Transition between Stationary and Moving High‐Field Domains in CdS in a Range of N‐Shaped Negative Differential Conductivity due to Field‐Quenching

Abstract

In CdS, unstable moving domains are observed, which exist in a current-voltage range between the range for stationary electrode-adjacent domains and the range for transitions to undeformed moving domains. These current -voltage ranges have been investigated in the same crystal by changing the electron concentration a t the boundary of a pseudocathode. Several CdS crystals, differently doped with Ag, Al, and &, have been investigated. An electro-optical method using the Franz-Keldysh effect was employed for domain observation. The influence of the distance between the electrodes on domain formation is discussed.I n CdS werden nichtstabile, wandernde Domiinen beobachtet, die in einem StromSpannungs-Bereich existieren, der zwischen dem Bereich der stationiiren, an der Elektrode anliegenden Domiinen und dem Ubergangsbereich zu undeformierten, wandernden Domiinen liegt. Diese Strom-Spannungs-Bereiche wurden in demselben Kristall durch hnderung der Elektronenkonzentration an der Grenzschicht einer Pseudokathode untersucht. Verschiedene CdS-Kristalle, .die unterschiedlich mit Ag, Al und Cu dotiert waren, wurden untersucht. Eine elektro-optische Methode, die den Franz-Keldysh-Effekt ausnutzt, wurde fur die Beobachtung der Domiinen verwendet. Der EinfluD des Elektrodenabstandes auf die Domiinenausbildung wird diskutiert.

Critical Conditions for Transitions between Stationary and Non‐Stationary High‐Field Domains in Semi‐Insulators

“…can be predicted, provided that the crystal is long enough and that the solution does not move too far from the singular pointf3) (small amplitude field instabilities). Such behavior was indeed observed for certain conditions in CdS [28]. There is good agreement between the quantitative results of this theory and the experiment.…”

“…Two theoretical approaches have been used to contribute to this question : An analysis of small perturbations close to the second singular point in the Poisson and transport equations [13, 17, 261 and a comparison of current-voltage characteristics for both types of domains 1271. Recently experimental evidence has been given [28] that stationary domains start to become unstable at a well defined current-voltage curve (for varying electron densities at the cathode). Here the high-field domains become inhomogeneous and these inhomogeneities move through part of the crystal, continuously changing their shape and causing current oscillations.…”

High‐field domains from dc to 60 GHz

“…For all these unstable moving domains a more detailed analysis of the complete transport system (1)-(3) is necessary and it is beyond the scope of this review. Here, we have to refer to the original publications, first presented by the team of the author [31][32][33][34]. We present here only the results of some of these discussions.…”

Stationary high‐field domains as tools to measure field‐dependent carrier densities and mobilities in CdS and work functions of blocking contacts

“…This solution is distinctly different from the well-known Schottky barrier, where the electron density declines steeply from the cathode. Within the domain, the electron density remains initially constant, until at the end of the domain, closer to the anode, the field decreases within a few Debye lengths and then remains constant again until it reaches the anode [9,10]. The space charge region is now shifted from the cathode to the end of the domain.…”

“…For a first theoretical analysis we used the field-of-direction method in 1961 [8]. Soon thereafter a large number of publications started to broaden the field with many theoretical and experimental investigations of the high-field domains [9][10][11][12][13][14][15][16], and separately of the Franz-Keldysh effect. Except for the further analysis of the stationary high-field domains by the research team of the author [17][18][19][20], the moving domains were almost exclusively analyzed by many other groups [21][22][23][24].…”