Both transistor and thyristor are semiconductor devices with alternating P type and N type semiconductor layers. They are used in many switching applications due to many reasons such as efficiency, low cost and small size. Both of them are three terminal devices, and they provide a good control range of current with a small controlling current. Both these devices have application dependant advantages.
Transistor
Transistor is made of three alternating semiconductor layers (Either P-N-P or N-P-N). This forms two PN junctions (a junction made by connecting a P type semiconductor and an N type semiconductor) and therefore, a unique type of behavior is observed. Three electrodes are connected to three semiconductor layers and middle terminal is called ‘base’. Other two layers are known as ‘emitter’ and ‘collector’.
In transistor, large collector to emitter (Ic) current is controlled by the small base emitter current (IB) and this property is exploited to design amplifiers or switches. In switching applications, the three layers of semiconductors act as a conductor when the base current is provided.
Thyristor
Thyristor is made of four alternating semiconductor layers (in the form of P-N-P-N) and therefore, consists of three PN junctions. In analysis, this is considered as a tightly coupled pair of transistors (one PNP and other in NPN configuration). The outermost P and N type semiconductor layers are called anode and cathode respectively. Electrode connected to inner P type semiconductor layer is known as the ‘gate’.
In operation, thyristor acts conducting when a pulse is provided to the gate. It has three modes of operation known as ‘reverse blocking mode’, ‘forward blocking mode’ and ‘forward conducting mode’. Once the gate is triggered with the pulse, thyristor goes to the ‘forward conducting mode’ and keep conducting until the forward current become less than the threshold ‘holding current’.
Thyristors are power devices and most of the times they are used in applications where high currents and voltages are involved. The most used thyristor application is controlling alternating currents.
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Difference between transistor and thyristor 1. Transistor has only three layers of semiconductor where thyristor has four layers of them. 2. Three terminals of transistor are known as emitter, collector and base where thyristor has terminals known as anode, cathode and gate 3. Thyristor is considered as tightly couple pair of transistors in analysis. 4. Thyristors can operate at higher voltages and currents than transistors. 5. Power handling is better for thyristors because their ratings are given in kilo watts and transistor power range is in watts. 6. Thyristor only requires a pulse to change the mode to conducting where transistor needs a continuous supply of the controlling current. 7. Internal power loss in transistor is higher than that of thyristor.
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