Current Filaments in Semiconductors

A current filament is a non-uniform radial distribution of current in the presence of a uniform electric field in a uniform sample. These filaments can have diameters in the 0.005 inch range. The current density at the center of the filament can be several orders of magnitude higher than the background current density in the rest of the sample. Filaments have been studied in devices exhibiting the current-controlled negative resistance associated with specific cases of two-carrier space-charge-limited current double injection. Electrons and holes are injected from opposite contacts into a semi-insulator in which the lifetime of carriers of one sign is much longer than the lifetime of carriers of the opposite sign. When forward biased this device exhibits a high-voltage, high-impedance (10
⁸
ohms) pre-breakdown region and a low-voltage, low-impedance (1 to 10
³
ohms) post-breakdown region. In the post-breakdown region current increases at a nearly constant voltage followed by a high-current power-law region. Current filaments have been studied throughout the post-breakdown region by recording the recombination radiation observed through one of the injecting contacts. Experimental and analytical studies of current filaments in silicon at 77°K and GaAs at 300°K are reviewed.