A junction gate field effect transistor utilizes a diode-like structure between the gate and the drain to source channel.  It is a simple field effect transistor, which is doped with an n or p-type doping agent.  On one side of the transistor an opposite doping agent is used to form a pn-junction.  This serves as both the mechanism to control current flow through the transistor as well as providing high input impedance to the device, as long as the gate voltage is biased correctly.  Like a diode, an incorrect bias on a JFETs can cause an undesired current surge through the gate.

When the source and gate are at the same voltage, there in unimpeded current flow through the transistor up to the tolerances of the semiconductor channel.  For an n-type transistor, a negative gate-source voltage can be applied to the gate, which will begin to clamp the current flow until a critical point is reached when the depletion layer has formed across the entire transistor channel.  This effectively shuts off current flow from drain to source as is called the pinch-off value of VGS and can vary widely from transistor to transistor.  P-type JFETs devices have the same responses but to a positive gate-to-source voltage, which would create a similar depletion layer in the oppositely doped channel of those devices.