Difference between Thyristor and Transistor

Both thyristors and transistors are semiconductor devices widely used in various applications. Read through this article to find out how a thyristor differs from a transistor.

What is a Thyristor?

A thyristor is a four-layer three-terminal semiconductor device. A thyristor consists of three PN-junctions. The three terminals of the thyristor are: anode (A), cathode (C) and gate (G). The gate is the terminal of the thyristor which is used to provide control signal to the device. The circuit symbol of the thyristor is shown in Figure-1.

A thyristor possesses high ratings of voltage and current and also has the ability to handle large power. The thyristor is switched ON by providing initial current to it and then it will remain in the ON state. A thyristor is used as an electronic switch in the high power circuits.

What is a Transistor?

A transistor is a three-layer three-terminal semiconductor device. It consists of two PN junctions. The terminals of the transistor are: emitter (E), base (B) and collector (C). The transistor is switched ON by providing a signal at the base terminal. The circuit symbol of the transistor is shown in Figure-2.

A transistor has low ratings of voltage and current and it has the ability to handle low power. A transistor can be operating as an electronic switch and an amplifier by providing proper biasing. The transistor can be operating in following three regions minus;

Active region as an amplifier
Saturation region as a closed switch
Cut-off region as an open switch

Difference between Thyristor and Transistor

The following table highlights the major differences between a thyristor and a transistor.

Parameter	Thyristor	Transistor
Definition	A thyristor is a four layer, three terminal semiconductor switching device which is used as a controlled rectifier and switch in the electronic circuits.	A transistor is three layer, three terminal semiconductor device which is used as a switch and an amplifier in the circuit.
Construction	Thyristor is formed by four layers of P-type and N-type materials arranged in an alternate manner.	Transistor is formed by sandwiching a layer of either N-type or P-type semiconductor material between two layers of P-type or N-type material respectively.
Terminals	A thyristor has three terminals named: anode (A), cathode (K) and gate (G).	A transistor also has three terminals named: emitter (E), base (B) and collector (C).
PN junctions	A thyristor has three PN junctions.	A transistor has two PN junctions.
Types	Based on the construction, thyristor is only of one type viz. PNPN.	Based on the construction, there are two types of transistors viz. NPN and PNP.
Conduction	Thyristor needs only a triggering pulse at the gate terminal to make it conducting and thereafter it remains conducting.	Transistor requires a continuous current at the base terminal for keeping it in a conduction mode.
Need of turn-off circuit (commutation circuit)	Thyristor needs an extra turn-off circuit in order to stop the conduction.	Transistor does not require any turn-off circuit.
Power rating	Thyristor has capability to handle large power. Thus, it is rated in kilowatts (kW).	A transistor has small power handling capacity. Hence, it is always rated in watts.
Switching time	Thyristor has a large turn-on and turn-off time, i.e., large switching time.	A transistor turns ON and turns OFF immediately, hence the switching time of the transistor is small.
Efficiency	Thyristor has higher efficiency than transistor.	Transistor has relatively low efficiency.
Size	Thyristor is larger in size and hence it is heavier than transistor.	A transistor is relatively smaller in size and less bulky.
Cost	Thyristors are more expensive than transistors.	Transistors are cheaper than thyristors.
Voltage drop	Thyristor has high voltage drop.	Transistor has low voltage drop than a thyristor.
Voltage and current ratings	A thyristor has high voltage and current ratings.	Transistor possesses low voltage and current ratings.
Internal power losses	A thyristor has low internal power losses.	For a transistor, the internal power losses are relatively higher.
Surge current capacity	Thyristor can handle large surge currents.	A transistor is very sensitive and cannot handle the large surge currents.
Suitability	Thyristors are best suited for low frequency and high power applications.	Transistors are best suited for high frequency and low power applications.
Applications	Thyristors are widely in controlled rectification and switching in high power circuits.	Transistors are used in amplification of signals and switching in low power circuits.

Conclusion

From the above discussion, it is clear that there are various differences between a thyristor and a transistor. Both of these semiconductor devices find a wide range of applications in different electronic circuits like electronic switches, amplifiers, controlled rectifiers, etc. Thus, the knowledge of these differences can help an individual in selecting either of them according to the requirements.