(a) Operation of JFET

Working of Junction Field Effect Transistor

 JFET is a voltage controlled device, this is because the drain current ID  is dependent on the value of gate to source voltage (VGS). Unlike the bipolar junction transistor (BJT), it is unipolar and hence operation depends only on the flow of majority carriers. 

1. UNBIASED JFET

A positive supply voltage VDS is connected between the drain and source terminals. In the n-type region, there are several electrons that are ready for conduction. Due to the positive supply at drain to source terminal, the electrons start flowing from source to drain. This constitutes the drain  current ID.These electrons pass through a narrow path between the two p-type layers, which is known as "channel" 

2. BIASED JFET

A positive supply is given between drain and source terminals and a negative supply between gate and source terminals in order to bias the JFET and understand it's operation.

(a) Operation of n-channel JFET when Vgs=0

When voltage between gate and source is zero, the positive supply (VDS) will result in the flow of current ID from source to drain as we observed in the unbiased condition. As the n-type material has a finite resistance, there will be a voltage drop across the channel. The depletion region of the reverse biased gate to source junction will slightly penetrate into the n-type region. The size of this depletion region depends completely on the value of reverse voltage VGS. Maximum drain current flows through source to drain when Vgs=0, thus this current is called as 'source saturation current' (Idss)

(b) Operation of n-channel JFET for a small negative Vgs

Due to the reverse voltage Vgs, the depletion region penetrates further into the n-type region and the channel width reduces further. The reduced channel width will reduce the drain current ID. Therefore, the drain current reduces with increase in Vgs. 

(c) Operation of n-channel JFET for a larger negative Vgs

If this reverse voltage is increased further, a point is reached where the two depletion regions touch each other resulting in zero drain current. The channel width is zero here. The gate to source voltage at which the drain current becomes zero is known as cut-off voltage Vgs(off). 

Pinch-off voltage: It is the value of drain to source voltage (VDS) beyond which the drain current attains a constant value. 

DRAIN CHARACTERISTICS OF JFET