Thyristors are a family of power semiconductor devices. Thyristors are used extensively in power electronic circuits. They are operated as bistable switches, operating from a non-conducting state to conducting state. The first Thyristor was produced in the year 1956. The most common type of Thyristor is silicon Controlled Rectifier (SCR).In this blog, we will discuss 

  • What is thyristors?
  • How it works
  • Classify thyristors with their control characteristics.

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What is thyristors?


A thyristor is a four-layer semiconductor device of a PNPN structure with three PN junctions. It has three terminals i.e. Anode, Cathode, and a Gate. An anode is a positive terminal and a Cathode is a negative terminal. The Gate controls the flow of current between anode and cathode. It works in electronic devices and equipment to control the electric power or current. It acts as a rectifier and can only transmit current in one direction.

Working Principal of  thyristors

The cross-section of a thyristor is shown in the figure. When the anode voltage is made positive concerning the cathode, the junctions J1 and J3 are forward biased and J2 is reverse biased. 



When the positive potential is increased beyond the breakdown voltage, the breakdown of junction J2 takes place and it starts conducting. Once the breakdown has occurred, it continues to conduct irrespective of the gate voltage, until the potential at the anode is removed or the current through the device is made less than the holding current. Now when a positive potential is applied at the gate terminal concerning the cathode, the breakdown of junction J2 takes place. To switch on the Thyristor quickly, an appropriate potential value has to be selected. The gate acts as a controlling electrode. When a small voltage known as gate pulse is applied to its gate, the device is triggered into a conduction state. This continues until the voltage across the device is reversed or removed. The gate trigger current varies inversely with the gate voltage and a minimum gate charge is required to trigger it. Thus the switching of Thyristors can be controlled through its gate pulse.

Classify thyristors with their control characteristics

  1. Emitter turn off Thyristors (ETO)
  2. Fast Switching Thyristors (SCR)
  3. Light Activated Silicon Controlled Rectifiers (LASCR)
  4. Gate Turn Off Thyristors (GTO)
  5. Reverse Conducting Thyristors (RCT)
  6. FET Controlled Thyristors (FET-CTH)
  7. MOS Turn Off Thyristors (MTO)
  8. Bidirectional Phase Controlled Thyristors (BCT)