I read that in transfer characteristics of JFET, Vds is kept constant and Vgs is varied. But when Vp for any device is constant and Vds is also constant then even Vgs should be constant.
Transfer characteristic of JFET
jfet
Related Solutions
Some JFET characteristics can look like this for positive and negative Vds: -
Note that the device can be regarded as a variable resistor up to a limit. It applies when |Vds| is much less than half a volt. A good document from TI (here) explains this: -
When VGS < 0.6 or so, a depletion region is formed between the gate and the channel (which is the conductive region between the drain and source). As VGS becomes more negative, this depletion region widens further into the channel, 'squeezing' the conductive region -- thereby reducing the current that flows.
With positive VGS (up to ~ 0.6 V or so), the depletion region narrows, and the device would run more current.
With VGS even greater than this, the gate-channel junction is forward biased, and a large gate-source current flows. This is generally not a useful mode of operation. Depending on the structure of the device, drain current may change also.
Best Answer
To the left is the transfer characteristic plotting Id against Vgs and to the right is the graph that shows the various conduction regions of the device.
The left hand plot is derived from the right hand plot by taking the points when Vds is at 10V (usually but not always) and translating across to the left hand graph so you get a plot of Id against Vgs.
Pinch off is where the device cross from being ohmic to saturating : -
Maybe you are confusing this voltage with Vgs(off)?