fingering effect
Due to second order effects, current may not change linearly with W/L.
Sometimes the effect of the STI can cause also current to change. STI stands for Shallow Trench Isolation, which is the way used to isolate the MOS device. It's performed by removing part of the substrate and applying oxide in this part. This ,however, causes stresses on the device altering its physics. When a single finger is used, the whole device suffers from the STI effect, but when fingers are used, only the outer fingers suffer from STI, while other fingers are shielded by the outer ones.