Thyristors for Dummies

Good evening, Habr. Let's talk about such a device as a thyristor. A thyristor is a semiconductor device with two stable states, having three or more interacting rectifying transitions. By functionality, they can be attributed to electronic keys. But there is one peculiarity in the thyristor, it cannot go into a closed state, unlike a regular key. Therefore, it can usually be found under the name - not a fully managed key.

The figure shows the usual view of the thyristor. It consists of four alternating types of electrical conductivity of the semiconductor regions and has three outputs: anode, cathode and control electrode.
The anode is contact with the external p-layer, the cathode is with the external n-layer.
You can refresh the memory of the pn transition here .


Depending on the number of conclusions, a classification of thyristors can be derived. In fact, everything is very simple: a thyristor with two terminals is called dinistors (accordingly, it has only an anode and a cathode). A thyristor with three and four pins is called triode or tetrode. There are also thyristors with a large number of alternating semiconductor regions. One of the most interesting is the symmetric thyristor (triac), which turns on at any voltage polarity.

Principle of operation

Typically, the thyristor is presented in the form of two transistors connected together, each of which works in an active mode.

In connection with this pattern, the extreme regions can be called emitter, and the central transition is collector.
To understand how the thyristor works, take a look at the current-voltage characteristic.

A small positive voltage was applied to the thyristor anode. Emitter junctions are included in the forward direction, and collector junctions in the opposite direction. (in fact, all the voltage will be on it). The section from zero to unity on the current-voltage characteristic will be approximately similar to the inverse branch of the diode's characteristic. This mode can be called the thyristor closed state mode.
As the anode voltage increases, the main carriers are injected in the base region, thereby accumulating electrons and holes, which is equivalent to the potential difference at the collector junction. With increasing current through the thyristor, the voltage across the collector junction will begin to decrease. And when it decreases to a certain value, our thyristor will go into a state of negative differential resistance (plot 1-2).
After that, all three transitions will be shifted in the forward direction, thereby transferring the thyristor to the open state (plot 2-3).
The thyristor will remain in the open state until the collector junction is biased in the forward direction. If the thyristor current is reduced, then the number of nonequilibrium carriers in the base regions will decrease as a result of recombination, and the collector junction will be biased in the opposite direction and the thyristor will go into the closed state.
When the thyristor is turned back on, the current-voltage characteristic will be similar to that of two diodes connected in series. The reverse voltage will be limited in this case by the breakdown voltage.

General parameters of thyristors

1. The turn - on voltage is the minimum anode voltage at which the thyristor goes into the on state.
2. Forward voltage is a direct voltage drop at the maximum current of the anode.
3. Reverse voltage is the maximum allowable voltage on the thyristor in the closed state.
4. The maximum allowable forward current is the maximum open current.
5. Reverse current - current at maximum reverse voltage.
6. Maximum electrode control current
7. On / off delay time
8. Maximum allowable power dissipation


Thus, in the thyristor there is a positive current feedback - an increase in current through one emitter junction leads to an increase in current through another emitter junction.
Thyristor is not a fully controlling key. That is, having switched to the open state, it remains in it even if it stops supplying a signal to the control transition if a current is supplied above a certain value, i.e., a holding current.


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