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Impurity avalanche breakdown in n-type GaAs layers prepared by t h e LPE, VPE and MBE methods has been studied in detail using voltage-controlled current-voltage (1-V) characteristics and luminescence measurements together with computer simulations. Scholl's model of the first-order non-equilibrium phase transition in a two-level system was found to be adequate to describe the main experimental results. The threshold voltage for breakdown depends on the initial free-electron concentration. The hysteresis width diminishes with decreasing binding energy of electrons in the excited states of donors. The changes in luminescence spectra observed during impurity breakdown reflect changes in the charge state of shallow impurities. The band corresponding to B A transitions disappears together with t h e decrease of the exciton radiative recombination rate.In the post-breakdown part of t h e I-V characteristics another region 01 nonlinear behaviour with hysteresis and sharp increase of instabilities was observed. Its origin has not yet been determined unambiguously.

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