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Power MOSFETs have a very low resistance from source to drain when switched on. But they also have a high gate capacitance. They can swich very quickly if their drive charges their gate capacitance fast enough.

Is it true that the chip is simply thousands of little MOSFETs connected all in parallel?

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A high gate capacitance can be a misleading topic. I don't think that it actually charges like a capacitor, but exhibits the frequency response. I may be wrong. What is true is that the gate capacitance will keep the VGS from developing at a higher frequency. By the way, isn't the transfer characteristic of a MOSFET just nice. I like the way it keeps some predictability to the signal.

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  • 3 months later...

Hi everyone.
Power MOSFETS do consist of many little MOSFETS connected in parallel. These are called "cells". For given die dimensions, companies will try to fit as many cells as possible (they connect in parallel so as to drop the drain - source resistance), while keeping a reasonable gate capacitance. What changes between companies is the cell structure (almost every company has patented their own - VMOS, SIPMOS, HEXFET etc), which will finally determine the gate capacitance as well.
As for how the "gate capacitance" charges, it does so pretty much like a capacitor (the structure metal/polysilicon - SiO2 - Si, IS a capacitor), but does not follow exactly the curve. That's because there are other parasitic capacitances (gate - source, gate - drain, gate - body etc.) which also charge giving a slightly different curve.

You might want to check out here:
for a more descriptive presentation on the topic.

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What do you suppose the gate capacitance amounts to. If they say it's in picofarad range, that is real low. I have seen picofarad capacitors and they seem much larger than the would be gate capacitance. So I wonder if the specification is intended for calculation purposes only. That is the voltage loss and behavior at higher frequencies.

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  • 2 months later...
Guest Yevgenip

I read an article (Also in Scientific American) about Super-Transistors. It talked about super-quick transistors and extremly high-voltage resisotrs. 8)
I'll post the article if someone is interested. :)

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