MOS IC-S:

Not all IC's are constructed using bipolar components, IC's are often designed to utilize either bipolar transistors or "Field-Effect transistors" (FETS).  The Field effect transistor is one in which the emitter-collector current is controlled by voltage rather than by a current.  Figure 3 shows the construction and operation of a MOSFET.

The FET may be constructed of a channel of either N-type or P-type silicon with a controlling gate sitting on top.  One end of the channel is called the source, and the other end is called the drain.  An N-channel FET has a P-type gate, so that when a positive voltage ios applied to the gate, the FET is forward biased.  There will be current flow between the source and the drain.  When a negative voltage is applied to the gate, the FET will be reversed biased, and the flow between the source and the drain will be pinched off.

The source and drain regions are diffused into the substrate.  A thin layer of silicon oxide is formed over the substarte and the appropriate windows are cut into it so that metal electrodes ) terminals) can be formed at the proper locations.  Note that the gate terminal is separated from the substrate by an extremely thin oxide layer, which is only 1 X 10^-10 metres thick, but it completely isolates the gate from the substrate.