Electrically positionable short-circuits

An electrical short-circuit for alternating-current (ac) microwave signals is physically positionable in direct response to an electrical bias control without an intermediary electromechanical converter. The electrically positionable short circuit includes at least first and second doped regions in a semiconductor, separated by a region in which the short circuit is formed between the doped regions by the bias. A first embodiment comprises discrete diodes connected between conductors at different locations, the discrete diodes having different forward junction voltages, so that varying the common bias voltage varies the number of conducting diodes and thus positions the short circuit in a stepwise manner. A FET embodiment includes various discrete MOSFETs having different conduction threshold voltages, and having their sources and drains connected at various points to the conductors to be short-circuited, so that variations of a common gate bias voltage selectively render one or more of the FETs conductive, thereby stepwise positioning the short-circuit. Distributed PIN and MOSFET structures provide continuous short circuit positioning as a monotonic function of bias. Two distinct modes of operation, pinchoff/resistive and a resistive/below-threshold are possible for the distributed FET embodiment. The electrically positionable short-circuit can be coupled to the conductors of a transmission line for effecting tuning, or can be coupled to a transmission line in such a way as to vary the signal path length and thereby provide phase shift.