Junction field-effect transistors (JFET)

Identify each type of JFET (whether it is N-channel or P-channel), label the terminals, and determine whether the JFET in each of these circuits will be turned on or off:

Identify each type of JFET (whether it is N-channel or P-channel), label the terminals, and determine whether the JFET in each of these circuits will be turned on or off:

Additionally, identify which of these four circuits places unnecessary stress on the transistor. There is one circuit among these four where the transistor is operated in a state that might lead to premature failure.

When a reverse-bias voltage is applied between the gate and channel of a JFET, the depletion region within expands. The greater the reverse-bias voltage, the wider the depletion region becomes. With enough applied V_GS, this expansion will cut off the JFET’s channel, preventing drain-source current:

Something not immediately apparent about this effect is that the formation of a wide depletion region necessary for cut-off of a field-effect transistor is also affected by the drain-to-source voltage drop (V_DS).

If we connect a gate-to-source voltage (V_GS) large enough to force the transistor into cutoff mode, the JFET channel will act as a huge resistance. If we look carefully at the voltages measured with reference to ground, we will see that the width of the depletion region must vary within the JFET’s channel. Sketch this varying width, given the voltages shown in the illustration:

Something really interesting happens if we reverse the polarity of the 20 volt source: the relationship between drain and source changes, and so does the depletion region profile. Sketch the new varying width of the depletion region with the 20 volt source reversed, and comment on what you see: