoelectric domains in GaAs has been studied by current noise measurements. In the nonlinear regime parametrically amplified thermal vibrations are observed peaking at subharmonic frequencies. At high pump powers the spectrum is continuous. The observed shift to lower frequencies of the electronic gain is only 10 per cent.

Domain phenomena in ZnS single crystals

Stationary and moving domains similar to those found by Böer and coworkers on CdS have been observed on ZnS single crystals by the help of the Franz-Keldish effect. The development of the stationary high-field domains was found to be strongly influenced by structural defects and in many cases the domains appeared to be bound to these defects. In this case the position of the high-field domain depends on the polarity of the applied voltage. Luminescence field quenching and electroabsorption show weak correlation. Different types of moving domains have been observed: an anode-to-cathode and two types of cathode-to-anode moving domains, showing different irradiation and voltage dependence. Moving domains can be quenched by infrared irradiation. The current waveform changes with ultraviolet intensity and wavelength as well as the field strength.

Acoustoelectric Phenomena in Piezoelectric Semiconductors

AbstractCurrent saturation and growth of stationary domains of high electric field are observed for many piezoelectric semiconductors. It is shown that such phenomena may be explained when a constant‐current pulse is applied to the sample. To study the steady state we include in the dc current the acoustoelectric term. With some simplifying assumptions, a simple theoretical calculation of the current—voltage characteristic and of the electric field repartition is given. A comparison is made with experimental results.

Influence of Boundary Conditions on High-Field Domains in Gunn Diodes

Using the method of the field of directions, the influence of the boundary conditions on stationary and moving high-field domains in Gunn diodes is analyzed and discussed. A criterion for self-induced instabilities, especially the Gunn oscillations, is given. It is shown that stationary domains must occur preceding the Gunn oscillations, and that such oscillations can only occur for slightly blocking contacts. The analysis given in this paper is similar to the one discussed for field-quenched CdS.

Trap-controlled Field Instabilities in Photoconducting CdS Caused by Field-quenching

The formation of stationary high-field domains adjacent to cathode or anode, dependent on the contact potential of the electrodes, their widening with increased applied voltage and their transition into two types of moving domains are discussed. Domains which move under deformation oft he domain profilea nd usually dissolve before they reacht he anode, and nearly undeformed moving domains are described. The structure and kinetics of these domains are directly observed using the Franz-Keldysh effect and photographs of typical domain forms are presented.

Experimental Evidence for a Reduction of the Work Function of Blocking Gold Contacts with Increasing Photocurrents in CdS

It is shown that stationary cathode-adjacent high-field domains, which occur in a range of negative differential conductivity, can be used to determine quite accurately the minimum electron density close to the cathode. From this density, the work function between cathode metal and photoconductor can be obtained for finite currents if hole currents and tunneling are negligible. Although the minimum electron density close to the cathode is at least two orders of magnitude below the bulk electron concentration (blocking cathode), gain factors well in excess of 100 have been observed, thus justifying the neglect of holes in the transport equation. For slightly doped CdS, as used in these investigations, it is concluded that tunneling can also be neglected. Using the method of stationary high-field domains, it is observed that, for a CdS-gold contact, the work function decreases markedly with increasing current densities. Values of about 0.4 eV for a current density of ${10}^{\ensuremath{-}4}$ A/${\mathrm{cm}}^{2}$, measured at 200\ifmmode^\circ\else\textdegree\fi{}K, are obtained.

Stationary High-Field Domains in the Range of Negative Differential Conductivity in CdS Single Crystals

In CdS crystals with an $N$-shaped negative differential conductivity range, stationary high-field domains adjacent to the electrodes are observed. With increasing applied voltage these steplike domains increase in width, staying attached to the cathode until they fill the entire crystal; then a still higher-field domain forms at the anode and increases in width. These domains can be explained within an earlier published theory, and allow the determination of electron densities at the cathode-CdS boundary and in the field-quenched region. The analysis of these stationary domains presents a new tool for work-function (metal-semiconductor) investigations.

Properties of a Free, Steadily Travelling Electrical Domain in GaAs

Experiments are described on the travelling domains of high electric field which can exist in n-type GaAs. A brief discussion of the appearance of stationary domains near the cathode and of the nucleation and transient behaviour of travelling domains is given, but the main topic is the steady-state behaviour of travelling domains. Measurements are reported on the dependence of domain velocity, and of the voltage drop across the domain, on the field E <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∞</inf> existing outside the domain. It is found that the velocity increases with E <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∞</inf> , but the voltage decreases. The resulting negative resistance is seen to have important consequences for the stability of the domain in real specimens.

Acoustoelectric Current Distribution and Current Saturation in CdS

The spatial distribution of electric field in photoconductive CdS is measured under the condition of current saturation. The presence of stationary high-field domains is related to the distribution of acoustoelectric current which, assuming Weinreich's relation holds, may in turn be related to the nonuniform growth and decay of acoustic energy in the samples. Detailed measurements on both homogeneous and inhomogeneous samples yield a better understanding of the mechanisms responsible for the current saturation and also provide a new method for determining the effective drift mobility of the carriers.

On the non-linear theory of electrical domains in hot electron semiconductors

The results are described of the non-linear treatment of the stationary electrical domains' motion. The domains arise in hot electron semiconductors if the N-type negative resistance were formed in the corresponding homogeneous system.