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This is the LTSpice model of the Fairchild 2N7002 NMOS transistor.

.model 2N7002 VDMOS(Rg=3 Vto=1.6 Rd=0 Rs=.75 Rb=.14 Kp=.17 
 mtriode=1.25 Cgdmax=80pCgdmin=12p Cgs=50p Cjo=50p Is=.04p 
 ksubthres=.1 mfg=Fairchild Vds=60 Ron=2 Qg=1.5n)

The equation of a saturated N-MOSFET is

enter image description here

Reading the SPICE .model, I had thought that for this Fairchild 2N7002, the parameters were Vth = 1.6 V and k' = 170 mA/V².

Then, using LTSpice, I've obtained the ID vs. VDS characteristic of the Fairchild 2N7002:

enter image description here

Acording to the equation, for VGS = 3V, I should get a current of ID = 170 / 2 x (3 - 1.6)² = 166 mA, but in the plot I get 140 mA.

Acording to the equation, for VGS = 4V, I should get a current of ID = 170 / 2 x (4 - 1.6)² = 489 mA, but in the plot I get 378 mA

Etc.

Moreover, if you treat Vth and k' as unknowns, you can compute them from the plot. For example, using the curves VGS = 4 V and VGS = 3 V, solving the system of equations 140 mA = k' / 2 × (3-Vth)² and 378 mA = k' / 2 × (4-Vth)². Then you get, k' = 0.1158 A/V² and Vth = 1.4452 V.

But if you use other curves, you get differente results. For example, using the curves VGS = 6 V and VGS = 5 V, solving the system of equations 700.36 mA = k' / 2 × (5-Vth)² and 1.088 A = k' / 2 × (6-Vth)². Then you get, k' = 0.08511 A/V² and Vth = 0.9431 V.

So, according to the plot, Vth and k' are not constants but variables.

So, why I get this difference? Which parameters are the true ones: those from the .model or those from the plot? Should I get this parameters from the datasheet instead?

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  • \$\begingroup\$ Have you tried includeding the gate, drain and source resistors in your handcalculations \$\endgroup\$ – Joren Vaes May 6 '17 at 17:06
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    \$\begingroup\$ You do not include source resistors and this is why you have a different result. For Rs= 0.75R and Vgs = 3V we have: Id = 0.5*170 *((3 - Id*0.00075) - 1.6)^2 and if I solve this I get this Id = 142.186mA exactly as in Ltspice \$\endgroup\$ – G36 May 6 '17 at 17:20
  • \$\begingroup\$ Not all those 2N7002 transistors will exactly fit the model, actually, it would be a miracle if you find one that does (within, for example 1 % difference model/measured), there will always be some variations. Measure 100 of them from different batches and the average might be close to what the model predicts. The 2N2007 is a switching transistor, its main purpose is to have a low enough Rds when Vgs is above a certain value. Look in the datasheet what can be expected. Is what you simulate or measure out out spec ? My guess is that it is not. \$\endgroup\$ – Bimpelrekkie May 6 '17 at 17:36
  • \$\begingroup\$ Also: you simulate a model and remember: a model is always a simplified version of reality. \$\endgroup\$ – Bimpelrekkie May 6 '17 at 17:40
  • \$\begingroup\$ Your simulating a model, its not going to reflect reality when you delve into it. The question you need to ask, is it good enough for the answers I need? if it is then use it. If it isn't then refine it and get the values from those you measure, or the datasheet \$\endgroup\$ – Voltage Spike♦ May 8 '17 at 17:48

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