In this paper, an experimented study is presented which determines the memory switching criterion for CdS/SiO/CdTe devices. The dc. characteristics obtained from isolated devices on various glass substrates, but having the identical CdS and CdTe semiconductors with different sandwiched SiO thicknesses reveal that the device impedance at OFF state is almost determined by the tunnel oxide thickness. But the forward and reverse threshold voltages are determined by the top contact area of the device. Physical arguments are presented which adequately explain the experimental results in this paper.