MOSFET speed difference due to source resistance?

I'm comparing datasheets for two MOSFETS - a Fairchild FCPF20N60 and an Infineon SPA20N60C3. They have similar specs except in switching times where the SPA20N60C3 appears to be hugely faster. Can this be mostly due to the fact that the Fairchild device is specced for Rg of 25 ohms whereas the Infineon product is specced for 3.6 ohms (note the decimal point)? Gate charges and input and output capacitances are similar although the reverse transfer cap of the Fairchild device is about twice that of the other one.

Reply to
Pimpom
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That's just how Fairchild rates things, for some reason. Until you get within range of the internal R_G, it should be roughly proportional to Rg.

Tim

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Seven Transistor Labs 
Electrical Engineering Consultation 
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Reply to
Tim Williams

The external Rg (pulse generator impedance) used for risetime testing is usually specified but varies all over the place. That does make it hard to compare fets. 25 ohms is common, namely a 50 ohm pulse generator with a 50 ohm termination. That's crazy for almost any mosfet, even small ones.

Fet internal Rg is often not specified, but it often dominates fast switching. I've seen similar mosfets where one is spec'd at 3 ohms and the other at 0.3.

SiC looks like the clear choice for fast, high voltage switching, like pockels cell drivers, until you see the internal Rg values, 5 ohms or so.

I get 60 volt, 1 amp, sub-ns edges from SOT23 mosfets that are spec'd for 10 or 20 ns. To go fast, you've just got to blast the gate and try it, then buy the exact same parts.

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John Larkin         Highland Technology, Inc 
picosecond timing   laser drivers and controllers 
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John Larkin

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