correct and simplify temp derivatives

src/spicelib/devices/vdmos/vdmosload.c

@@ -297,17 +297,21 @@ VDMOSload(GENmodel *inModel, CKTcircuit *ckt)
             /*  Calculate temperature dependent values for self-heating effect  */
             if (selfheat) {
                 Beta = here->VDMOStTransconductance;
-                dBeta_dT = Beta * model->VDMOSmu * pow(Temp, model->VDMOSmu-1) / pow(model->VDMOStnom, model->VDMOSmu);
+                dBeta_dT = model->VDMOStransconductance * here->VDMOSm 
+                           * model->VDMOSmu * pow(Temp/model->VDMOStnom, model->VDMOSmu) / Temp;
                 rd0T = here->VDMOSdrainResistance;
                 if (model->VDMOStexp0Given)
-                    drd0T_dT = rd0T * model->VDMOStexp0 * pow(Temp, model->VDMOStexp0-1) / pow(model->VDMOStnom, model->VDMOStexp0);
+                    drd0T_dT = model->VDMOSdrainResistance / here->VDMOSm 
+                               * model->VDMOStexp0 * pow(Temp/model->VDMOStnom, model->VDMOStexp0) / Temp;
                 else
-                    drd0T_dT = rd0T * (model->VDMOStrd1 + model->VDMOStrd2*(2*model->VDMOStnom*Temp - model->VDMOStnom*model->VDMOStnom));
+                    drd0T_dT = model->VDMOSdrainResistance / here->VDMOSm 
+                               * (model->VDMOStrd1 + 2 * model->VDMOStrd2 * (Temp - model->VDMOStnom));
                 rd1T = 0.0;
                 drd1T_dT = 0.0;
                 if (model->VDMOSqsGiven) {
                     rd1T = here->VDMOSqsResistance;
-                    drd1T_dT = rd1T * model->VDMOStexp1 * pow(Temp, model->VDMOStexp1-1) / pow(model->VDMOStnom, model->VDMOStexp1);
+                    drd1T_dT = model->VDMOSqsResistance / here->VDMOSm 
+                               * model->VDMOStexp1 * pow(Temp/model->VDMOStnom, model->VDMOStexp1) / Temp;
                 }
             } else {
                 Beta = here->VDMOStTransconductance;