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MOSFETS (Difference between inversion and enhancement mode)

Discussion in 'General Electronics Discussion' started by Thyrex, Dec 21, 2010.

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  1. Thyrex

    Thyrex

    5
    0
    Dec 21, 2010
    Hi,

    I was just wondering if anyone could help by clarifying a few points on MOSFETS please.

    I appreciate that they can operate in 5 modes:
    Flatband, Accumulation, Depletion, Inversion, Strong Inversion

    The classic 'turned on for no applied gate voltage' "depletion mode MOSFET" I assume, is simply a MOSFET operating in depletion mode.

    However, the two commonly used terms to describe MOSFETS are "delpetion mode" and "enhancement mode" and I was wondering whether the enhancement mode MOSFET was constructed differently, or if it simply referred to one of the 5 modes of operation.

    I suspect there is a simple answer to this but the more I try and research it, the more confused with terminology I get - so any pointers greatly appreciated!

    Thanks
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,271
    2,718
    Jan 21, 2010
    Enhancement vs depletion mode tell you what happens at Vgs = 0. In reality, they are not 2 different things, the gate threshold voltage can be pretty much anything the designer wants. Logic level mosfets have it closer to 0 (as do devices designed to operate at very low voltages.

    Note that jfets are always depletion mode devices because the diode junction at the gate can't be driven in the "enhancement" direction more than 0.6V (and thus an enhancement mode device would be useless)

    One complicating factor is that of the 4 types of mosfet, D enhancement, N depletion, P enhancement, and P depletion, only 3 are possible (you can't have a P channel depletion mode mosfet). Depletion mode mosfets are pretty rare except for some dual gate devices used for RF.

    But the terms flatband, accumulation, inversion, and depletion actually refer to the gate charge and how the channel is affected.
     
  3. Thyrex

    Thyrex

    5
    0
    Dec 21, 2010
    Thanks, so the a single device can be an enhancement mode or depletion mode FET.

    In terms of constructing one from a simple MOS capacitor - assume I had one fabricated on n-type substrate - am I right to say that this could be used to make a P-Channel MOSFET (as you can't have an n channel forming in n substrate) and by your above argument, this must be a P Channel Enhancement Mode MOSFET (as P depletion mode doesn't exist)?

    Many Thanks.
     
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