David Hess (davidwhess@gmail.com) on 6/22/11 wrote:
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>
>Now I am confused. How does that explain the negative temperature coefficient
>for MOSFET channel resistance? It can not be because of threshold voltage change
>because it occurs in saturation. It does however become a positive temperature coefficient at very high voltages.
>
>Or does the above only apply to power MOSFETs and not to the submicron ones used in logic processes?
>
>How does this relate to gain increasing in bipolar transistors with increased temperature?
>Secondary breakdown is a very real problem in bipolar power transistors.
>

The temperature coefficient of the channel resistance of MOSFETs is *positive*, not negative.

For power applications this is a major advantage of MOSFETs, because it allows their connection in parallel without the danger that the current will pass mostly through a single device.

For logic circuits this leads to the familiar behavior that they are faster at low temperatures, where the channel resistance is small, so the RC products are also small, thus the switching times become smaller.

Nevertheless, there are certain operating points (e.g. close to breakdown or in subthreshold mode) where the channel resistance may *seem* to have a negative temperature coefficient. However, in those cases, what you see is not the current that corresponds to the drain-source voltage divided by the channel resistance, but a current whose largest part is due to other effects, e.g. due to the parasitic bipolar transistor that exists in parallel with any MOSFET. Whenever the leakage current through the parasitic bipolar transistor is not negligible compared to the drain current, you will see a negative temperature coefficient.