The relationship between physical and electrical parameters is reviewed. Some of the newer technologies are discussed. The interdigitated structure is compared with the two main alternatives offered at present. At the same time a basic correction is made to the well established approximation of power gain as applied to power devices. The necessity for this correction is also developed with respect to modules, leading to the conclusion that thin film modules offer probably the only chance to utilize fully the intrinsic power gain capability of devices. As the useful limit of silicon appears now in sight, numeric examples are used wherever possible.