I must apologize - what I called the "linear region" I really should
have called "saturation region" or "active mode"  This "partially on"
region makes the FET act like a constant current source/sink. It is in
this region where the relationship between gate-source voltage and
drain current has a positive temperature coefficient, so that the hot
FET in a parallel combination will draw more of the current and get
hotter.

I got mixed up and thought that the three contrasting regions were
cutoff,ohmic, and linear. Ohmic and linear are actually just two words
for the same region. The three regions are cutoff, ohmic, and
saturation (or active). Saturation is a confusing term which I tend to
avoid because it is in some sense the opposite of BJT saturation. In
BJT's, saturation means that the base drive is more than sufficient to
turn the BJT fully on. In FETs, saturation means that the channel is
not inverted over its full length and so the available carriers are
fully in use and changing the drain-source voltage doesn't get you any
more current (hence the channel is "saturated" with current)

Sean


On Wed, Oct 6, 2010 at 8:14 PM, ivp <joecolquitt@clear.net.nz> wrote:
>> Just one caveat to the FETs, that property [re: hotspots] is only
>> guaranteed in the full-on (a.k.a ohmic) region. If the FET is
>> partially on (or in the linear region), then the behavior is usually
>> the opposite and the hotter FET will take more of the current
>
> I thought a FET in the linear region could be characterised as a
> variable resistor, and temperature rise leads to resistance rise which
> leads to current fall, and therefore increase in current through less-
> heated paralleled devices
>
> So, as well as good hard gate drive, paralleled FETs should have
> good thermal bonding ?
>
> Joe
>
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