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Like Thyristors, transistors are also used a switching device. Transistors are the tiny electronic component that changed the world, we can find them in every device like TVs, mobiles, laptops, calculators, and earphones etc. As they are adaptable or versatile it doesn’t mean that they can do, we can use them as amplifying and switching device but they cannot handle higher current, also a transistor required a continuous switching current. So, for all these issues and to overcome these problems we use Thyristors.  
Generally, SCR and Thyristor are used interchangeably but SCR is a kind of Thyristor. Thyristor includes many types of switches, some of them are SCR (Silicon Controlled Rectifier), GTO (Gate Turn OFF), and IGBT (Insulated Gate Controlled Bipolar Transistor) etc. But SCR is the most widely used device, so the word Thyristor become synonymous to SCR. Simply, SCR is a kind of Thyristor.
SCR or Thyristor is a four-layered, three-junction semiconductor switching device. It has three terminals anode, cathode, and gate. Thyristor is also a unidirectional device like a diode, which means it flows current only in one direction. It consists of three PN junction in series as it is of four layers. Gate terminal used to trigger the SCR by providing small voltage to this terminal, which we also called gate triggering method to turn ON the SCR.

                                    Symbol                                     schematic diagram

Two Transistor Analogy of Thyristor 
Here, the equivalent circuit of two transistor shows that the base of PNP transistor T1 is fed by collector current of NPN transistor T2 and collector current of transistor T1 feeds base of transistor T2. Hence, conduction of both the transistor depends on each other. So, until one of the base of any transistor get base current it will not conduct even if the voltage is present at anode and cathode. The main difference between the transistor and Thyristor is, transistor turns off as the base current is removed while the Thyristor remains ON just by trigger it once. For applications like alarm circuit which need to trigger once and stay ON forever,
cannot use transistor. So, for overcome these problems we use Thyristor.