high frequency analog behavior in gpdk045

Hello,

 I try to use the 1.8V nmos in the gpdk045 in a class B (F) PA. For this purpose, the drain current should be approx. the positive half of a sine (obtained by providing a strong (half) sine at the gate, dc-biased at approx. the treshold voltage, and under the assumption that Vds does not enter deeply into the knee region which would reduce Ids).

For W=200 um; L=150 nm; at 1 GHz; Ids is a "clean" positive half sine wave.

But at 15 GHz, it is not at all, Ids becomes a peak with a peak current much lower than the maximum of the half sine wave current at 1GHz; even though the drain source voltage stays high (about 1.9V) so it is not a low Vds that causes the drastic current decrease.

In fact, Vds does not change much from its bias point at all (a sine with an amplitude of 0.1V instead of the approx. 1.8V at 1 GHz).

I did a DC sweep first (with save::oppoint M0 included in a .scs-file) to find the parasitic drain capacitance (to ground); which is strongly voltage dependent (drain/bulk diode) and small at a high drain voltage.

This capacitance becomes in parallel with the load and forms a current divider in parallel with the ideal current source; but even with the maximal Cdd; the current should still be (much more than) about 47% of the maximal drain current and since this network is "linear"; it seems strange that the current waveform could be deformed from a half sine to a small peak...

I can't find an explanation for this behavior; and it seems that with these waveforms, I won't be able to design a "classical" (=non-switching) 15 GHz PA (although I already made a class E PA at 15 GHz in this gpdk).

The waveforms are similar at lower DC drain voltages.

If I substitute the same model parameters into ADS, I do obtain approx. half a sine, and the waveforms agree much more with my  expectations, but I am not sure whether the resulting model in ADS is correct since I had to insert the parameters by hand (automatic importer failed).

I am sorry for mentioning ADS again but I am just not sure where to look for a solution.

Kind regards,

Joris