The reduction in size, noise and voltage is realized in a MOS solid-state imaging device. The reflectivity of such a mirror display device may be enhanced by providing a second switchable polarizer between the display device and the polarizing mirror. The second active layer has a higher bandgap than the first active layer such that a two-dimensional electron gas layer arises between the first active layer and the second active layer. In the second speed range with higher speeds than in the first speed range, a vector modification of the input voltage is carried out, for example for controlling the speed of a fan motor. Electric energy is transformed through induced currents of the transformer, and a small amount of energy attributed to an exciting current of the transformer is used for voltage boosting, thereby attaining a relatively high output power.