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A Mosfet Primer — Parallax Forums

A Mosfet Primer

william chanwilliam chan Posts: 1,326
edited 2008-04-13 15:58 in General Discussion
Today's question for some mosfet experts,

1. When should you use a P-Type mosfet instead of the cheaper N-Type mosfet?
2. Why are most mosfets enhancement types?
3. Why are TO-92 mosfets so difficult to find?
4. Why mosfet datasheets always quote Vgs figures and not Vgd figures?
5. Why can't we we swap the Drain and Source terminals?

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Comments

  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2008-04-09 17:20
    william chan,

    This almost sounds like a school assignment.....



    1. When should you use a P-Type mosfet instead of the cheaper N-Type mosfet?
    ·· This is design specific


    2. Why are most mosfets enhancement types?
    ·· The physical construction of a depletion MOSFET is identical to the enhancement MOSFET, with one exception.
    ·· The conduction channel is physically implanted (rather than induced)!
    ·· Thus, for a depletion MOSFET transistor, the channel conducts even if vGS=0
    ·· Enhancement types are used so that a voltage threshold must be reached before the MOSFET transistor will conduct.


    3. Why are TO-92 mosfets so difficult to find?
    ·· I'm not sure that they are.


    4. Why mosfet datasheets always quote Vgs figures and not Vgd figures?
    ·· See the explanation in question #5 as to why, but basically the controlling voltage of the transistor is between Vgs... not Vgd


    5. Why can't we swap the Drain and Source terminals?
    ·· In the actual MOSFET silicon, the Source and Drain can be swapped, but because it's not a good idea to leave the substrate of the MOSFET floating (<-- can cause latch-up) it is often tied to the Source.· Doing so actually defines what the Source and Drain are.· With a MOSFET transistor, the Source and Drain form a reverse biased diode to the Substrate across a PN junction region.· When you tie the Source to the Substrate you also short out one of these PN junction diodes.· The remaining "active" diode between the Drain and substrate is what prevents you from "swapping" the Drain and Source terminals.· When you do swap the terminals, the once reversed biased PN junction diode is now forward biased and allowed to conduct.· For a transistor specifically designed to handle back-EMF protection,·an aditional (large) PN juction area is added to the design.



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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • LeonLeon Posts: 7,620
    edited 2008-04-09 20:48
    There are several TO-92 MOSFETs, the 2N7000 and BS170 spring to mind.

    Leon

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  • william chanwilliam chan Posts: 1,326
    edited 2008-04-10 00:36
    For question 1,

    I would think that if you need to drain current from a load (control the negative supply to the load),
    you should use a N-Type mosfet.
    If you need to source current into a load (control the positive supply to the load), you need a P-Type mosfet.

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  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2008-04-10 01:15
    william chan,

    That's why I said that it would be design specific.· Depending on what your requirements were and what type of application you were doing, would indicate when and where you would use a specific kind of transistor.

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2008-04-10 02:34
    I think that the Source and Drain of a J-FET (MPF102, &c.) can be swapped.

    Here are links to a TO92, P-channel logic-level MOSFET --
    http://www.bgmicro.com/index.asp?PageAction=VIEWPROD&ProdID=11725
    http://docs.bgmicro.com/pdf/bss92.pdf

    I buy from their on-line store often; they accept PayPal, too.

    Also, MOSFETs have that·zener diode (back-biased) across the Drain and Source, reversing that would forward-bias that diode.
  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2008-04-10 03:47
    PJ Allen,
    ·
    "I think that the Source and Drain of a J-FET (MPF102, &c.) can be swapped" - Right, but that is a different device than a MOSFET
    ·
    J-FET's are constructed differently and their function is opposite of a MOSFET... i.e. when you apply power to the gate·of a J-FET they
    turn OFF and tend to stay OFF because you have effectively charged a capacitor (<-- reverse bias·PN junction)... that's why you need
    a resistor from the Gate to Source/Drain to keep the·"capacitor" charged or discharged.
    ·
    ·
    The construction of an NMOS MOSFET has a P+ substrate, each Source and Drain are N+, so you form·two·PN junctions or diodes.· One at the Source, and one at the Drain.· The·Gate is capacitive and separated by an oxide layer (insulator/dielectric).· As I mentioned earlier, the Source
    terminal is connected to the Substrate effectively shorting out the PN diode that is formed so the only "diode effects" come from the PN junction located at the Drain.
    ·
    A PMOS MOSFET is constructed the same way, except an additional layer is required (an N+Well) that the entire device sits in.· The Source and Drain are then·P+, forming a similar PN junction as described above.··
    ·
    http://en.wikipedia.org/wiki/MOSFET
    ·
    ·
    The construction of a p-channel JFET has a P+ substrate, each Source and Drain are connected to the Substrate.· Because the substrate is·a poor conductor·(<-- semi-conductor), you basically have a diffusion resistor.· The Gate is connected to an N+ region in-between the Source and Drain contacts.· Since this connection forms a reverse-biased·PN junction diode, under normal conditions it functions as a capacitor.· When the capacitor is "charged" it·induces·a depletion region that "pinches off" the flow of electrons between the Source and Drain contacts.
    ·
    An n-channel JFET works the same way except that the substrate (or N+Well)·is N+, and the Gate is P+
    ·
    http://en.wikipedia.org/wiki/JFET
    ·

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.

    Post Edited (Beau Schwabe (Parallax)) : 4/10/2008 3:53:43 AM GMT
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2008-04-10 12:18
    Yep, just thought I'd make mention of the J-FET (in case that was·why "w" was asking about the possibly in re. MOSFETs.)

    I'd love to find a stash of·RCA dual-gate FETs (in boxes.)
  • LeonLeon Posts: 7,620
    edited 2008-04-10 14:04
    Similar (but much better) devices to the old 40673 are still made, like the BF981. They are all surface-mount, though.

    Leon

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  • william chanwilliam chan Posts: 1,326
    edited 2008-04-13 00:20
    I think the most confusing thing about mosfets must be the P-Type mosfets,
    where the Source-Terminal is actually draining the current and the Drain-Terminal is actually sourcing the current.
    Why did they label things wrongly?

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  • jmalaysiajmalaysia Posts: 97
    edited 2008-04-13 00:34
    Real answer to #2)

    Depletion mosfets require a constant gate source in an 'off' position, thus wasting power, which is generally undesireable.
  • CCraigCCraig Posts: 163
    edited 2008-04-13 15:58
    Unless your working with AC (RF), then it's mostly desirable.
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