Heating of electrons by electric fields smaller than that required for generation of domain oscillations was investigated in samples of ΕL2-rich semi-insulating GaAs. Current-voltage characteristics were measured as a function of temperature between 268 K and 330 K. They exhibit a sublinear shape which is interpreted as a result of an enhanced electron capture on the ΕL2. The capture rate and the electron temperature as a function of the electric field was determined. A fitting procedure gave the value of electron capture cross-section on the ΕL2 to be 2.7 x 10 -13 cm' which agrees with literature data.PACS numbers: 72.20.Ht Long known spontaneous current oscillations in semi-insulating (SI) GaAs caused by high electric field domains propagation [1] have also been investigated during last years [2,3]. The mechanism responsible for this phenomenon is electric field enhanced capture of electrons from the conduction band on a deep centre. In particular, it is proposed that in undoped SI GaAs the level involved is the EL2 [4]. This idea was first put forward by Kamińska et al. [5] who explained the phenomenon as the following.When the electric field E applied to the sample is sufficiently high, the electrons in the conduction band gain enough energy to come over the configurational barrier which separates a free electron from its bound state on the EL2. This leads to a decrease in the free electron concentration and thus to a negative differential conductivity (NDC). The observed current oscillations are a result of drowning the system into the NDC regime.This reasoning was recently put into a quantitative model by Johnson et al. [2] and Surma and Łusakowski [6]. The principal idea is to describe the electron gas by an electron temperature Te which is different from the crystal lattice temperature Τ. The present paper deals with the range of electric fields below that (777) 778 A. Zduniak, J. Łusakowski' G. Nowak required for the oscillations. An analysis of the experimental data allows to calculate Te as a function of Ε and shows that the capture rate of electrons from the conduction band on the EL2 increases with Ε.All the experiments described below were performed on samples cut from one LEC wafer of undoped SI GaAs. A sample with six contacts was used for low field Hall measurements which gave the temperature dependence of the zero electric field mobility (which is constant for electric fields below 3 kV/cm [7]). Current-voltage (I-V) characteristics were measured as a function of the temperature on another sample with two bar-shaped contacts (Fig. 1). The known value of the electron mobility allowed to calculate the concentration of free electrons n as a function of the temperature and the electric field. Under the assumption that the occupancy of the EL2 changes only as a result of transitions to and from the conduction band, the condition of a stationary state takes the form ncn (Nt -nt ) = en nt , where cn is the capture rate, en is the emission rate of the inverse process, Nt and nt is the concentration...