cweakinv, add model parameter model->VDMOSsubshift

src/spicelib/devices/vdmos/vdmos.c

@@ -83,6 +83,7 @@ IFparm VDMOSmPTable[] = { /* model parameters */
 
  /* weak inversion */
  IOP("subthres",  VDMOS_MOD_SUBTHRES,   IF_REAL, "Current(per volt Vds) to switch from square law to exponential subthreshold conduction"),
+ IOP("subshift",  VDMOS_MOD_SUBSHIFT,   IF_REAL, "Shift of weak inversion plot on the vgs axis"),
 
 /* body diode */
  IOP("bv",    VDMOS_MOD_BV,    IF_REAL,   "Vds breakdown voltage"),

src/spicelib/devices/vdmos/vdmosdefs.h

@@ -332,6 +332,7 @@ typedef struct sVDMOSmodel {       /* model structure for a resistor */
     double VDMOSa;
     double VDMOScgs;
     double VDMOSsubth;
+    double VDMOSsubshift;
     double VDMOSmtr;
 
     /* bulk diode */
@@ -377,6 +378,7 @@ typedef struct sVDMOSmodel {       /* model structure for a resistor */
     unsigned VDMOScgsGiven   :1;
     unsigned VDMOSaGiven   :1;
     unsigned VDMOSsubthGiven   :1;
+    unsigned VDMOSsubshiftGiven   :1;
     unsigned VDMOSmtrGiven   :1;
 
     unsigned VDMOSDbvGiven   :1;
@@ -443,6 +445,7 @@ enum {
     VDMOS_MOD_RB,
     VDMOS_MOD_MTRIODE,
     VDMOS_MOD_SUBTHRES,
+    VDMOS_MOD_SUBSHIFT,
     VDMOS_MOD_BV,
     VDMOS_MOD_IBV,
     VDMOS_MOD_NBV,

src/spicelib/devices/vdmos/vdmosload.c

@@ -15,7 +15,7 @@ VDMOS: 2018 Holger Vogt
 #include "ngspice/suffix.h"
 
 static double
-cweakinv(double n, double vgst, double vds, double lambda, double beta, double vt, double mtr);
+cweakinv(double n, double shift, double vgst, double vds, double lambda, double beta, double vt, double mtr);
 
 
 int
@@ -369,22 +369,22 @@ VDMOSload(GENmodel *inModel, CKTcircuit *ckt)
                  * numerical differentiation for gd and gm with a delta of 2 mV */
                 if (model->VDMOSsubthGiven && (here->VDMOSmode == 1)) {
                     double delta = 0.001;
-                    cdrain = cweakinv(model->VDMOSsubth, vgst, vds, model->VDMOSlambda,
+                    cdrain = cweakinv(model->VDMOSsubth, model->VDMOSsubshift, vgst, vds, model->VDMOSlambda,
                         Beta, vt, model->VDMOSmtr);
                     /* gd */
                     double vds1 = vds + delta;
-                    double cdrp = cweakinv(model->VDMOSsubth, vgst, vds1, model->VDMOSlambda,
+                    double cdrp = cweakinv(model->VDMOSsubth, model->VDMOSsubshift, vgst, vds1, model->VDMOSlambda,
                         Beta, vt, model->VDMOSmtr);
                     vds1 = vds - delta;
-                    double cdrm = cweakinv(model->VDMOSsubth, vgst, vds1, model->VDMOSlambda,
+                    double cdrm = cweakinv(model->VDMOSsubth, model->VDMOSsubshift, vgst, vds1, model->VDMOSlambda,
                         Beta, vt, model->VDMOSmtr);
                     here->VDMOSgds = (cdrp - cdrm) / (2. * delta);
                     /* gm */
                     double vgst1 = vgst + delta;
-                    cdrp = cweakinv(model->VDMOSsubth, vgst1, vds, model->VDMOSlambda,
+                    cdrp = cweakinv(model->VDMOSsubth, model->VDMOSsubshift, vgst1, vds, model->VDMOSlambda,
                         Beta, vt, model->VDMOSmtr);
                     vgst1 = vgst - delta;
-                    cdrm = cweakinv(model->VDMOSsubth, vgst1, vds, model->VDMOSlambda,
+                    cdrm = cweakinv(model->VDMOSsubth, model->VDMOSsubshift, vgst1, vds, model->VDMOSlambda,
                         Beta, vt, model->VDMOSmtr);
                     here->VDMOSgm = (cdrp - cdrm) / (2. * delta);
                     here->VDMOSgmbs = 0.;
@@ -888,14 +888,15 @@ scalef(double nf2, double vgst)
  */
 
 static double
-cweakinv(double n, double vgst, double vds, double lambda, double beta, double vt, double mtr)
+cweakinv(double slope, double shift, double vgst, double vds, double lambda, double beta, double vt, double mtr)
 {
-    double nf2 = 0.1; /* empirical setting of sin 'speed' */
-    double n1 = n + (1 - n) * scalef(nf2, vgst); /* n < n1 < 1 */
+    vgst += shift * (1 - scalef(2, vgst));
+    double n = slope / 2.3 / 0.0256; /* Tsividis, p. 208 */
+    double n1 = n + (1 - n) * scalef(2, vgst); /* n < n1 < 1 */
     double first = log(1 + exp(vgst / (2 * n1 * vt)));
     double second = log(1 + exp((vgst - vds * mtr * n1) / (2 * n1 * vt)));
     double cds =
-        beta * n1 * 2 * vt * vt * (1 + scalef(nf2, vgst) * lambda * vds) *
+        beta * n1 * 2 * vt * vt * (1 + scalef(2, vgst) * lambda * vds) *
         (first * first - second * second);
     return cds;
 }

src/spicelib/devices/vdmos/vdmosmask.c

@@ -51,6 +51,9 @@ VDMOSmAsk(CKTcircuit *ckt, GENmodel *inst, int which, IFvalue *value)
         case VDMOS_MOD_SUBTHRES:
             value->rValue = model->VDMOSsubth;
             return(OK);
+        case VDMOS_MOD_SUBSHIFT:
+            value->rValue = model->VDMOSsubshift;
+            return(OK);
         case VDMOS_MOD_TYPE:
             if (model->VDMOStype > 0)
                 value->sValue = "vdmosn";

src/spicelib/devices/vdmos/vdmosmpar.c

@@ -125,6 +125,10 @@ VDMOSmParam(int param, IFvalue *value, GENmodel *inModel)
             model->VDMOSsubth = value->rValue;
             model->VDMOSsubthGiven = TRUE;
             break;
+        case VDMOS_MOD_SUBSHIFT:
+            model->VDMOSsubshift = value->rValue;
+            model->VDMOSsubshiftGiven = TRUE;
+            break;
         case VDMOS_MOD_BV:
             model->VDMOSDbv = value->rValue;
             model->VDMOSDbvGiven = TRUE;

src/spicelib/devices/vdmos/vdmosset.c

@@ -75,6 +75,9 @@ VDMOSsetup(SMPmatrix *matrix, GENmodel *inModel, CKTcircuit *ckt,
         if (!model->VDMOSsubthGiven) {
             model->VDMOSsubth = 0;
         }
+        if (!model->VDMOSsubshiftGiven) {
+            model->VDMOSsubshift = 0;
+        }
         if (!model->VDMOSmtrGiven) {
             model->VDMOSmtr = 1.;
         }