Electrical breakdown in GaAs Schottky diodes creates a granular filamentary structure of submicron dimensions between the contacts. The filament exhibits very fast (<2 nsec) electrical switching between stable resistance states. Irrespective of the contact metal, the filament becomes superconducting when in the low-resistance state. Electron transport in all the resistance states is intergrain tunneling between metallic inclusions and is governed by an activation energy. Switching is not based on metal transport. © 1975.