We report on atomically resolved scanning tunneling microscopy images and tunneling spectra of ͑110͒ cleavage surfaces of semi-insulating GaAs without illumination at room temperature. With help of simple model calculations we extract the physical mechanisms involved in the tunneling processes from and into semi-insulating GaAs. Atomically resolved images can only be observed at negative voltages, while no tunneling into empty states is possible without illumination. This is explained, on the one hand, by the absence of a carrier inversion at the semiconductor surface without illumination under the nonequilibrium tunneling contact conditions. On the other hand, at negative voltages in the noncontact mode an accumulation at the surface occurs and leads to tunneling of electron from the valence band states into the empty tip states. This current is limited by the tunneling through the vacuum barrier and the scanning tunneling microscopy images are found to show the occupied dangling bond states above the arsenic atoms. In the point contact mode the current is limited by tunneling through the space charge region without and with illumination. The implications of the results for the investigation of low-conductivity materials by scanning tunneling microscopy are discussed.