Integrated Circuits and Materials

In a MOS (Metal-Oxide-Semiconductor) structure, the source and drain are two terminals connected to the semiconductor material (usually silicon) and play crucial roles in the operation of MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors).  The source is the terminal through which charge carriers (electrons in n-channel MOSFETs or holes in p-channel MOSFETs) enter the channel. It is typically grounded or connected to a lower voltage in n-channel MOSFETs, or to a higher voltage in p-channel MOSFETs. The source is called this way because it is the source of carriers that flow through the channel. On the other hand, the drain is the terminal where the charge carriers exit the channel. It is connected to a higher voltage in n-channel MOSFETs, or a lower voltage in p-channel MOSFETs, to create a flow of carriers from the source to the drain. When a voltage is applied between the drain and the source, a current flows from the source to the drain, provided the MOSFET is in the "on" state. During operation, the flow of carriers between the source and drain is controlled by the gate, which is insulated from the semiconductor by a thin oxide layer. By applying a voltage to the gate, an electric field is created in the underlying semiconductor, forming a conductive channel between the source and drain. In an n-channel MOSFET, a positive gate voltage attracts electrons to form a conductive channel, while in a p-channel MOSFET, a negative gate voltage attracts holes to form the channel. 

The n-well region cannot be kept floating. A specific contact, that can be seen on the right side of the n-well in Figure 4996, serves as a permanent connection to high voltage.

Figure 4996. Incorrect and correct polarization of the n-well. [1]

[1] Etienne Sicard, Basics of CMOS Cell Design, 2007.