How to calculate transient frequency from y-parameter in matlab

[tarjina]

Hi All,
I have my DC and small-signal AC analysis from TCAD Sentaurus. Now, I have to plot transit frequency vs. drain current.
I am calculating transient frequency in two ways.
1. ft= gm/(2*pi*cgs)
2. ft= f0/imag(y21/y11); f0=3e8 also used during simulation.

Now, I have two different graphs that completely looks different.


matlab code for method 2 as follows:

[varnames83, data83]=read_plt('Id_n83_ac_des.plt'); %read all data in matrix as well as data names
CDCG83       = data83(:,strcmp(varnames83,'c(d,cg)'));
CCGCG83       = data83(:,strcmp(varnames83,'c(cg,cg)'));
aDCG83       = data83(:,strcmp(varnames83,'a(d,cg)'));
aCGCG83       = data83(:,strcmp(varnames83,'a(cg,cg)'));
CSCG83       = data83(:,strcmp(varnames83,'c(s,cg)'));
aSCG83       = data83(:,strcmp(varnames83,'a(s,cg)'));
y2183         =aDCG83+1i*2*pi*3e8*CDCG83; % formed by the equation given in Sentaurus user guide
y1183         =aCGCG83+1i*2*pi*3e8*CCGCG83;
ftac83        =3e8/imag(y2183./y1183);
p11=plot(IDS83(1:126)*1e9 ,-ftac83/(1e6),'-','LineWidth',3.0);
pl1.Color= [0 0 1];

Can anyone tell me what is going wrong? If I change the

ftac83        =3e8/imag(y2183./y1183);

to

ftac83        =3e8/1i*(y2183./y1183);

I have the following figure:

But the changed code gives me an warning: imaginary values are ignored during plot. So, that should be an incorrect result because ft is calculated from imaginary part of y-parameter.


I am scratching my head here; also the clock is ticking for me. Any help is appreciated, thanks a lot in advance.