Thyristors are semiconductors that are, essentially control systems  for switching and controlling power. The ones used most are  Silicon-Controlled Rectifiers (SCRs), Diode AC Switches (Diacs), Triode  AC Switches (Triacs), and Uni-Junction Transistors (UJTs). Here’s a  breakdown of each one and how they function:

1. Silicon-Controlled Rectifiers (SCRs): Three-terminal semiconductors  used for controlling the flow of current in electrical circuits. This  device consists of four layers of alternating P-type and N-type  semiconductor material. The device operates in three states: off state,  forward blocking state, and forward conducting state. In the off state,  the device blocks the flow of current. When a voltage is applied to the  gate terminal, the device enters the forward blocking state, and the  current starts flowing through the device when the voltage across the  anode and cathode exceeds a certain threshold value. SCRs are widely  used in power control applications, such as motor control, heating  control, and lighting control.
    
2. Diode AC Switches (Diacs): Two-terminal semiconductors that are used  for controlling the flow of AC current in electrical circuits. The  device consists of two layers of alternating P-type and N-type  semiconductor material. Operates in two states: off state and on state.  In the off state, the device blocks the flow of current. When a voltage  is applied to the device, the device enters the on state and conducts  current in both directions. Diacs are commonly used in AC voltage  control applications, such as dimmer circuits and fan speed controllers.
    
3. Triode AC Switches (Triacs): Three-terminal semiconductor devices  that are used for controlling the flow of AC current in electrical  circuits. The device consists of two SCR devices connected in parallel  in an inverse parallel configuration. The device operates in four  states: off state, forward blocking state, forward conducting state, and  reverse blocking state. In the off state, the device blocks the flow of  current. When a voltage is applied to the device, the device enters the  forward blocking state, and the current starts flowing through the  device when the voltage across the anode and cathode exceeds a certain  threshold value. Commonly used in AC voltage control applications, such  as light dimmers and motor speed controllers.
    
4. Uni-Junction Transistors (UJTs): Three-terminal semiconductor devices  that are used for generating trigger pulses in electrical circuits. The  device consists of a single P-type material and two N-type material.  Operates in two states: off state and on state. In the off state, the  device blocks the flow of current. When a voltage is applied to the  device, the device enters the on state and conducts current between the  emitter and one of the base terminals. UJTs are commonly used in  applications that require precise timing, such as triggering of SCRs and  Triacs.
    

By understanding the operation of each of these devices, control  system engineers can choose the appropriate device for their specific  application to achieve the desired control and power handling  requirements.

REFERENCES:

The Silicon-Controlled Rectifier (SCR) | Thyristors | Electronics Textbook.  (n.d.). Www.allaboutcircuits.com.  https://www.allaboutcircuits.com/textbook/semiconductors/chpt-7/silicon-controlled-rectifier-scr/

What is a DIAC – Operation Applications Circuits» Electronics Notes. (n.d.). Www.electronics-Notes.com. https://www.electronics-notes.com/articles/electronic_components/scr/what-is-a-diac.php

Electrical4U. (n.d.). TRIAC: What is it? (Definition, Operation & Applications) | Electrical4U. Https://Www.electrical4u.com/. https://www.electrical4u.com/triac/

Unijunction Transistor – Construction, Working Principle, and Characteristic Features.  (n.d.). Www.tutorialspoint.com.  https://www.tutorialspoint.com/unijunction-transistor-construction-working-principle-and-characteristic-features