Explain the construction of n channel and p channel FET. Differentiate FET with BJT.

Construction of n-channel and p-channel FET

Field-effect transistors (FETs) are unipolar transistors, meaning they rely on the majority carriers in a semiconductor to conduct current. In contrast, bipolar junction transistors (BJTs) rely on both majority and minority carriers to conduct current.

n-channel FET

An n-channel FET is constructed on a p-type substrate. The source and drain terminals are connected to two n-type regions, and the gate terminal is connected to a metal electrode that is located between the source and drain regions.

When a positive voltage is applied to the gate terminal, electrons are attracted to the gate electrode from the n-type source and drain regions. This creates a channel of electrons between the source and drain regions, allowing current to flow between the terminals.

p-channel FET

A p-channel FET is constructed on an n-type substrate. The source and drain terminals are connected to two p-type regions, and the gate terminal is connected to a metal electrode that is located between the source and drain regions.

When a negative voltage is applied to the gate terminal, holes are attracted to the gate electrode from the p-type source and drain regions. This creates a channel of holes between the source and drain regions, allowing current to flow between the terminals.

Differences between FETs and BJTs

Characteristic	FET	BJT
Majority carriers	Yes	Both majority and minority carriers
Type of channel	n-channel or p-channel	npn or pnp
Gate terminal	Yes	No
Base terminal	No	Yes
Collector terminal	No	Yes
Emitter terminal	No	Yes
Current flow	Controlled by the gate voltage	Controlled by the base current
Input impedance	High	Low
Output impedance	Low	High
Gain	Voltage-controlled	Current-controlled
Applications	Amplifiers, switches, memory devices	Amplifiers, switches, oscillators

Advantages of FETs over BJTs

• FETs have a higher input impedance than BJTs, which makes them more suitable for use in high-impedance circuits.
• FETs have a lower output impedance than BJTs, which makes them more suitable for use in low-impedance circuits.
• FETs have a higher gain than BJTs, which makes them more suitable for use in amplifiers.
• FETs are less susceptible to noise than BJTs, which makes them more suitable for use in low-noise applications.

Disadvantages of FETs over BJTs

• FETs are more expensive to manufacture than BJTs.
• FETs are more susceptible to damage from electrostatic discharge (ESD) than BJTs.
• FETs have a lower breakdown voltage than BJTs.