MOS transistor structure

A MOSFET (Metal Oxide Semiconductor Field Effect Transistor), or MOS, as is commonly called, is an electronic device which converts change in input voltage into a change in output current. The basic structure of a MOS transistor (as seen sideways) is as shown in figure 1. The substrate is a lightly doped semiconductor. Source and Drain regions are heavily doped regions of type opposite to substrate. In-between source and drain is a region called channel. Above the channel is a very thin layer of oxide. 

The voltage is applied to input terminal, which is called "Gate" terminal. If sufficient voltage is applied at the gate terminal, a channel gets formed between source and drain terminals. Depending upon the nature of channel formed, MOS is termed as N-MOS or P-MOS.

N-MOS: For an N-MOS, substrate is P-type, source and drain regions are N-type. Application of a positive voltage at Gate terminal with respect to substrate will result in formation of channel of electrons.

P-MOS: For a P-MOS, substrate is N-type, source and drain regions are P-type. Application of a negative voltage at Gate terminal with respect to substrate will result in formation of channel of holes.