A controllable semiconductor switch comprises an anode, a cathode and a switchable amorphous semiconductor element interposed between the anode and cathode. The switchable amorphous semiconductor element has a high electrical resistance for substantially blocking current between the anode and cathode, and it, upon the application of a voltage above a threshold voltage value to the anode and cathode of the semiconductor switch, is capable of having at least portions thereof between the anode and cathode substantially instantaneously changed to a low electrical resistance for substantially conducting current between the anode and cathode. The anode or cathode or both may comprise a semiconductor which regulates the threshold voltage value of the semiconductor switch. Where the cathode or anode comprises a semiconductor and the other a metal, the semiconductor switch may be asymmetric in its operation, it having a higher threshold voltage value for one polarity of the voltage applied to the anode or cathode than for the opposite polarity. By controlling the semiconductor anode and/or cathode the threshold voltage value of the semiconductor switch may be varied or regulated to desired values.