Differentiate BJT and FET.

BJT (Bipolar Junction Transistor)

• Structure: A BJT is a three-terminal device consisting of two PN junctions. The three terminals are the emitter, base, and collector.
• Operation: BJTs operate by the flow of majority and minority carriers across the PN junctions. When a forward bias is applied to the base-emitter junction, majority carriers (electrons) are injected from the emitter into the base region. These electrons then diffuse across the base region and are collected by the collector-base junction, which is reverse-biased.
• Characteristics: BJTs are characterized by their current gain, which is the ratio of collector current to base current. BJTs also have a high input impedance and a low output impedance.

FET (Field-Effect Transistor)

• Structure: A FET is a three-terminal device consisting of a semiconductor channel with a source and drain terminal at each end. The third terminal, the gate, is a metal electrode that is insulated from the channel.
• Operation: FETs operate by the modulation of the channel conductivity by the gate voltage. When a positive voltage is applied to the gate, a depletion region is formed under the gate, which reduces the channel conductivity. This reduces the drain current. When a negative voltage is applied to the gate, the depletion region is reduced, which increases the channel conductivity and the drain current.
• Characteristics: FETs are characterized by their transconductance, which is the ratio of drain current to gate voltage. FETs also have a high input impedance and a low output impedance.

Comparison of BJT and FET

Feature	BJT	FET
Structure	Three-terminal device with two PN junctions	Three-terminal device with a semiconductor channel
Operation	Majority and minority carrier flow across PN junctions	Modulation of channel conductivity by gate voltage
Characteristics	High current gain, high input impedance, low output impedance	High transconductance, high input impedance, low output impedance

Applications of BJT and FET

BJTs are used in a wide variety of applications, including amplifiers, switches, and oscillators. FETs are also used in a wide variety of applications, including amplifiers, switches, and logic gates.

Conclusion

BJTs and FETs are two important types of transistors with different structures, operating principles, and characteristics. They are used in a wide variety of applications in electronics.