Q. How does the value of Idss and Vp change with the change in Vgs

The value of Vp and and Idss can be controlled by adjusting the value of the gate to sorce voltage ie Vgs. This is done by setting Vgs more and mire negative. In other words the gate terminal will be set at lower potential levels when compared to the source. As a result of setting a negative Vgs,the saturation level Idss is reached earlier at a lower level when compared to Vgs = 0 v.The resulting saturation level for Id has been reduced and in fact will continue to decrease as Vgs is made more and more negative. The pinch off voltage also continues to drop in a parabolic manner as we decrease the value of Vgs.