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Temperature dependences of dark current, i(T ), thermally stimulated current, TSC, and current-voltage, I-V, characteristics have been investigated in p-type TlGaSe 2 single crystals having different technological origins, using different contacts, in a wide temperature range and at different heating rates. Two types of initially undoped crystals with high resistivity, differing in deviation from the stoichiometry (type 1 and type 2), have been investigated. It is shown that the mechanism of the current flow strongly depends on the technological origin of the sample. Negative TSC is observed in type 1 samples for the first time and attributed to the simultaneous release of holes and electrons from acceptor and donor states in the same temperature interval. In these types of crystals, the dark current appears due to the double injection of electrons and holes from the contacts and at high heating rates β 8-10 K min -1 has a thermally activated character. The parameters of subsequent impurities have been obtained using the TSC theory. The shallow donor states with extremely low capture cross sections were shown to contribute to the thermally activated processes in these types of crystals. In type 2 crystals low-frequency current oscillations, LFCO, have been observed for the first time in the temperature range 120-180 K. These types of crystals are characterized by nonlinear N-type I-V characteristics. Mechanisms are proposed to explain the nonlinear I-V characteristics and current oscillations.

Low-frequency current oscillations (LFCOs) have been observed for the first time in the ferroelectric semiconductor TlGaSe 2 . It is shown that LFCOs can be observed in specially undoped high-resistivity single crystals and in crystals after annealing for some hours within the incommensurate phase. LFCOs are shown to be due to non-linear current -voltage (I-V) characteristics of the crystals. The origin of the non-linearity of the I-V characteristics is based on the formation of electret states due to the capture of carriers by deep impurity centres. The possible influence of electret states on the peculiarities of different physical properties observed in these crystals is discussed.

The effect of thermal annealing on dark conductivity and thermally stimulated conductivity in ferroelectric-semiconductor TlGaSe 2 within the incommensurate phase is investigated. It is shown that both types of conductivities are drastically changed after annealing of the crystal for some hours within the incommensurate phase. The results obtained lead to the conclusion that the main effect of annealing is a large decrease of impurity capture cross sections leading to the dramatic increasing of relaxation times.