12.13 : Biasing of FET

Biasing a Junction Field Effect Transistor (JFET) is crucial for setting operational parameters and ensuring efficient functioning in electronic circuits. JFETs are characterized by using a single carrier type in N-channel or P-channel configurations, where the channel is surrounded by PN junctions. These junctions are central to the device's ability to control current flow.

In an N-channel JFET, the structure consists of N-type material forming the channel on a P-type substrate, with the gate made of P-type material. Biasing this type involves applying a negative voltage to the gate relative to the source. This negative voltage repels electrons in the channel, creating a depletion region that narrows the channel and reduces its conductivity. This manipulation of the channel's width controls the electron flow and, consequently, the current.

Conversely, a P-channel JFET features a P-type semiconductor channel with N-type gate material on either side. In this structure, applying a positive gate voltage to the P-type channel repels the holes, increasing the depletion region and decreasing conductivity. This reverse biasing is critical in controlling the current through the device.

Effective biasing is fundamental for JFETs used as switches or amplifiers. The applied gate-source voltage precisely adjusts the depletion region's width, managing the current flow through the channel. Moreover, the biasing of JFETs also influences characteristics like the AC drain resistance, which typically ranges several hundred ohms, and parameters such as transconductance and amplification factor.

Proper biasing prevents the JFET from operating in undesirable states like saturation or cutoff, optimizing its performance and durability in circuits.