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A Thyristor or SCR (Silicon Controlled Rectifier) is a semiconductor device that allows the flow of electric current only in one direction. It behaves like a diode, but unlike the latter, the pass-through current can be driven.
The main component of the thyristor is silicon, a very common element on Earth.
Using sophisticated technologies, atoms of different elements are inserted into the purest silicon. This operation is called “Doping”.
In this way, the presence of impurities in its crystal lattice, allows the silicon to become a semiconductor.
Silicon belongs to group 14 of the periodic table of elements and has 4 valence electrons. If the doping atoms belong to group 13, which have 3 valence electrons such as boron or gallium, we can obtain a type P semiconductor and create a hole. If we use instead elements of group 15 (with 5 valence electrons) like phosphorus or arsenic, we can generate a type N semiconductor, leaving a free electron in the crystal lattice.
If we connect a type P with a type N semiconductor, we can produce a P-N junction. The separation zone is called depletion region.
In this thin layer, free electrons of the given N layer will occupy the holes of P layer, creating a new region where there aren’t any free electrons or holes.
Once we reach this equilibrium, side N depletion region becomes positively charged , while side P becomes negatively charged. An electric field is therefore generated, which is a barrier for an additional exchange and it behaves as an insulation.
This is a P-N diode.
If we power the two far ends with a voltage superior than the potential barrier, with the negative pole on the N layer, which is the cathode, and the positive pole on the P layer, which is the anode, we can observe the direct polarization phenomenon.
We can see the movements of new electrons inside the N layer, and the free electrons going through the depletion region, filling the holes of the P layer and then continue to close the circuit.
With reverse bias (therefore reversing the polarity of the power supply), the depletion region will become wider and it will block the electric current flow.
We can understand that a diode works in a unidirectional way, which means that it allows the current flow only in one direction.
The Thyristor differs from the diode in having four layers of alternating semiconductor wafers in the
“P-N - P-N” configuration which forms three depletion regions: where the anode is the outer P layer, the cathode the opposite N layer, and the so-called Gate, which is to be found at the end of the intermediate P semiconductor.
As you can see, it would be impossible to make the electric current flow because in direct polarization it would increase the central depletion region, and in reverse bias the other two would increase.
In order to conduct electric current in direct polarization, the thyristor must overcome the central depletion region with a method called gate triggering.
A second power supply is connected between the cathode and the gate, the electrons pass through the first N layer, overcome the first forward bias and flow into the P layer.
Now the P semiconductor has many more electrons than the N semiconductor above!
For this reason the P layer acts like an N layer, towards the upper N layer; so the electrons pass through the central depletion region.
The three lower layers now look like a large N semiconductor and the electric current can cross the last P-N junction and close the circuit. The secondary power supply can now be removed because the thyristor is behaving like a P-N junction diode and will continue to operate without an additional gate triggering.
The gate triggering works as a switch to allow or to not allow the passage of current in the thyristor, allowing its driving.
Due to this characteristic thyristors are used in inverters, AC voltage converters and controllable voltage rectifiers, because they can supply adjustable DC voltages from a fixed AC voltage.
In fact, by providing an alternating current to a thyristor (which has a sinusoidal alternating voltage), it can only supply a single half-wave rectified current, called pulsating direct current. Furthermore, by delaying the gate triggering, the thyristor can be triggered at any instant of the positive...

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