A plasma device with large diameter and short interelectrode distance has been designed and implemented. Theoretical modeling and simulations have been carried out for different interelectrode distances, and experimental results obtained under different pressures p, both with argon atmosphere. The device produces direct-current (dc) discharges in the parallel-plate electrode configuration, with gallium phosphide (GaP) semiconductor at one side and SnO2-coated glass conducting material at the other side, separated by gas medium with width of 50 mu m to 500 v. The device can be operated under different values of interelectrode distance d, applied voltage U, and gas pressure p. Current-voltage characteristics and breakdown voltages have been found experimentally and theoretically. In addition, theoretical breakdown curves have been derived from simulations. The theory can also identify the space-charge density, thermal electron velocity, reduced electric field strength (E/N), electron density ne, and secondary-electron emission (gamma). Comparison between experiment and theory shows that the theory can estimate the breakdown very well for low pressure and small interelectrode gap.