In this work, we report the fabrication of germanium arsenide (GeAs) field-effect transistors with ultrathin channel and their electrical characterizations in a wide temperature range, from 20 K to 280 K. We show that the p-type electrical conductivity and the field effect mobility of GeAs transistors increase with the temperature and that at lower temperatures the electrical conduction of the GeAs channel is dominated by the 3D variable range hopping but becomes band-type at higher temperatures, after the formation of a highly conducting two-dimensional (2D) channel. The presence of this 2D channel, limited to few interfacial GeAs layers, is confirmed by the observation of an unexpected peak in the temperature dependence of the carrier density per area at about 75 K. Such a feature is explained considering a model based on a temperature-dependent channel thickness. Indeed, at higher temperatures, the carrier injection from the contacts increase and the ionization of defects is favoured enabling the formation of a 2D highly conductive channel close to the dielectric interface, which screens the electric field from the gate and confine it to the first few layers of the material. The formation of the 2D channel is corroborated by numerical simulations, that show excellent agreement with the experimental data, and by the estimation of 0.4 nm Debye screening length at room temperature.