Noise due to field- and current instabilities in CdS

Abstract

In this review paper, we summerize the field and current instabilities that occur when the conductivity decreases more than linearly with increasing field, or increases sufficient steeply with increased current density. In both cases well defined transition ranges exist that causes a chaotic development of field-or current-inhomogeneities, respectively. These are the ranges in which substantial additional low-frequency electronic noise is generated that shows a typical 1/f behavior. Field instabilities occur in a range of an N-shaped current-voltage characteristic that result in high-field domains which can be stationary or move through the device and cause a stationary or oscillating reduction in current. The latter, as the Gunn effect, enjoys technical application as an ac generator. In CdS the negative differential conductivity regime is trap-controlled and thereby kinetic effects are slowed down so that they can be observed visually, using the Franz-Keldysh effect. During such observation, chaotic effects can be seen before the periodic oscillations are organized, and cause a substantial increase in low-frequency noise. The chaotic initiation of well organized periodically moving high-field domains are discussed in a phase-portrait analysis of the nonlinear dynamics for pattern formation in semiconductor devices. Also in an S-shaped characteristic that is initiated by substantial Joule’s heating and yields current channel formation, current instabilities can occur that lead to low frequency noise. The related phenomena are briefly summerized in this review. The experimental evidence of these chaotic developments of field- and current-instabilities is shown during the oral presentation in a movie, using electro-optical or electro-thermo-optical effects for visualization.