During manufacture, integrated circuits are tested at varying frequencies and temperatures to categorise them into speed grades. However, why don't all ICs come out the same and work the same? They all come from the same photolithographic mask, right? Am I missing something?
Electronic – Why do chips not always “meet the grade”
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The illustration is correct. VGS must be above Vth for the NMOS to be ON, otherwise it will be OFF. Therefore in your circuit the maximum possible voltage at the source is VDD-Vth, otherwise the NMOS would be OFF. Note that in your circuit VGS=VDD-(VDD-Vth)=Vth, so it is ON, but the voltage at the source cannot increase further because it would turn OFF the NMOS (and the source voltage would decrease, turning it ON again). It therefore comes to an equilibrium where VGS=Vth.
The fact that there is three NMOS instead of one doesn't change this situation, because all 3 NMOS have the same VGS voltage.
If you want to use an NMOS as a switch and have the full VDD on the load you should place it in the low side as in the following picture.
simulate this circuit – Schematic created using CircuitLab
It is important to understand that Q2 is in Saturation mode and not cut off mode, therefore some current flows through it (channel length modulation) which cannot be neglected. Because of the current flowing through the NMOS there is voltage drop even when Q1 is off. For Quantitative analysis we cannot ignore the current from NMOS in the saturation region.
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Best Answer
Modern ICs are REALLY small. Tolerances are huge during processes such as ion implantation and oxide growth. At such small sizes, these things can't be treated as anything but a probabilistic process. Lines also tend to be smeared due to the feature size being the minimum possible given the wavelength of light. When you get worst-case performance in a bunch of these different steps, then you get a non-functioning IC.
Companies don't design the IC so that it functions at the worst-case - it would be too costly. So instead they do Monte Carlo simulation of the manufacturing parameters, estimate a yield, and do testing after the fact.
Typical "design corners":
Source: What I remember from my IC manufacturing class.