Silicon Controlled Rectifier (SCR) :

An SCR is a controlled rectifier made up of p-type and n-type semiconductor material belonging to the thyristor family. It consists of three terminals anode, cathode, and gate, and works similar to a diode when a pulse is applied to the gate terminal. It is a unidirectional device that can conduct current only in one direction.

Bipolar Junction Transistor (BJT) :

A BJT is also a semiconductor device with PN junctions that can amplify the signals. It is also a three-terminal device emitter, collector, and base. BJTs are mainly used in switching and amplifier circuits that increase the strength of a weak signal, whereas SCRs are used in rectifier and power control applications.

Metal Oxide Semiconductor Field Effect Transistor (MOSFET) :

MOSFETs are similar to BJTs used in switching and amplifier circuits. It is a three-layer, three/four-terminal (drain, source, and gate) unipolar device. The main difference between BJT and MOSFET is that BJT is a current-controlled device whereas the flow of current in MOSFET is controlled by voltage i.e., it is a voltage-controlled device.

Insulated Gate Bipolar Transistor (IGBT) :

An IGBT is formed by combining the characteristics of BJT and MOSFET. It combines the low ON-state losses of BJT and the very simple gate drive of MOSFET. The structure of IGBT is similar to MOSFET. It has high voltage and current handling capabilities with high-speed switching and low gate current performance.

Comparison Between SCR, BJT, MOSFET, and IGBT :

Parameter	SCR	BJT	MOSFET	IGBT
Full Form	SCR stands for silicon-controlled rectifier.	BJT stands for bipolar junction transistor.	MOSFET stands for metal oxide semiconductor field effect transistor.	IGBT stands for insulated gate bipolar transistor.
Symbol
Terminals	It has three terminals anode, cathode, and gate.	It has three terminals emitter, collector, and base.	It has three terminals emitter, collector, and gate.	It has three terminals emitter or source, collector or drain, and gate.
Type of Device	It is a majority carrier device.	It is a bipolar device.	It is a majority carrier device.	It is a majority carrier device.
Gate or Base Control	Gate has no control once it is turned ON.	Base has full control over the operation.	Gate has full control over the operation.	Gate has full control over the operation.
Voltage and Current Ratings	The voltage and Current Ratings of SCR are 10kV and 4kA respectively.	The voltage and Current Ratings of SCR are 2kV and 1kA respectively.	The voltage and Current Ratings of SCR are 1kV and 50A respectively.	The voltage and Current Ratings of SCR are 1.5kV and 400A respectively.
Switching Frequency	It has a low switching frequency (up to 500Hz).	It has a medium switching frequency (up to 10kHz).	It has a high switching frequency (up to 100kHz).	It has a medium switching frequency (up to 20kHz).
Gate Drive	Current	Current	Voltage	Voltage
Snubber Circuit	Unpolarized	Polarized	Not essential	Not essential
Voltage Blocking Capability	It has both symmetric and asymmetric voltage blocking capability.	It has asymmetric voltage blocking capability.	It has asymmetric voltage blocking capability.	It has asymmetric voltage blocking capability.
Temperature Coefficient	Negative	Negative	Positive	Generally flat but positive at high current
Paralleling	SCR can be connected easily in parallel.	BJTs cannot be easily connected in parallel due to the base-emitter voltage mismatch problem.	Paralleling MOSFETs is easy.	IGBTs are also easily paralleled due to good control over the variation of IGBT parameters from one device to another.
Applications	The applications of SCR are AC to DC converters, controlled rectifiers, ac voltage controllers, etc.	The applications of BJT are amplifiers, oscillators, UPS, SMPS, choppers, etc.	The applications of MOSFET are SMPS, robotics, microprocessors, automobile industry, motor controllers, etc.	The applications of IGBT are inverters, welding machines, UPS, SMPS, etc.