MOSFET PSP model usage of NF parameter

[hpretl]

When I do a simple testbench in ngspice simulating just a single LV-NMOS (I assume it is the same for PMOS and HV variants, but I have not checked it), I see that the model parameter rg is not reacting to the number of fingers I set. This is not ideal, as the PSP model can model extrinsic resistances.

From the PSP model doc:

The calculation of rg is:

In the model file of IHP's PDK, the PSP model is instantiated like that:

This means that essentially the nf (number of fingers, here called ng, which is not consistent) parameter is never passed on to the PSP model, instead only the width of a single finger is used in the model, and the multiplier parameter m is then increased by nf as passed on to the model as mult=ng*m. In this way, the intrinsic model does not know about the true nf and m parameters. This is also confirmed by looking at the model parameters in ngspice, which always shows nf=1.

Maybe this can be changed in a future release? It looks like some capabilities of the PSP model are bypassed in this way.

[hpretl]

@bmurmann what do you think? Maybe this also explains some other model things you identified?

[bmurmann]

Potentially, but the issues I wrote about where also present for ng=1.

[KrzysztofHerman]

@hpretl I have followed the $R_G$ implementation and I found the exact lines of code in the file PSP103_module.include, which define it:

1522:  RG_p =  RSHG_i * (`oneThird * W_f / NGCON_i + XGWE) / (NGCON_i * L_slif) + (RINT_i + RVPOLY_i) / (W_f * L_f) + NF_i * RGO;
1709:  RG_i   = `CLIP_LOW(RG_p, 0.0);
3065:  rg  =  RG_i / MULT_i;

Also I found this:

1721: MULT_i  = `CLIP_LOW(MULT * NF_i, 0.0); // Note: NF_i is set to 1 for local model

Assuming that $R_G$ scales with m its value should consider the $N_F$ parameter passed indirectly as mult
Could @miesli or @metroid120 comment on that ?

[miesli]

Hi,

yes the "m" or "ng" or "nf" issue is already well known for the PSP model in this PDK. Check also #25 for this.

Basically, the PDK was extracted and scaled like this in the SG13G2 PDK and somehow they decided that this was the best way to do that. @hpretl you are definitely right, that this does not use the PSP model correctly but without a new extraction with new measurements it is not easy to change since this requires most likely to verify all characteristics again.

Best regards

[dwarning]

We are speaking about psp Version 103.6.0.

3065: rg = RG_i / MULT_i;

The RG_i scaling by MULT_i is used here for OP variable calculation - not for contribution.

For that we have around line 1826:

if (RG_i > 0.0) begin ggate = 1.0 / RG_i;
This value is used for matrix entry and noise analysis. The scaling is done in macro CollapsableR by multiplying with MULT_i.
So in the end - even with our confusing instantiation - RG should be correct scaled by NF because NF is part of the mult instance parameter.
@hpretl : Could you provide an simple test case to check our assumptions.

[hpretl]

My not-very-sophisticated test is this (so maybe I am just interpreting things wrong):

I take a single MOSFET test bench, and look at the model parameters after op using show. Setting ng=1 or ng=2 reports the same Rg, which should not be the case, I think.

Here the netlist:
mosfet_diode_sizing.spice.zip

[dwarning]

Thanks for the test case.
I think the problem is the reporting of the OP variable rg. I put in few strobes into the va code.
For ng=1:
OSDI(debug) n.xm1.nsg13_lv_nmos: RG_p 100.192
OSDI(debug) n.xm1.nsg13_lv_nmos: RG_i 100.192
OSDI(debug) n.xm1.nsg13_lv_nmos: RG_i 100.192 MULT_i 1 rg 100.192
OSDI(debug) n.xm1.nsg13_lv_nmos: RG_i 100.192 lp_rg 100.192

For ng=2:
OSDI(debug) n.xm1.nsg13_lv_nmos: RG_p 200.385
OSDI(debug) n.xm1.nsg13_lv_nmos: RG_i 200.385
OSDI(debug) n.xm1.nsg13_lv_nmos: RG_i 200.385 MULT_i 2 rg 100.192
OSDI(debug) n.xm1.nsg13_lv_nmos: RG_i 200.385 lp_rg 200.385

From code I see that RG_i goes into ggate. ggate will be then multiplied with MULT_i in CollapsableR and brings the correct current into branch.

rg seems buggy maybe it is repaired in newer versions.