Why does a MOSFET fail on fast signals, even if they're much longer than the turn on + turn off times?

Question

When running at a fast signal, why does the MOSFET below not turn off, even when gate voltage is well below threshold (even negative)? Although the signal is fast (7 MHz, 140 ns period) it is < 1/10th the turn on / turn off time.

For that matter, it doesn't seem to fully turn on either.

The datasheet shows gate threshold voltage to be between 0.8V and 3V. The troughs are well below 0.8V, and the peaks above 3V. Furthermore, increasing amplitude to 4V (so that gate voltage goes from -4V to 4V) didn't substantially change things: we still see R1 current between 0.93mA and 1.26mA, and drain voltage never above 2.4V.

The cause can't be attributed to input capacitance, as the input is an ideal voltage source.

Decreasing the frequency to 1 MHz solves the problem:

But why? The datasheet shows turn off and turn on times of 10 ns, well within the 140 ns period of Vsig. Why does the MOSFET fail for fast signals, even if their period is > 10x turn off and turn on time?

Answer 1

While you may not have drawn a resistor in series with V2, the mosfet model most certainly contains one.

See LTSPICE wiki: https://ltwiki.org/LTspiceHelp/LTspiceHelp/M_MOSFET.htm which in the table contains an R_g parameter as seen in this snippet:

While your model could have a 0 ohm resistance, I posit that it doesn't because of the behavior you are showcasing.

I suggest you inspect the model, find the gate resistance, set it to zero, then place a discrete gate resistor, and observe gate voltage again. Then you will see the effect of the capacitances.

Or, just observe the current out of V2 and you should see the unmistakable current caused by the input capacitance and, more notably, the miller capacitance.

For instance, I grabbed a random model and put a 10MHz sinewave through it, just as you did.

See my results with the model, as supplied, and one where I over-write the gate resistance to 0 using a .model statement: First: mosfet model as supplied. Second: mosfet model with gate resistance set to zero.