Abstract
The non-ohmic hopping conductivities of a large number of samples of lightly doped by various impurities (P. As, Sb, B and Ga) and weakly compensated n-and p-type Si were investigated. The current-voltage (I-V) characteristics appeared to be different in two temperature regions typical for the studies of Ohmic hopping conductivity: the low-temperature hopping conductivity region with a constant activation energy ε3 (the ε3 region) and the relatively high-temperature hopping conductivity saturation region (the s region). In the ε3 region the conductivity increases with increasing field owing to a Poole-Frenkel-like lowering of the activation energy. In the s region the conductivity decreases with increasing field and shows a negative differential conductance (low-frequency current oscillations have been observed) according to Aladashvili et al. (1988. JETP Lett., 47, 390) as predicted by Nguyen and Shklovskii. It is shown that low-frequency current oscillations are associated with ‘hopping’ domain periodic formation and travel in the bulk of the sample; the hopping domain is of triangular shape parallel to the external field, with equal leading and back edges. Estimates give the velocity c = 0.25 cm s−1, the domain width d = 150μm and the average field E = 300 V cm−1. The mobility of the hopping carriers is measured.