Thyristors are multifunctional semiconductor devices that have unique current- and voltage-handling capabilities. They can switch from a non-conducting OFF state to a highly conducting ON state in a variety of ways, enabling flexible control of electrical power and conversion between AC and DC.

Internal Structure

The thyristor consists of four alternating layers of P-type and N-type semiconductor material forming three PN junctions in series. The outer layers are known as the anode and cathode terminals. Additional gate terminals can be attached to the inner layers, allowing external signals to trigger the transition between the blocking and conducting modes.

Based on the number of gates, thyristors are classified into: 
• Two-terminal device without a gate (DIAC)
• Three-terminal device with one gate (TRIAC) 
• Four-terminal device with two gates (SCS)

Volt-Ampere Characteristics  

The volt-ampere curves of a thyristor are highly nonlinear and have two distinct regions – reverse blocking mode and forward blocking mode. 

In the reverse blocking region, all PN junctions are biased in the reverse direction. A small leakage current flows through the device.

In the forward blocking region, the thyristor blocks current flow until the breakover voltage is reached. At that point, the resistance abruptly falls and the thyristor switches to its ON-state.  

Switching Actions

Thyristors can be switched from the OFF-state to ON-state in several ways:

• Breakover voltage triggering
• Gate signal triggering - for gated devices like TRIACs 
• Light triggering - for light-activated thyristors
• dv/dt triggering due to rate of rise of voltage 

Once turned ON, the device current must be reduced below the holding current value for it to switch OFF. This latching capability provides flexibility for power control applications.


Thanks to their robustness and unique switching capabilities, thyristors find use in a wide array of applications:

• Power switching and control - drives, power supplies, motor starters, light dimmers
• Power conversion - HVDC transmission, AC/DC rectifiers, inverters
• Overvoltage protection 
• Digital electronics - logic gates, memory cells
• Pulsed lasers and arc welders

Thyristors continue finding new applications as power semiconductor technology advances. They remain indispensable for efficient power conversion, switching and control.

With the expanding use of renewable energy sources such as solar and wind, thyristors play a vital role in grid integration and management of these irregular power generation assets. Sophisticated thyristor systems provide continuous dynamic control over transmission of renewable energy onto the smart power grid.

Emerging applications include high-density, high-efficiency power electronics for electric vehicles. Thyristor converters and inverters are ideal to withstand high voltages in EV traction motor drives. Gate turn-off thyristors (GTOs) allow pulse-width modulation to achieve smooth and reliable speed control for EVs.

In consumer electronics, light dimmers based on thyristors have been augmented by TRIAC dimmers enabling efficient lighting control. Solid-state circuit breakers using thyristors provide intelligent shut-off capability combined with current limiting functions for smart homes.   

The excellent thermal stability and insensitive gate drives offered by thyristors have opened up cutting-edge applications in avionics controls. Thyristors tolerate operation under extremes temperature, pressure and vibration - making them indispensible for aerospace applications.

Advances in silicon carbide and gallium nitride based power semiconductors will further expand capabilities of high-power, high-temperature thyristors. They will find newer applications from HVDC transmisison systems to electric aircraft propulsion drives leveraging their unique capabilities.  

In summary, innovations in thyristor materials and circuit topologies continue to fuel their adoption into newer application domains - while retaining mission-critical roles in existing industrial and power infrastructure.