Stationary High-field Domains Research Articles

Cadmium Sulfide Is a Very Unusual Model Semiconductor

CdS has three states that are thermodynamically stable, one at low fields and two at high fields when stationary high-field domains are initiated in a range of negative differential conductivity, one when the domain is attached to the cathode then the CdS is n-type. And the other one when the domain is attached to the anode and the CdS is turned p-type. When the photoconductivity of CdS is field quenched stronger than linearly, a band of lowest conductivity is introduced between the electrodes, the field in this band is increased to maintain current continuity. This is the high-field domain, that limits the current to a low, constant value. With increased bias the domain expands, but current and domain field remain constant. As long as the domain is attached to the cathode, stationarity is achieved by the limited supply of electrons from the blocking cathode. When the domain is expanded to reach the anode, then a new, higher field domain is generated at the anode that expands toward the cathode and stability is maintained because of the limited supply of holes from the blocking anode. The Minimum Entropy principle forces the current to remain constant by adjusting the width of the higher field, anode-adjacent domain: the transition from the cathode to the anode adjacent domain is not visible in the current that remains saturated, but the conductivity changes from n- to p-type, and now attains the third stable thermodynamic state. This is the first time CdS is p-type that can never be achieved by doping (where the CdS is self-compensating by a strong intrinsic donor). This is very unusual since the current in both cases is carried by drift alone: j = e n μn Fc = e p μp Fa and is forced to remain the same, even though carrier density and mobility are substantially different. All of this is accomplished by adjusting the width and the field of the high-field domain. There is one more unusual coincidence, because both domains require that they are created in a range of overcritical negative conductivity. That needs to have a defect distribution of donors and acceptors for providing this negative differential conductivity at a similar field for electron- and for hole-quenching. It is given by field excitation from Coulomb attractive traps that all lie in the kV cm−1 range. Cadmium sulfide which has an extensive distribution of Coulomb attractive electron and hole traps that can produce such conditions that no other known semiconductor can provide. This is believed to make CdS such an unusual model semiconductor that can perform the many applications, foremost in combination with p-type solar cells without pn-junction that with a thin layer of CdS create highly efficient solar cells.

Theoretical analysis of the DC avalanche breakdown in GaAs MESFET's

The channel avalanche breakdown in GaAs MESFET's has been investigated using nonstationary electron dynamics and an ionization coefficient taken as a function of average electron energy. Stationary high-field domains of different shapes and peak-field localization are calculated at the breakdown, depending on technological parameters, device geometry or gate bias. Design rules are given to obtain maximum saturated output power and a full-channel current breakdown voltage comparable to the one near pinchoff. In particular, it is found that both a recessed channel geometry and an increased gate-drain distance should yield the best device performances with a doping level not higher than about 1.2-10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">17</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> and a channel current I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dss</inf> between 275 and 330 mA/mm.

Design criteria for GaAs MESFETs related to stationary high field domains

Using a modified theory of the high-field domains which takes into account the field-dependent diffusion we show that the existence of the high-field domain at drain side of the gate in GaAs MESFETs leads to a new set of design criteria which should be met to achieve the optimum performance. We derive these criteria and estimate the drain-to-source breakdown voltage and the maximum power of the device as functions of the doping density and device dimension. We also estimate the optimum gate length, the thickness of the active layer, the drain-to-gate separation and the doping level as functions of frequency. It is demonstrated that a larger than conventional drain-to-gate separation might be necessary for power devices to house a fully developed highfield domain and to avoid a premature breakdown at the drain. Our estimates indicate that the maximum power at 10 GHz can reach a theoretical limit of about 20 watts for class A operation.

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.

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.

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.