Unused MOSFETs

Discussion in 'General Electronics Chat' started by MuPlusSigma, Mar 21, 2016.

  1. MuPlusSigma

    Thread Starter New Member

    May 22, 2014
    Is there a correct way to connect a spare, unused FET out of a package of multiple unconnected FETs?

    This question was asked before:
    But I'm hoping for more of a consensus.

    I had been thinking that shorting gate to source, and then tying drain to ground, made sense. But then I couldn't think of any problem with just tying all three to ground.

    The linked thread is solid in support of doing something, not leaving the FET unconnected, but there are multiple ideas as to what to do: Connect gate to source by resistor, or tie gate to drain and ground the source. There is no indication that either of these is wrong.

    Are any of these ideas the best? Are any of these ideas bad? Are they all simply sufficient, and that's all they need to be?

    That thread is the first thing on google when I search "unused fets", so maybe we can do a little better.
  2. #12


    Nov 30, 2010
    The last guy in that Thread got things confused. Everybody else said pretty much the same as each other. Short the gate to the source. It's the gate that runs loose if not tied to something. If you want a universal rule to cover all mosfets, j-fets, enhancement mode and depletion mode, N type and P type, connect all 3 terminals together and ground them. No matter what kind of transistor it is, it can't conduct anything if all 3 terminals are at the same voltage.

    There might be an exception if the whole silicon chip was at a voltage which is not zero. Then you could run into voltage break-down problems by grounding an unused transistor. I can't think of how to make a universal rule which includes every possible option for voltage.

    Hang on...consider the lowest voltage pin on the chip to be ground and this works for any voltage.
    Last edited: Mar 22, 2016
    MuPlusSigma likes this.