Modern CMOS chips integrate millions of active devices (NMOS and PMOS) side by side in a common silicon substrate.

Circuits are designed with these devices to implement complex logic or analog functions.

In designing such circuits it is usually assumed that the individual devices do not interact with each other except through their circuit interconnections.

Thus individual devices should be electrically isolated from each other.

This is accomplished by various methods –

a. Junction isolation

b. Oxide isolation

c. LOCOS (LOCal Oxidation of Silicon)

d. Trench isolation