John Larkin wrote...
The fets are pretty much infinitely fast inside, so it's just a matter
of gate drive. We're driving them with a bunch (emphasis bunch) of
fast TinyLogic gates. ...
I assume you're switching one 2n7000 to ground. What's the load, a
50-ohm or other resistive pullup, with no coax cable, etc., just a
low-capacitance scope probe?
Nice looking pulse, huh? It almost looks faked.
A textbook pulse. One of the issues in fast-driving power MOSFETs is
their gate spreading resistance. Depending upon the structure, some
of the FET's area is quickly reached by the gate signal, for fast turn
on and turn off, but the remaining area is "further away," with an RC
delay. This portion won't turn on until a little later in the pulse,
an aspect that doesn't have much effect on the turn-on waveform unless
the FET is running at very high currents and the reduction of Ron with
time is apparent. But at turnoff the "near" portion can be turned off
quickly, just as it was turned on, but some of the "far" portion, which
sadly is in parallel to the output after all, is RC-delayed and turns
off slowly, and the waveform can have a gradual-recovery appearance.
BTW, this is another aspect that spice models fail to handle properly.
Your 2.5ns turnoff is impressive. Does this indicate the 2n7000 parts
you were using don't suffer from the gate-spreading resistance problem?
Which manufacturer's FET were you using? Also, don't you prefer 2n7002
SMT parts rather than 2n7000 TO-92 through-hole for your fast circuits?
The list of 2n7002 manufacturers and the variants they offer is quite
impressive (A, C, D, E, F, K, L, LT1, M, T). And the datasheet output
characteristics plots show even more variation than I saw in the 2n7000
families.
What's your experience with these? For example, Fairchild's 2n7002MTF
type says it has "Improved Inductive Ruggedness," which sounds useful,
although I'm not sure what that means. Better avalanche capability?
This transistor's online datasheet features nearly unreadable plots,
http://www.fairchildsemi.com/pf/2N/2N7002MTF.html but it appears it's
capable of switching 2.5A, more than most other 2n7000 or 2n7002 parts.
They charge more, 7.5 cents rather than 5.6 cents, qty 1000. :>)