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The Width and Length of the oxide layer in a Mosfet is fixed in the factory, and there is not usually any need to try and obtain those values.
The thing that we do need is their ratio
e.g. if W=2 and L=1 then we need W/L
So for that example, the answer is 2
Note that there are no units in the answer for a ratio.
Example 2:
W=1 μm
L=500nm
What is the answer for W/L ?
The answer is 1000/500 = 2 with no units
Notice that we first had to be sure that the units were consistent
If you look at the default EC Nmos settings, you will see that:
W= 1μm
L=200nm
The W/L which we have there is 1000/200 ie 5
It is quite easy providing you put things in place correctly before doing the calculation.
Now ... from part 4 we had the formula:
Kn=0.5KP(W/L)
If we were to make our W/L =2, then Kn would equal KP
In EC, using 2 for W/L will make calculations easier for obtaining the Kn value which the Nmos will use to determine its gain (Transconductance characteristic). You can alter that ratio at any time later when you have had plenty practice.
So now you can go in to the Nmos settings and put in any values for W and L .....but just be sure that W/L =2
You can then put in any value for KP
(e.g. 1mA/V²) or whatever...... knowing that your KP value can be used as Kn in Mosfet formulae such as
Id=Kn(Vgs-Vth)² for the simplified Saturation current which was mentioned in part 4
Keep (W/L) = 2 for now, using any values.
Then KP is the same as Kn .... Kn is used in many formulae. It is often called the Conductance parameter.
It is different for every Mosfet and is not likely to be found on datasheets.
Now that we are able to choose between Omhic (for full switching), or Saturation (for amplification ....which the above details are mostly leading us to), we can now work towards a basic Nmos amplifier setup.
We will immediately recognise the Saturation state of the gate bar on the Nmos schematic and set about adjusting the Nmos W, L, KP, Vgs and Vth etc. with a view towards fixing the drain in a set position ready for a signal implant to be amplified.
Earlier Part 4 is here http://everycircuit.com/circuit/4863487332581376
Next part 6 is here http://everycircuit.com/circuit/4940688430202880
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