JFET

The current source between the Drain and the Source is controlled by the voltage across the Gate and the Source.

The ratio dID/dVGS (change in the Drain Current over the change in the Voltage across the Gate Source) is called the forward transfer admittance, symbol yfs.

To make an N-channel JFET work, the Gate/Source voltage must always be less than the Drain and Source voltages. That means that VGS must be negative. If VGS becomes more negative, ID will decrease. The voltage at which the drain current becomes zero, is called the pinch-off voltage.