Re: My Battery Monitor

[Victor Tikhonov <vtikhono@lsil.com>]

That exactly what I stated in my reply - for power fets multiple drains
and one source
are used, so they cannot be swapped. For small signal transistors, if
you get
technical, n+ doped regions (except in IC) have the same profile and are
at the
opposite ends of p- substrate, so electrically are identical.

For enhanced n-ch transistor simply the most positive end becomes a
drain.
P substrate usually connected internally to one n+ region, thus forming
p-n junction
between it and another n+ region (so here is your diode). So this first
n+ region
then can be used only as source. Often though substrate has own terminal

(I'm sure everyone have seen 4 pin fets), and in this case there are no
diodes formed.

Sorry if getting too technical is off general discussion practice.
I'm in listening mode now...

Victor

BTW, I do failure analysis of ICs for living...


>
> This is news to me.
> We always used the 'intrinsic diode' or body drain diode in Power FETs
> for flyback current in an H bridge control circuit (as in a shunt
> motor control).
> The old FETS had a crappy diode so they were typically not used.
> The Drain and source regions are at opposite ends of the silicon.
> The gate is closest to the source and when positive bias is applied an
> inversion layer is formed at the surface of the P-base region.  This
> forms a channel between the source region and the drift region through
> which current can flow.
> If you look at the structure of any MOSFET (JFET, HexFET, TrenchFET)
> the drain and source are distinct structures in the design and cannot
> be swapped.
> So before you build your own motor control, I don't suggest you swap
> them!!!!!
> Rod Hower
>