Difference Between Thyristor and Transistor:

As already mentioned, transistors and thyristors are both semiconductor devices. They are now widely employed in switching operations because of their numerous advantages such as noiseless operation owing to absence of moving parts, very high switching speed (say 10⁹ operations per second), high efficiency, low maintenance, small size, little weight and trouble free service for long period, large control current range (say from 30 A to 100 A) with small gate current of few mA over mechanical switches or electromechanical relays. However, transistors and thyristors both have their own areas of applications. Thyristors have some advantages over transistors, as enumerated below :

1. Thyristor is a four layer device while the transistor is a three layer device.
2. Due to difference in fabrication and operation it is possible to have thyristors with higher voltage and current ratings.
3. Rating of a transistor is always in watts while that of a thyristor is in kWs i.e., thyristors have better power handling capacity.
4. A thyristor needs only a pulse to make it conducting and thereafter it remains conducting. On the other hand a transistor needs a continuous current for keeping it in a conducting state.
5. Internal power losses in a thyristor are much smaller than those in a transistor.
6. Circuits using power transistors will be smaller in size and less costlier than circuits using thyristors.
7. Commutation circuit, which is costly and bulky is required in case of thyristors.
8. Power transistors have no surge current capacity and can withstand only a low rate of change of current whereas the thyristors have surge current rating and, therefore, can withstand high rate of change of current compared to transistors.
9. Power transistors switch on faster than SCRs, and turn­off problems are practically nonexistent, and therefore, power transistors can be employed in very high fre­quency applications. Thyristors, on the other hand, can be used in comparatively low frequency applications.
10. Transistors have little operating experience in high power applications. Power transistors or Darlington pairs are more susceptible to facture. Thyristor circuits, on the other hand, have a proven record of many years of reliable operation.

Transistors, however, have lower voltage drop, and need no turn-off circuit. Power transistors are linear devices better suited for some linear power applications such as audio and high frequency power amplifiers, regulator systems etc.