Electrical instability in semi-insulating (SI) GaAs plates of the semiconductor gas-discharge structure (SGDS) is studied experimentally in air and neon as a function of pressure, interelectrode distance, and cathode area diameter. While being driven with a stationary voltage, it generates current and discharge light-emission instabilities with different amplitudes of the oscillation in Ne up to atmospheric pressure. The presence of the deep electronic levels of defects, give rise to the N-shaped negative differential resistance (NDR) of the material, and as a consequence, to oscillations in the discharge light emission when a dc voltage of a high enough magnitude is applied to the GaAs cathode. A SGDS with an N-shaped current–voltage characteristic (CVC) has been analyzed using both the current and discharge light emission data showing the electrical instability in the GaAs cathode. It is observed that application of high feeding voltage to the cathode gives rise to a nonuniform spatial distribution of the discharge light emission, which disturbed the operation of the system. The experiment also presents a new method to study and visualization of the electrical instabilities in the high-resistivity GaAs plates of large diameter.