It is based on considering the effects of external bias conditions on charge distribution in MOS system and on conductance of free carriers on one hand, and the fact that the current flow depends only on the majority carrier flow between the two device terminals. Various second-order effects observed in MOSFETs are next dealt with. Subsequently, the complementary MOS (CMOS) inverter is taken up. Its DC characteristics, noise margin and the small-signal characteristics are discussed. Various load configurations of MOS inverters including passive resistance as well as transistors are presented. The differential inverter involving double-ended inputs and outputs are discussed. The complementary switch or the transmission gate, the tristate inverter and the bipolar  devices are briefly dealt with.

2.1.1  nMOS and pMOS Enhancement Transistors

Figure 2.2 depicts a simplified view of the basic structure of an n-channel  enhancement mode transistor, which is formed on a p-type substrate of moderate doping level. As shown in the figure, the source and the drain regions are made of two isolated islands of n⁺-type diffusion. These two diffusion regions are connected via metal to the external conductors. The depletion regions are mainly formed in the more lightly doped p-region. Thus, the source and the drain are separated from each other by two diodes, as shown in Figure 2.2. A useful device can, however, be made only by maintaining a current between the source and the drain.  The region between the two diffused islands under the oxide layer is called the channel region. The channel provides a path for the majority carriers (electrons for example, in the n-channel device) to flow between the source and the drain.

The channel is covered by a thin insulating layer of silicon dioxide (SiO₂). The gate electrode, made of heavily doped polycrystalline silicon (polysilicon or poly in short) stands over this oxide. As the oxide layer is an insulator, the DC current from the gate to the channel is zero. The source and the drain regions are indistinguishable due to the physical symmetry of the structure. The current carriers enter the device through the source terminal while they leave the device by the drain.