That transistor has 0.7nC of gate charge that has to be switched. To transfer that amount of charge in 10ns requires 70mA of drive current.
Hmm. So there doesn't seem much point in using the MOSFET if I need 70mA to drive it so I can sink 100mA to light up my LED. I guess I can understand that the 100mA could be continous, were the 70mA would only be needed for for the 10ns during switching.
I looked at a few MOSFET driver chips, but I didn't find a fast one that could run on a 3.3v supply, which probably makes sense given the properties of typical MOSFETs.
So I'm thinking now of just using a bunch of digital buffer gates hooked up parallel, each with their own current limiting resistor on it. The
74AC125 apparently can be run at 10 MHz, and by using all four gate I should be able to get my 100 mA. And I can power it with 3.3v. The data sheet does not specify the rise and fall times, just the propagation delay.
Depends a lot on how far the signal has to go. Chances are you will have to add a high frequency amp to the detector (see Electronic Design Welcome for an example).
That was an excellent link! Thanks very much. The part they use in that circuit is available from Digikey, and is in a reasonable package too. The only downside is that I can't run it with a +3.3v supply. So I looked around on the web, and there are lots of articles on photodiode amplifiers, although the other examples aren't designed for 10MHz. Then I saw there are several entire books on photodiode amplifiers. (sigh) What I'm really interested in is doing the FPGA code, not the analog electronics. Oh well.
Thanks again for your help, I think you've improved my chances for success dramatically.
-Kevin