What are thyristors?

With four layers of alternating P- and N- materials, a thyristor is a solid-state semiconductor device. It acts as a type of bi-stable switch and it conducts when the gate receives a current trigger and continues to conduct until the voltage is reverse biased across the device or by some means until the voltage is removed. The silicon-controlled rectifier is considered synonymous with thyristors by some sources. Thyristors are defined as more complex by other sources as it contains at least four-layer of alternating N-type and P-type substrate. They find large applications in the control of electrical power supplies, electric motor speed control, and ranging of light dimmers because thyristors can control large amounts of power supplies and voltage with a small device. The circuit symbol of the thyristor is given below.

Introduction to thyristors

It is a type of three-terminal, four-layered semiconductor device with each layer consisting of N and P-type materials. For instance, P-N-P-N. The main terminals are the anode-to-cathode terminal which are across all four layers. The gate terminal is the control terminal that is attached to p-type material near the cathode. Thyristors have three modes of operation, which are :

1. Reverse blocking mode: The voltage is applied in such a direction that would be blocked by a diode.
2. Forward blocking mode: Voltage is applied in such a direction that helps the diode to conduct but the thyristor is not triggered into for conduction.
3. Forward conducting mode: In this mode, the thyristor is triggered into conduction and it continues to conduct until the forward current flow drops down a value called threshold value which is known as "holding current".

Functions of the gate terminal

Three PN junctions are there in a thyristor named $J_1, J_2, and J_3$ from the anode. The junctions $J_1 and J_3$ are forward biased and junction $J_2$ is reverse biased when the anode terminal is at positive potential namely $V_{AK}$ with respect to the cathode and no voltage is applied at the gate-controlled terminal. No conduction takes place now as the junction $J_2$ is reverse biased (turn-off). The avalanche breakdown of junction $J_2$ happens and the thyristor starts conducting when the voltage $V_{AK}$ is increased beyond a threshold value of break over voltage $V_{BO}$. The breakdown of junction $J_2$ occurs when a positive potential $V_G$ is applied at the gate terminal with respect to the cathode terminal.  The thyristor can be switched on quickly by selecting an appropriate value of $V_G$. Irrespective of the gate terminal voltage value, the thyristor continues to conduct once avalanche breakdown has occurred until the current flow through the device becomes less than the holding current value or the potential $V_{AK}$ is removed. The gate voltage $V_{GT}$ and the gate current $I_{GT}$ characterize the gate pulses. The gate current varies inversely with gate pulse width.

Types of thyristors

• AGT (Anode gate thyristor): A type of thyristor which has a gate on n-layer near to the anode
• ASCR or Asymmetrical silicon controlled rectifier
• BCT (Bidirectional control rectifier): It is a bidirectional type of switching device which has two thyristor structures with separate gate contacts.
• BOD (Breakover diode): Triggered by avalanche current, it has a gateless thyristor.
• DIAC: It is a type of bidirectional trigger device.
• Dynistor: It is a type of unidirectional switching device.
• Shockley Diode: It is a type of unidirectional trigger and switching device.
• SIDAC: It is a type of bidirectional switching device.
• ETO: It is an emitter turn-off thyristor.
• GTO: It is a gate turn-off thyristor.
• IGCT: It is an integrated gate-commutated thyristor.
• LASCR: It is a light-activated SCR.
• MCT: It is a MOSFET-controlled thyristor. For on and off-state control, it has two additional FET structures.
• RCT: They are reverse conducting thyristors.
• SCR: Silicon-controlled rectifier is a type of phase control rectifier.
• TRIAC (Triode for alternating current): It is a type of bidirectional switching device which contains two thyristor structures with common gate contact.
• QUADRAC: It is a special kind of thyristor that is made up of a combination of DIAC and TRIAC.

Reverse conducting thyristors

A reverse conducting thyristor is not capable of reverse blocking because it has an integrated reversed diode. Wherever reverse or freewheel diodes are used, these devices are advantageous in use. Since the SCR and the diode never conduct at the same time, so they do not produce any heat and can be easily cooled and integrated. In frequency changers and inverters, the reverse conducting thyristors are often used.

Photothyristors

These types of thyristors are only activated by light. Their insensitivity to electrical signals is the advantage of photothyristors which can cause faulty operations in electrically noisy environments. A light-activated thyristor has an optically sensitive region in its gate terminal into which the electromagnetic radiations are coupled with a fiber called optical fiber. Light-triggered thyristors are advantageous in high voltage applications such as HVDC (high voltage direct current) because no electronic boards are required to provide the potential of the thyristors to trigger it. Two common types of photothyristors include the light-activated SCR and the light-activated TRIAC. When exposed to light, a LASCR acts as a switch and turns on. The LASCR is on even if the light is removed but the power is not removed and the polarities of the anode and cathode are not reversed yet.

Context and Applications

This topic is significant in the professional exam for undergraduate, graduate, and postgraduate courses.

• Bachelors in Electrical Engineering
• Bachelors in Electronics and Telecommunication
• Masters in Electrical Engineering
• Masters in Electronics and Telecommunication

Practice Problems

1. How many layers does a thyristor contain?

1. 2 layers
2. 3 layers
3. 4 layers
4. 5 layers

Answer: Option c

Explanation: With four layers of alternating P- and N- materials, a thyristor is a solid-state semiconductor device.

2. How many terminals does a thyristor have?

1. Two-terminals
2. Three-terminals
3. Four-terminals
4. Five-terminals

Answer: Option b

Explanation: Thyristor is a type of three-terminal, four-layered semiconductor device with each layer consisting of N and P-type materials. For instance, P-N-P-N.

3. Which terminal is the control terminal in the thyristor?

1. Gate terminal
2. Source terminal
3. Drain terminal
4. None

Answer: Option a

Explanation: The gate terminal is the control terminal that is attached to p-type material near the cathode.

4. Which of the following layers of transistors can be termed thyristor?

1. PNPN transistor
2. NPN transistor
3. PNP transistor
4. FET-CTH

Answer: Option a

Explanation: A transistor contains alternatively 4 layers of P- and N-type, for instance, PNPN layer transistor.

5. QUADRAC type thyristor is a combination of which two type thyristors?

1. SCR and TRIAC
2. DIAC and TRIAC
3. DIAC and SCR
4. RCT and phase control rectifier

Answer: Option b

Explanation: QUADRAC is a special kind of thyristor that is made up of the combination of DIAC and TRIAC.

Related Concepts

• Thyristor controlled reactor
• Insulated gate bipolar transistor
• Latch-up
• Thyristor drive