The Different BJT Vs FET Transistors

 

BJT (Bipolar Junction Transistor) and FET (Field-Effect Transistor) are the two main types of transistors used in electronic circuits. They work according to different principles and have their own characteristics:

 

BJT (Bipolar Junction Transistor):

Operation Principle:

Ø A BJT is a current controlled device. It regulates the flow of current between its collector and emitter terminals based on the current flowing at the base terminal.

Structure:

Ø BJT has three layers of semiconductor material: the emitter, base, and collector. These layers are either N-P-N (NPN) or P-N-P (PNP) configurations.

Current Gain:

Ø BJTs have a current gain (β) which represents the ratio of the collector current to the base current (Ic/Ib). This current gain can be quite high, allowing small base currents to control large collector currents.

Voltage Requirements:

Ø BJTs typically require a small base-emitter voltage (around 0.7 V for silicon transistors) to start conducting.

Applications:

Ø Commonly used in amplifiers, both audio and radio frequency.

Ø Switching applications in digital circuits.

Ø Used in analog circuits where high gain is needed.

FET (Field-Effect Transistor):

Operation Principle:

Ø FET is a voltage-controlled device. It regulates the flow of current between its source and drain terminals based on the voltage applied to its gate terminal.

Structure:

Ø FETs have three terminals: gate, source, and drain. The current flows between the source and drain terminals, controlled by the voltage applied to the gate.

Current Gain:

Ø FETs do not have a direct current gain like BJTs. The voltage at the gate terminal determines the current between the source and drain.

Voltage Requirements:

Ø FETs are voltage-driven and require a voltage applied to the gate to control the current flow. There is virtually no current flow into the gate.

Applications:

Ø Commonly used in high-impedance input circuits.

Ø Switching applications in digital circuits.

Ø Used in amplifiers, especially at higher frequencies.

Ø Found in integrated circuits (ICs) due to their high input impedance.

 

Key Differences:

Control: BJT is current-controlled, FET is voltage-controlled.

Current Gain: BJTs have a current gain (β), FETs do not have a direct current gain factor.

Voltage Requirements: BJTs require a small base-emitter voltage to conduct, FETs require a voltage applied to the gate.

Applications: While they have overlapping applications, BJTs are often used in analog amplification, and FETs are often used in high-impedance applications and digital circuits.

Both transistors have their advantages and are chosen based on the specific requirements of a circuit.