A basic error I was making about MOSFETs.

MOSFETs are Metal-Oxide, Field Effect Transistors, and are a main type of transistor in modern circuit design. Their properties are well-documented.

I had a basic misconception about their behavior, that concerned how they go into saturation, and to what extent their Drain voltage affects their behavior, even after saturation.

This error started, because I had read a specifications-sheet, about the circuit-simulation program SPICE, according to which MOSFETs are simulated as having parameters, which are denoted by keywords that SPICE recognizes, but the meanings of which can easily be misread by any person, who does not know a lot about circuits. This specifications-sheet names the parameter ‘Lambda’.

What I had mistakenly thought, was that this parameter both defines the Drain voltage at which saturation takes place, as well as the degree to which channel-length modulation takes place afterward. I had justified this idea, with the additional idea, that the Bulk voltage of a MOSFET, plays more strongly near the Source and Drain of a MOSFET, than the relative Gate voltage does. This was factually wrong.

I had quoted another article on the Web, that defines channel-length modulation, and which may explain the phenomenon of saturation while doing so.

The fact is that the established literature now seems 100% accurate in describing both phenomena. And a practical consequence of this is, that the circuits which are labeled ‘Current Mirrors’ in circuit-design, will work very well. Because, either MOSFET that forms part of such a current mirror, will saturate in a trivially easy way.

The following would be my own updated synopsis, of both phenomena. It assumes an n-Channel, enhancement-mode MOSFET:



I added a bit of complexity, to how channel-length modulation sets in, so that ‘exactly at the saturation point’, the two definitions / simulations of Drain current would be equal. But, because (‘λ’) is usually assumed to be small, the impact of this modification, or of its absence, should also be small.

Conclusion:

By default, a MOSFET operates in its saturation-region, in which Gate voltages vary Drain current. The ability of the MOSFET to operate in its ‘triode region’, is as limited as the Drain voltage is in practice, not to exceed the saturation-point.

Dirk

 

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