The dependence of the electrical instabilities in the GaAs photodetector of an ionization-type semiconductor photographic system on control parameters is investigated. It is found that active properties of GaAs photodetectors that manifest themselves at high electric fields play an important role in the observed phenomena. It is established that the presence of the deep electronic levels of defects, the so-called EL2 centres, give rise to the N-type negative differential conductivity of the material and, as a consequence, to oscillations in current when a dc voltage of a high enough magnitude is applied to a GaAs photodetector. At the same time, instabilities of spatially non-uniform distributions resulting in the formation of multiple current filaments with increasing voltage above the critical value have been observed. It should also be noted that the current oscillations in the unstable regions may negatively affect the characteristics of devices based on semi-insulating GaAs.