The current instability in a planar gas discharge system is studied experimentally in a wide range of the gas pressure, p (44-550Torr), interelectrode distance, d (45-330 mu m), and diameter, D (5, 9, 12, 18, 22 mm), of the electrode areas of the semiconductor cathodes. While being driven with a stationary voltage, it generates current instabilities with different amplitudes of the oscillation. Through spatially uniform irradiation of the semiconductor cathode, non-stationary states which are non-homogeneous can be generated in a system. It is shown that under the experimental conditions the discharge gap played only a passive role and was not responsible for the appearance of the current instability. At the same time, for different diameters, D, of the electrode an expanded range of current oscillations is observed. The pronounced N-type negative differential resistance is observed in the current-voltage characteristic of a planar eras discharge system with a large-diameter semiconductor cathode.