JFETS/MOSFETS and applied potential

[gfa239]

Explain how and why the weak signal (potential) applied to the gate G modifies (or modulates) the strong (current) between the source S and the drain D in the JFET . In particular you must indicate what happens to the depletion layer and the conducting channel when
a. (10) Positive potential is applied to the gate
b. (10) Negative potential is applied to the gate

Hints:
1. This is a p-channel JFET.
2. It may be useful to draw a comparison to the vacuum tube amplifier.


I understand that applying positive potential causes a reduction in the drain current and that the gate voltage (VGG) becomes more positive till all current is cutoff. How does that relate to the vacuum tube amplifier ? And does that mean that the depletion layer decreases in size? I'm not sure i have a good grasp of this concept.

 

[Valkarie]

A: a. When a positive potential is applied to the gate of a p-channel JFET, the electric field created by the gate attracts the majority carriers (electrons) from the source and drain towards the gate. This reduces the number of electrons available for conduction between the source and drain, thus reducing the current between them. As a result, the depletion layer at the source and drain increases in size, blocking the flow of current even further. This effect is similar to that of a vacuum tube amplifier, where a positive voltage applied to the grid reduces the current flowing through the tube. b. When a negative potential is applied to the gate of a p-channel JFET, the electric field created by the gate repels the majority carriers (electrons) away from the gate. This increases the number of electrons available for conduction between the source and drain, thus increasing the current between them. As a result, the depletion layer at the source and drain decreases in size, allowing the flow of current. This effect is similar to that of a vacuum tube amplifier, where a negative voltage applied to the grid increases the current flowing through the tube.