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I’m struggling to understand why a negative voltage applied to the bulk contact of an n-type MOSFET increases the threshold voltage. I can see why it happens due to the equations, but I can’t wrap my head around how it actually works.

I have 2 questions:

  1. Applying a negative voltage to the bulk makes the potential difference (and thus the field?) between the gate and the bulk greater, which should make it easier to attract electrons under the gate. Why is this wrong?

  2. Intuitively, the negative voltage at the bulk terminal repels electrons, pushing them towards the gate. Doesn’t this mean that the electron concentration under the gate increases more rapidly, reducing the threshold voltage?

Edit after John's answer:

Thank you, I think I get it now, but what would happen if we instead considered a MOS capacitor, so with no n+ doped regions? There I can't see how the threshold would increase.

Edit, answer to the MOSCAP edit: in the case of a MOS capacitor, since we have no n+ doped regions, I think the negative bulk potential helps the gate accumulate electrons, reducing the threshold Voltage.

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  • $\begingroup$ Would Electrical Engineering be a better home for this question? $\endgroup$
    – Qmechanic ♦
    Commented May 29, 2022 at 12:45
  • $\begingroup$ @Qmechanic before my question was edited I mentioned that I asked in the EE site as well, but the question remained unanswered $\endgroup$
    – ale_zec
    Commented May 29, 2022 at 12:46
  • $\begingroup$ Crossposted from electronics.stackexchange.com/q/621390/52589 $\endgroup$
    – Qmechanic ♦
    Commented May 29, 2022 at 12:47
  • $\begingroup$ @Qmechanic thanks for adding that, didn't know I needed to mention it $\endgroup$
    – ale_zec
    Commented May 29, 2022 at 12:49
  • $\begingroup$ What's the "threshold" of a MOS capacitor? $\endgroup$
    – John Doty
    Commented May 29, 2022 at 12:55

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The electrons are "squeezed" into the space bounded by the gate and the body. In the common enhancement-mode MOSFET, the body has a built-in negative potential which blocks electrons in the source from entering the channel. Positive potential on the gate can counteract this, allowing electrons to flow. But negative potential on the body can, in turn, increase the potential barrier, blocking the flow.

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  • $\begingroup$ Thank you. So applying a negative body potential is like reverse biasing the source-body pnj. The 2 effects I mentioned aren't exactly wrong then, maybe just not relevant since the channel is 'built' using electrons from the source, right? $\endgroup$
    – ale_zec
    Commented May 29, 2022 at 12:39
  • $\begingroup$ Yep. Note that you can make a depletion mode MOSFET by (lightly) doping the channel to make it N-type. Then, electrons are "naturally" present in the channel, and current flows at zero gate bias. If you connect the MOS gate to source, and use the body as the gate, you have a JFET. $\endgroup$
    – John Doty
    Commented May 29, 2022 at 12:52
  • $\begingroup$ Thank you John, I get it now, super helpful. Could you address the edit about the MOS capacitor if you're able? $\endgroup$
    – ale_zec
    Commented May 29, 2022 at 12:53

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