12.13 : 场效应晶体管（FET）的偏置

A junction field effect transistor is used in electronic circuits to control electrical currents.

JFETs consist of an N-type or P-type silicon bar containing PN junctions on the sides and come in two main variations: N-channel and P-channel.

In the crystal structure of an N-type JFET, a thin layer of N-type material is on a P-type substrate. The gate is formed on top of the N-channel using P-type material. Lead wires are attached at the end of the channel and the gate.

In an N-channel JFET, a negative gate voltage repels the channel electrons, creating a depletion region and reducing channel conductivity.

Conversely, in a P-channel JFET, a positive gate voltage repels the channel holes, resulting in reduced channel conductivity through reverse biasing.

By controlling the gate-source voltage, the width of the depletion region can be adjusted, effectively managing the current flow through the channel.

The AC drain resistance, typically ranging in several hundred ohms, determines the amount of current flowing through the channel. Other key parameters of a JFET include transconductance and amplification factor.