In almost all modern electronic circuits, transistors are the key active element. By reducing the dimensions of MOS transistors and the wires connecting them in integrated circuits (ICs), it has been possible to increase the density and complexity of integrated systems. Figure 1.1 illustrates the reduction in feature size over time. It is expected that a chip designed in a 35-nm IC process will include more than 1011 transistors in a few years [1]. But up to now, the scaling progress was essentially attributed to the improvements in manufacturing technology. However, as the physical limits are being met, some changes to the device structures and new materials will be necessary. Next, we will describe the transistor structure and the different equivalent models that are generally used for simulations.