Semi-conductor Device

A thyristor is a solid-state semiconductor device with four layers of alternating P- and N-
type materials. It acts exclusively as a bistable switch, conducting when the gate receives a
current trigger, and continuing to conduct until the voltage across the device is reversed
biased, or until the voltage is removed (by some other means).
A three-lead thyristor is designed to control the larger current of the Anode to Cathode path
by controlling that current with the smaller current of its other lead, known as its Gate. In
contrast, a two-lead thyristor is designed to switch on if the potential difference between its
leads is sufficiently large (breakdown voltage).
Some sources define silicon-controlled rectifier (SCR) and thyristor as synonymous. Other
sources define thyristors as more ornately constructed devices that incorporate at least four
layers of alternating N-type and P-type substrate.
The first thyristor devices were released commercially in 1956. Because thyristors can
control a relatively large amount of power and voltage with a small device, they find wide
application in control of electric power, ranging from light dimmers and electric motor speed
control to high-voltage direct-current power transmission.
Thyristors may be used in power-switching circuits, relay-replacement circuits, inverter
circuits, oscillator circuits, level-detector circuits, chopper circuits, light-dimming circuits,
low-cost timer circuits, logic circuits, speed-control circuits, phase-control circuits, etc.
Originally, thyristors relied only on current reversal to turn them off, making them difficult to
apply for direct current; newer device types can be turned on and off through the control gate
signal. The latter is known as a gate turn-off thyristor, or GTO thyristor.
A thyristor is not a proportional device like a transistor. In other words, a thyristor can only
be fully on or off, while a transistor can lie in between on and off states. This makes a
thyristor unsuitable as an analog amplifier, but useful as a switch.