The Basic Four Layer Device•The basic thyristor is a 4-layer device with two terminals, the anode and the cathode. •It is constructed of four semiconductor layers that form a pnpn structure. •The device acts as a switch and remains off until the forward voltage reaches a certain value; then it turns on and conducts. Conduction continues until the current is reduced below a specified value. •This basic thyristor is also known as a silicon unilateral switch (SUS), Shockley diode, or 4-layer diode.
Two Transistor Model of 4-Layer Diode•The 4-layer diode (also known as Shockley diode and SUS) is a type of thyristor, which is a class of devices constructed of four semiconductor layers. •The pnpn structure can be represented by an equivalent circuit consisting of a pnp transistor and an npn transistor.
Holding Current•Once the 4-layer diode is conducting (in the on state), it will continue to conduct until the anode current is reduced below a specified level, called the holding current, IH. •When IAfalls below IHthe device rapidly switches back to the off state and enters the forward-blocking region.Switching Current•The value of the anode current at the point where the device switches