OE-10.2 MOS & CMOS Circuit Design Process
You can compare N-well, P-well and twin-tub processes, draw stick diagrams, and apply lambda-based design rules to produce a layout.
Stick diagrams are the sketch between a circuit schematic and a full layout - they capture topology and relative placement without committing to dimensions, and they are the fastest way to think about area. Lambda-based rules exist so a design can be scaled to a different process without redrawing, which is a nice lesson in abstraction. The well choice determines which transistor type sits in native substrate and which in a diffused tub, with consequences for performance and latch-up.
Work through these
N-well process
The first fabrication route, defined by which type of region is created in the substrate. The three processes below differ in exactly that choice.
P-well process
The complementary route, with the opposite region created. Comparing the two is the point rather than memorising either.
Twin-tub process
Doing both, which costs an extra step and lets each device be optimised independently. It is what modern processes actually do.
MOS layers
What is physically stacked on the wafer, layer by layer. It is the vocabulary the drawings in the next items are written in.
Stick diagrams
A quick, colour-coded sketch that captures topology without dimensions. It is the fastest way to think about a layout, and it is worth doing by hand.
Lambda-based design rules
Design rules expressed in a scalable unit rather than fixed dimensions, so a layout survives a process change. It is a neat idea and the reason old layouts can be reused.
Layout diagrams
The real thing, with dimensions, that gets sent for fabrication. Going from the sketch above to this is the practical exercise of the topic.
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