Functioning of the Tiristor.

Following the first image it is supposed that between plate and cathode does not exist any tension as well as between the government electrode, or door and the cathode therefore there exist certain areas devoid of charges, well definite, in each of the unions PN, and indicated like J1, J2, J3. If a tension is applied between plate and cathode being the positive plate with regard to the cathode, the unions J1 and J3 polarize in direct sense and become narrower, while the union J2 polarizes in inverse sense, and his depletion area becomes more wide, as it is possible to see in the second image. Under these circumstances it does not happen practically current (except the familiar and insignificant current of escape).
And so, here we have the same surprise: If this new element that we have just mounted was behaving like the diodes placed in series, it is clear that the step of the current should take place, since it would be equivalent to the step of one and another diode successively; but after the material close semiconductor was if in his different classes, it happens that this forecast is not fulfilled and is created, as we were saying, a resistance area in J2 that it does that this element is blocked in direct sense.
We have said that in fact what has been done is not to join two diodes, one after other, but rather to join two transistors so that the scheme of this element would be something like what is seen in the third image, where every transistor preserves somehow his characteristics and, obviously, we have left a crystal to which he is salted by an electrode to which is to the one that we have called earlier a door (G). Well, all right: let's see that it happens if supporting the same tension source between plate and cathode that we saw in the first image, we apply a positive impulse to him in the same door electrode, as one sees in the fourth image. Now we see that the current happens across the union J3 polarized in direct sense, the cathode towards the door or electrode of government, as it would happen in a transistor NPN. Three low regions composed by the glazing N2, P2 and N1 can be considered to be a transistor NPN in which P2 contituye the base; N1 the collector, and N2, the issuer. Current government - cathode electrode issuer - base of a transistor is equivalent to current, and for the effect transistor departs from the issuer's current crosses the union J2 and happens to the collector. The electrons flow across the union J2 is a cause of the region of depletion become closer and that, therefore, his resistance redusca.
As a result of everything exhibited the proportion of the tension plate - cathode that appears in the union J2 diminishes, which allows to increase the tension of the unions J1 t J3. The direct polrización of J1 increases and a certain number of hollows crosses the union, which turns out to be represented by means of the white arrow in the fifth image.
The regions P1, N1 and P2 can be considered to be in turn a transistor PNP, for what, due to the effect of the normal transistor, some of the hollows of the issuer P1 flow towards the coletor P2 across the base N1 (white arrow in the sixth image). The latter hollows flow across the union J2 does that his width redusca moreover and, for consiguient, the electrons flow increases in the transistor formed by the regions N2, P2 and N1.
This accumulative effect initiated on the positive impulse applied between electrode of government and cathode, continues quickly until the union J2 disappears completely, with which the effective resistance plate - cathode of the tiristor becomes very small and a direct current of big intensity can ciircular across him.
The tiristor remains in the enclosed conduction state after eliminating the positive impulse between electrode of government - cathode that made it happen to the conduction state. In practice, a small positive impulse applied to the electrode of government, depocos volts of largeness, is sufficient to unblock the tiristor. Nevertheless, the duration of the impulse has to be it sufficiently long to achieve with it that the comerry anodic one reaches a certain minimal value of maintenance before it eliminates the impulse.
The blockade of the tiristor can only be obtained reducing the tension plate - cathode below his maintenance level, which is obtained normally limiting the tension plate - cathode to zero.

He keeps on reading on Tiristores:

Curves typical of the Tiristor.
Variety of Tiristores.
Code of designation of diodes controlled semiconductors (Tiristores).

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