jfet, gate at zero volts

Discussion in 'Homework Help' started by john arclight, Apr 18, 2015.

  1. john arclight

    Thread Starter New Member

    Mar 25, 2015
    Im studying jfets and mosfets at the moment. And Im a little puzzled. This is a paragaph that i was reading for an N channel jfet:
    " When no voltage is apllied to the gate, the jfet acts like a resistor. The only limit to channel current is the resistance of the channel, and current flows through the jfet. However, if a negative voltage is applied the gate lead of the jfet, the PN junction is reverse biased. As a result,a depletion region developes in the jfet and opposes the flow of current through the device."

    Wouldn't that clearly indicate that the gate at 0 volts acts like a conductor NOT a resistor?
    Thank you so much for any input
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    All conductors have an intrinsic resistance.
    jFET's are semiconductors, whose conduction resistance can be modified by applying a Gate voltage.

    The FET you describe is a Depletion Mode FET.
  3. crutschow


    Mar 14, 2008
    A conductor is simply a substance that conducts electricity.
    Its resistance can vary from zero for a superconductor to many megohms for a high resistivity material.
  4. john arclight

    Thread Starter New Member

    Mar 25, 2015
    Well i understand that all conductors have resistance to current flow....
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    So it's a matter of semantics and intended use. A good conductor is called a conductor; a bad conductor is called a resistor :).
  6. john arclight

    Thread Starter New Member

    Mar 25, 2015
    Right...a matter of "context"...makes sense i suppose.
    Thank you so much for the input.
  7. Papabravo


    Feb 24, 2006
    This is why you have to look at the datasheet very carefully. In the datasheet will be an explicit statement of the range of on resistance for and N-channel FET with 0 volts on the gate with respect to the source. Depending on your expectations that value could either higher or lower than what you expect.
  8. MrChips


    Oct 2, 2009
    It is all relative. A conductor is a poor resistor. A resistor is a poor conductor. Or you can have something in between.

    Just think of the source-drain channel as a path for charge carrier to flow regardless of whether it is a resistor or conductor.

    Read up on the difference between a depletion-mode and enhancement-mode JFET.


    What you are describing is a depletion-mode JFET where the channel allows charge carrier to flow. When the gate is reversed biased the channel is "pinched off" and hence restricts the current flow.