MOSFET devices for analog layout usually exist in two forms; those that
require matching and those that do not. For devices that do not require matching, simply
folding the device into smaller fingers creates a smaller layout and reduces some device
parasitics (gate resistance and source/drain to bulk capacitances). For devices that do
require matching, current mirrors, differential pairs, etc., each device is broken up into
fingers of the same width, which are interdigitated together to form a compound device.
Usually, for two devices of the same size that share a source or drain, a common-centroid
layout with dummy devices is used. If more devices must be matched, an interdigitation
pattern with dummy devices is used. There are many other ways to create a MOSFET in
layout that are very application specific. For example, power transistor layouts will be
quite different from any of the above layouts because of the amount of current that power
devices must source.