＜Operation of JFETs＞
JFET : Junction Field-Effect Transistor
(1) In the N-channel junction field-effect transistor (Fig. 3-3 (a)), when a voltage is applied between the drain and the source, electrons flow from the source to the drain.
(2) When a reverse bias is applied between the gate and source, the depletion layer expands and suppresses the electron flow in (1). (Narrowing path of electron flow)
(3) If the reverse bias voltage between the gate and source is further increased, the depletion layer blocks the channel and the flow of electrons stops.
As shown above, voltage applied between gate and source controls the condition between drain and source. So FETs are voltage-driven devices.
Note: Direction of current flow is opposite to that of electron flow. The mechanism of widening of the depletion layer is the same as for diode.