Gas breakdown in air in a planar gas discharge system was studied experimentally at various distances L between the electrodes and different diameters R of the electrode areas of the semiconductor cathodes. The cathode was irradiated on the back-side with light in a particular wavelength range that was used to control the photoconductivity of the material. The current-voltage characteristics of the gas discharge system with a large-diameter GaAs semiconductor cathode were studied in a wide range of the gas pressure p (28-550 Torr), interelectrode distances L (45-330 mum), and conductivities of the cathode. We show that for all experimental conditions the current density increases over the entire range of voltages U > U-dc as the diameters R of the electrode areas increase. It is shown that, for arbitrary geometric dimensions of the semiconductor cathode (in the range of L/R under investigation), the ratio of the breakdown electric field to the gas pressure holds constant (E-dc/p)(min) approximate to const. at the breakdown curve minimum. The breakdown voltage U-dc is a function of both the product of the gas pressure p by the distance L and the ratio L/R (U-dc = f (pL, L/R).