Hi folks, I was using HSPICE today and did a .OP analysis, which gives a bunch of small-signal parameters. Are there cases where this small-signal parameters aren't going to make sense? I'm basically looking for parameters such as un, Cox, Vt, and lambda, to use for rough hand analysis. I have a modern short-channel device, and the parameters .OP gives don't seem usable for hand analysis.
The goal here is to take the parameters given by .OP, and use them for hand analysis.
Here is an example from the web of what kind of thing I should get:
subckt element 0:m1 model 0:nch id 252.5184u ibs 0. ibd -24.7482f vgs 1.4400 vds 2.4748 vbs 0. vth 700.0000m vdsat 740.0000m beta 922.2733u gam eff 500.0000m gm 682.4823u gds 2.4642u gmb 220.2702u cdtot 5.0000f cgtot 23.9418f cstot 18.5418f cbtot 4.000e-16 cgs 18.5418f cgd 5.0000f
For this web example, I can fill in the classic sat eqn. ids=k*W/(2*L)*(Vgs-Vt)^2 and get a close result for ids. Now in my case, with the short channel device, the parameters are not giving anything close for ids to what HSPICE gave.
Above Vdsat=Vgs-Vt=1.44-0.7=0.740. My MOSFET results don't even make sense...Vgs=0.9, Vth=0.6, but the tool is giving a Vdsat much lower than
0.3! I don't understand. :(
One book I have suggests extracting these from I-V curves, which I can do... I just want to understand why the small signal parameters HSPICE is giving don't work with the hand-analysis equations.
I'd be happy to explain this in more detail, but if you've done a .OP analysis to get small signal MOSFET parameters I think this should be a good start.
P.S. I took BETA to be un*Cox*W/L which seemed to work in the above example. Is this right?