In the above picture why it is written "negative voltage of -1 V has been applied between the gate and source terminals for a low level of VDS".My question is what happens if we use high level of VDS??
This line is not clear to me?

 

My question is wht happend if we use high level of V[sub]DS[/sub]??

Nothing. Maximum drain current for an N channel JFET is with 0V gate bias. Positive gate bias won't increase current.

Contrast that with a depletion mode N channel MOSFET. You can increase drain current with positive bias and decrease with negative.

 

For a given Vgs, the value of Vds at which pinch-off occurs, I call Vdsp, it's Vdsp=Vgs-Vp. And that's the parabola shaped curve, locus of Vds pinchoff voltages, for given Vgs.

So if using an external voltage source or resistors, you can set where your Vds DC bias will be, and choose your current, as a percentage of Idss, the saturation current, which is always in the datasheet. Go over Vdsp for a set Vgs, and your max current levels out regardless of Vds.
http://web.cecs.pdx.edu/~jmorris/ece321/ECE321 Winter 2008/J & B Added Textbook Material/Section 4.12 Insert p194.pdf
I need to start learning semi-conductor physics on the electron/hole level.

 

#1 what is the part number on that data sheet? is it programmable? let's be real: there is no "jfet" there are parts and they don't all operate the same - they are not in general and are not compatible.

#2 test curves are made by published test circuits. they are telling you that using their circuit (which is not seen above) the curve will certainly be produced (with their error, if it is 5% for a jfet your in big trouble!)

#3 "a datasheet" said the resistance on Rds = (delta Vds)/(delta Id). now: if you are conducting from S to D (which is the standard application), then you many need that info, obviously, "the most efficient". Biasing potential between pins of any and all 3 transistor is an important concept - they can be + or - biased to each other for certain effects. if you are viewing a product - you want to know their published curve isn't cheated and want to know how they set up the circuit.

A "circuit simulator" should have something like at least 15 intrinsic properties for a transistor so it can simulate the action (voltage and current reaction on all three leads) based upon the environment (the push or pull on all three leads, and the temperature, and so on). A data sheet is more meant to describe properties of a particular product to those who are familiar (ie, have a circuit simulator and can check out every alternative themselves, or have a book describing every alternative - but let me say 15! (15 factorial) is quite a large number of curves and will fit in no book).