We report on a reversible avalanche breakdown due to free-carrier multiplication caused by cross band gap impact ionization in a GaAs microcrystal. The n GaAs microcrystal (length 1 mum, diameter 1 mum) was embedded between n(+) GaAs layers serving as electric contacts. We guided an electric pulse to the sample and determined, from the reflected and transmitted pulse, the I(V) characteristic. The breakdown was indicated by a sudden current rise and voltage drop and a hysteresis effect and, furthermore, by electron-hole recombination radiation. We reached the threshold field for ionization by making use of a high-field domain whose formation was based on the Gunn effect. The microcrystal could reproducibly be switched into the nonequilibrium avalanche state. Our analysis indicates that the effect provides a basis for the development of an ultrafast electric switch. (C) 2003 American Institute of Physics.