@Stefan Schippers: I am performing AC analysis on a single resistor loaded FET (PMOS). As shown in the attached, phase response looks fine but magnitude response is upside down. I havent seen this one before and cant see anything strange in my setup. Attached is my .sch file. Do you see any reason for this? Thanks

Can you try to plot in ngspice ?

Thanks @Stefan Schippers for confirming you see this strange behavior also on your side. I investigated a bit further. Results are sumarised in the attached which show results are expected for L = 1um but as you increase L they get more and more unexpected (for both NMOS / PMOS). @Tim Edwards: This looks to me like a modelling issue for longer channel devices. What do you think and has this been reported before as I would be surprised if it hasnt been observed in the past. Thanks.

@Diarmuid Collins I have run the sims with ngspice-42 and there is no difference at all in the above results. So there are no regressions in current ngspice-44

Thanks @Stefan Schippers. So you see this behaviour with models which are not from SW130? For example with default BSIM models? Attached are the DUTs I used for PMOS / NMOS analysis.

@Diarmuid Collins yes I placed a standard

devices/pmos4.sym

set its model name to

cmosp

and attached the following attribute to it:

device_model="
* PTM 65nm PMOS
.model  cmosp  pmos  level = 54
+version = 4.0          binunit = 1            paramchk= 1            mobmod  = 0
+capmod  = 2            igcmod  = 1            igbmod  = 1            geomod  = 1
+diomod  = 1            rdsmod  = 0            rbodymod= 1            rgatemod= 1
+permod  = 1            acnqsmod= 0            trnqsmod= 0
+tnom    = 27           toxe    = 1.95e-009    toxp    = 1.2e-009     toxm    = 1.95e-009
+dtox    = 0.75e-9      epsrox  = 3.9          wint    = 5e-009       lint    = 5.25e-009
+ll      = 0            wl      = 0            lln     = 1            wln     = 1
+lw      = 0            ww      = 0            lwn     = 1            wwn     = 1
+lwl     = 0            wwl     = 0            xpart   = 0            toxref  = 1.95e-009
+xl      = -30e-9
+vth0    = -0.365       k1      = 0.4          k2      = -0.01        k3      = 0
+k3b     = 0            w0      = 2.5e-006     dvt0    = 1            dvt1    = 2
+dvt2    = -0.032       dvt0w   = 0            dvt1w   = 0            dvt2w   = 0
+dsub    = 0.1          minv    = 0.05         voffl   = 0            dvtp0   = 1e-009
+dvtp1   = 0.05         lpe0    = 0            lpeb    = 0            xj      = 1.96e-008
+ngate   = 2e+020       ndep    = 1.87e+018    nsd     = 2e+020       phin    = 0
+cdsc    = 0.000        cdscb   = 0            cdscd   = 0            cit     = 0
+voff    = -0.126       nfactor = 1.9          eta0    = 0.0058       etab    = 0
+vfb     = 0.55         u0      = 0.00574      ua      = 2.0e-009     ub      = 0.5e-018
+uc      = 0            vsat    = 70000        a0      = 1.0          ags     = 1e-020
+a1      = 0            a2      = 1            b0      = -1e-020      b1      = 0
+keta    = -0.047       dwg     = 0            dwb     = 0            pclm    = 0.12
+pdiblc1 = 0.001        pdiblc2 = 0.001        pdiblcb = 3.4e-008     drout   = 0.56
+pvag    = 1e-020       delta   = 0.01         pscbe1  = 8.14e+008    pscbe2  = 9.58e-007
+fprout  = 0.2          pdits   = 0.08         pditsd  = 0.23         pditsl  = 2.3e+006
+rsh     = 5            rdsw    = 165          rsw     = 85           rdw     = 85
+rdswmin = 0            rdwmin  = 0            rswmin  = 0            prwg    = 3.22e-008
+prwb    = 6.8e-011     wr      = 1            alpha0  = 0.074        alpha1  = 0.005
+beta0   = 30           agidl   = 0.0002       bgidl   = 2.1e+009     cgidl   = 0.0002
+egidl   = 0.8
+aigbacc = 0.012        bigbacc = 0.0028       cigbacc = 0.002
+nigbacc = 1            aigbinv = 0.014        bigbinv = 0.004        cigbinv = 0.004
+eigbinv = 1.1          nigbinv = 3            aigc    = 0.69         bigc    = 0.0012
+cigc    = 0.0008       aigsd   = 0.0087       bigsd   = 0.0012       cigsd   = 0.0008
+nigc    = 1            poxedge = 1            pigcd   = 1            ntox    = 1
+xrcrg1  = 12           xrcrg2  = 5
+cgso    = 1.5e-010     cgdo    = 1.5e-010     cgbo    = 2.56e-011    cgdl    = 2.653e-10
+cgsl    = 2.653e-10    ckappas = 0.03         ckappad = 0.03         acde    = 1
+moin    = 15           noff    = 0.9          voffcv  = 0.02
+kt1     = -0.11        kt1l    = 0            kt2     = 0.022        ute     = -1.5
+ua1     = 4.31e-009    ub1     = 7.61e-018    uc1     = -5.6e-011    prt     = 0
+at      = 33000
+fnoimod = 1            tnoimod = 0
+jss     = 0.0001       jsws    = 1e-011       jswgs   = 1e-010       njs     = 1
+ijthsfwd= 0.01         ijthsrev= 0.001        bvs     = 10           xjbvs   = 1
+jsd     = 0.0001       jswd    = 1e-011       jswgd   = 1e-010       njd     = 1
+ijthdfwd= 0.01         ijthdrev= 0.001        bvd     = 10           xjbvd   = 1
+pbs     = 1            cjs     = 0.0005       mjs     = 0.5          pbsws   = 1
+cjsws   = 5e-010       mjsws   = 0.33         pbswgs  = 1            cjswgs  = 3e-010
+mjswgs  = 0.33         pbd     = 1            cjd     = 0.0005       mjd     = 0.5
+pbswd   = 1            cjswd   = 5e-010       mjswd   = 0.33         pbswgd  = 1
+cjswgd  = 5e-010       mjswgd  = 0.33         tpb     = 0.005        tcj     = 0.001
+tpbsw   = 0.005        tcjsw   = 0.001        tpbswg  = 0.005        tcjswg  = 0.001
+xtis    = 3            xtid    = 3
+dmcg    = 0e-006       dmci    = 0e-006       dmdg    = 0e-006       dmcgt   = 0e-007
+dwj     = 0.0e-008     xgw     = 0e-007       xgl     = 0e-008
+rshg    = 0.4          gbmin   = 1e-010       rbpb    = 5            rbpd    = 15
+rbps    = 15           rbdb    = 15           rbsb    = 15           ngcon   = 1
"

I think these models come from https://mec.umn.edu/ptm

Thanks @Stefan Schippers. Thinking further about this, it actually makes perfect sense. As shown in the attached "badly" drawn circuit, the output is unloaded and so only sees Cbd to vssa (very small). There is a feedforward path through Cgd which creates the zero (wz = gm/Cgd). As L increases, wz moves in. For relatively large L, wz < wp to give the increasing magnitude response as reported and attached. This came as a surprise to me as practically speaking the output is always loaded with some sort of cap which keeps wp < wz. In fact if you re-run the sim with the output loaded with 100fF the increasing magnitude response (zero dominated) becomes decreasing since wp < wz (see attached). I see similiar behaviour in the commercial PDK I am working on at present. @Tim Edwards please ignore my modelling concern, all seems to be in order (im clearly overly paranoid about modelling and tend to suspect it when I see an unexpected result!) In fact, if nothing else, this little exercise was good to give high confidence w.r.t accurate modelling of device parasitics such as Cgd / Cbd.