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The theory of operation, construction, and electrical characteristics of a new family of silicon thyristors for use as bidirectional (ac) switches are discussed. With these p-n-p-n devices, load current flow in either direction can be controlled by the application of a low voltage, low current pulse between a gate trigger terminal and one of the load current terminals. Blocking current and voltage characteristics are similar to those of a silicon-controlled rectifier (SCR); but unlike SCR's they can switch load current of either polarity. Devices have been made which, in the blocking state, will support several hundred volts with very little current flow; yet, when in the conducting state, will carry many amperes with a voltage drop of approximately a volt. Static power switching with these devices in such applications as lamp dimming, temperature controls, small motor speed controls, etc., appears to be particularly important. For triggering, these new devices depend on the manipulation of lateral current paths. The potential differences created by the flow of current in these paths determine, in turn, the lateral distribution of injected charge carriers within the device during the triggering interval. This paper presents an analysis of lateral biasing effects and the parameters which control them.