AVR-Chat

Here are a couple of application notes that show the power MOSFET 
structure, how the diode comes about, and that it's there to stay:

    http://www.vishay.com/docs/70572/70572.pdf
    http://www.vishay.com/docs/71933/71933.pdf
    http://www.irf.com/technical-info/appnotes/an-1084.pdf

These are for currently popular, power MOSFETs which are "enhancement" 
mode devices ... normally off, turned on.

There's also a "depletion" mode device that's normally on, turned off, 
much like a standard JFET device.  These were actually called Insulated 
Gate Field Effect Transistors (IGFET) back in the old days.  I don't 
know that these devices are available except as small signal devices, or 
if they're even available at all any more.  It's been a while since I 
needed to use a depletion type.  There is no Source-Drain body diode in 
this type device and I believe the last time I did, it was because the 
Source-Drain diode was problematic in that application.

Power MOSFETs do make very inexpensive, high current diodes.  Has anyone 
ever experimented with them with respect to current sharing in parallel 
device configurations, as diodes, like we would when using them in 
parallel as MOSFETs?


REB


Graham Davies wrote:
> --- In AVR-Chat@yahoogroups.com, Robert Adsett <subscriptions@...> wrote:
>   
>> The diode is inevitable in
>> the MOSFET structure isn't it?
>>     
>
> No, I don't think so.  Diodes between both source and drain and the substrate are usual, but you can get rid of those too if you don't want them (in the extreme case, using silicon-on-insulator technology).
>
> Most of what I know about MOSFETs is from IC design.  The substrate is generally connected to ground.  N-Channel MOSFETs with their source grounded thus have the diode, but N-Channel MOSFETs with their source connected somewhere else, for example in a NAND structure, don't.  Or, rather, they have the diode but it doesn't go to the source.
>
> I've no idea how this works with individual MOSFETs, which tend to have very complicated structure these days.  I would absolutely not make any assumptions that are not backed up by the manufacturer's data sheet.
>
> Graham.
>
>
>
>
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