Recent results on wave propagation phenomena in planar transmission lines on semiconducting substrates are presented. Special emphasis is laid upon Schottky coplanar lines which are theoretically described by an equivalent circuit. The waveguides are used as optically controlled phase shifters. Under large signal conditions, solitons can be generated and microwave bistability occurs. Finally, we show that Schottky coplanar lines can be applied as high-speed traveling-wave photodetectors, and novel photonic switching devices are proposed.