Understanding Field Effect Transistors (FETs): Classification, Structure, and Principles

Classification

1. Junction FETs (JFETs)

• Utilize a reverse-biased PN junction between the gate and channel to control current flow.

• Available in N-channel (electron-dominated) and P-channel (hole-dominated) variants.

• Simple structure and robust performance make them suitable for analog circuits and low-power applications .

2. Metal-Oxide-Semiconductor FETs (MOSFETs)

• Enhancement-mode: Requires a threshold voltage to form a conductive channel (normally off).

• Depletion-mode: Conducts at zero gate voltage and can be modulated by positive/negative bias (normally on) .

• Feature an insulated gate (SiO₂ layer) for higher input impedance and lower noise.

• Subclassified into:

Structure

• Source (S): Supplies majority carriers.

• Drain (D): Collects carriers from the channel.

• Gate (G): Controls channel conductivity via an electric field.

Working Principle

• JFETs: Reverse-biasing the gate-channel junction expands the depletion region, narrowing the channel and reducing current .

• MOSFETs:

• Enhancement-mode: A positive gate voltage induces an N-type inversion layer in the P-substrate, creating a channel.

• Depletion-mode: A pre-implanted channel allows current flow at Vgs=0, which decreases with negative bias .