Stable Space-Charge Layers in Two-Valley Semiconductors

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Abstract
When electric fields of several thousand volts are applied to certain samples of n‐type GaAs and InP, space‐charge layers have been observed which are stable until they drift into the anode. Calculations of the possible stable configurations have been made using the two‐valley model of McCumber and Chynoweth for GaAs. Under various voltage and sample length conditions, four types of configurations are possible: a high‐field domain (or dipole layer), a low‐field domain, an accumulation layer, and a depletion layer. Only in the limit of very high voltages do the domain configurations resemble the superposition of a stable accumulation layer and a stable depletion layer. The calculations show that the high‐field domain is the only type that should have been observed under past experimental conditions. When a device contains a high‐field domain, an increase in voltage reduces the current. This negative conductance effect is most pronounced in samples of GaAs where the value of doping density times sample length squared (NL2) is about 109 cm−1. The minimum field for sustaining a high‐field domain decreases rapidly from the critical field for samples 30 μ long to about one‐half the critical field for longer samples.