A thyristor is a semiconductor device with two stable states and three (or more) interacting rectifying junctions. In terms of functionality, the thyristor is classified as electronic, not fully controllable keys. How does this device work and what is it like?
Thyristor classification
A typical thyristor has three leads in the form of an anode, a cathode and a gate electrode, where the anode is a contact with the outer p-layer, and the cathode is a contact with the outer n-layer. The classification of thyristors is carried out depending on the number of available leads: for example, a device with two leads (anode and cathode) is called a dinistor, and a device with three or four leads is called a triode or tetrode tinistor. One of the most interesting devices is considered to be a triac (symmetrical tinistor), which turns on at any voltage polarity.
There are tinistors with even more semiconductor alternating regions.
Typically, this device is represented by two interconnected transistors operating in an active mode. The extreme regions of the thyristor are called emitter, while its central junction is called collector. The thyristor is turned on by supplying a pulse to the control circuit of positive polarity (relative to the cathode). The duration of transient processes in this case depends on the nature and current of the load, amplitude, applied voltage, rate of current rise, and so on. For a visual explanation of the operation of a thyristor, the current-voltage characteristics of the device are usually used.
Thyristor operation
A small positive voltage is applied to the anode of the device. In this case, the collector junction is turned on in the opposite direction, and the emitter junctions are switched on in the forward direction. On the current-voltage characteristic, the section from zero to one is approximately equal to the reverse branch of the current-voltage characteristic of the diode (the closed state of the device). With an increase in the anode voltage, the injection of basic carriers begins, causing the accumulation of holes and electrons, which is equivalent to the potential difference at the central junction.
After increasing the current with the thyristor, the voltage present at the collector junction will decrease.
With a decrease in voltage to a certain level, the thyristor goes into a state called negative differential resistance. Then all transitions of the thyristor are shifted in the forward direction, making it open. The device will be in it until the collector junction is displaced in the same direction. Reverse connection of the thyristor gives the same current-voltage characteristic as two diodes that were connected in series. In this case, the reverse voltage will be limited by the breakdown voltage.
