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
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    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
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    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

    Expert

    Mar 14, 2008
    13,013
    3,233
    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
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    Well i understand that all conductors have resistance to current flow....
     
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    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
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    Right...a matter of "context"...makes sense i suppose.
    Thank you so much for the input.
     
  7. Papabravo

    Expert

    Feb 24, 2006
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    1,790
    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

    Moderator

    Oct 2, 2009
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    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.

    http://en.wikipedia.org/wiki/Depletion_and_enhancement_modes

    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.

    http://en.wikipedia.org/wiki/JFET
     
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