The transistor has three terminals - Emitter, base and collector whereas the SCR has three terminals - Anode, cathode and gate.
The transistor is a three layer, two junction device whereas the SCR is a four layer, three junction device.
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The transistor is switched on by continuous base current whereas the SCR is switched on by gate pulse.
The SCR remains in on condition after removing gate pulse.
The input base current controls the output current in the transistor whereas the input gate current controls the anode current in the SCR.
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The on state voltage drop in the transistor is in the order of 0.3 to 0.8 voltages whereas it is approximately 1.0 to 1.5 voltage in the SCR.
The transistors are available in comparatively low voltage and current ratings whereas the SCRs are available in high voltage and high current ratings.
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Commutation circuit does not necessary for transistor. Commutation circuit requires for the DC applications
Transistor : Applicable for high frequency, SCR : Applicable for low frequency