
Studies have revealed that for transmitting voltages of very high magnitude over large distances, #HVDC(High voltage Direct Current) transmission is more economical. So, there is a necessity to convert the generated high voltage AC in the power generation stations to high voltage DC. A thyristor seems to be capable of doing this effectively, even at high voltages and currents. It is a four-layered, 3 terminal solid-state semiconductor device that can act as a bistable switch, rectifier, and many more. Let's get to know in detail about this compact and compelling device.
What is a thyristor?
If a power supply is connected to a normal transistor, one of the two junctions between the P and N layers will be reverse biased. Thus, a continuous secondary voltage is connected between the emitter and base terminals to bring the transistor into the ON state. But, this secondary voltage source means high power loss, particularly for high power applications.
As a solution to this, William Shockley proposed the thyristor for the first time in 1950. The name Thyristor is a blend of THYRatron and transISTOR. It can operate even after disconnecting the secondary voltage source. It is basically a semiconductor device that has 4 alternate P and N layers. The basic form of a thyristor has three terminals named Cathode, Anode, and Gate. The gate terminal is connected to the P-layer near the cathode. It controls the flow of charges between the anode and cathode terminals.

How does Thyristor work?
To understand the working of a thyristor, let's know in brief about the diodes. In a diode, two semiconductor materials, one doped with P-type impurities and the other with N-type impurities are joined to form a PN junction. At the junction, there forms a depletion layer due to the initial migration of electrons and holes, thereby restricting the further movement of charge carriers across the junction. The potential developed across the junction due to the initial migration of charge carriers is the Barrier potential.

When a voltage source greater than the barrier potential is connected across the diode, the charge carriers overcome the block and the diode starts conducting. This is termed as the forward bias. When the polarity of the voltage source connected across the diode is reversed, the charge carriers move further away from the junction and no conduction takes place. This state is recognized as a reverse bias.
Coming to the thyristor, four alternate P and N layers of semiconductors are joined. Consider a P-N-P-N #thyristor connected to a voltage source. In this case, at least one of the three junctions(the middle junction) of the thyristor would be reverse biased. To make the device conducting, the depletion region should be broken down. This is achieved using the Gate-enabling process.

A secondary voltage source is connected between the gate and cathode terminal and electrons are injected into the P-region. Eventually, the P-region becomes flooded with electrons and becomes an N-region. The bottom three layers are now N-type and the topmost layer is P-type. Thus, it functions as a PN junction diode and starts conducting and continues to conduct even if the secondary voltage source is removed. This is because sufficient #electrons are injected into the P-region and they have made their way into the N-region.
Thus, when a sufficient positive signal pulse or #current is given to the gate terminal of a thyristor, it starts conducting. The minimum value of current known as Holding current (Ih) is necessary to operate the thyristor in ON state. At the #semiconductor level, for the thyristor to turn ON (or latch on) and start conducting, the sum of current gains of both the common base transistors must exceed unity. The current so required is known as the Latching current.
How to switch off the thyristor?
To switch off a thyristor, the value of current through it must be changed, so that the sum of current gains is less than one. Turn off begins after the value of the current becomes less than the Holding current. They can also be switched off by connecting them in a tuned LC circuit, where they are subjected to fluctuating voltage instead of steady voltage.
Switching characteristics

By giving the power supply, we obtain the V-I characteristics between the cathode voltage Va and anode current Ia. From the characteristics obtained, it is inferred that the thyristor can have three modes of operation. They are
Reverse Blocking Mode: