Double-well structure & Triple-well(Deep N-well) Structure

To understand the concept of the body terminal in transistors, it’s important to look at the cross-sectional structure of transistors, especially the well structure. The diagram in the Figure illustrates this structure. Typically, silicon wafers have a p-type substrate.

Differences between Double-welland Deep N-well structure

Double-Well Structure (Figure a):

  • This is a fundamental structure used in MOS (Metal-Oxide-Semiconductor) fabrication.
  • In an NMOS (N-channel MOS) transistor, the source and drain regions are within an n-type region that is inside a P-well.
  • For a PMOS (P-channel MOS) transistor, the source and drain regions are within a p-type region inside an N-well.
  • Generally, the entire P-well is connected to the ground (GND), and the N-well is connected to the supply voltage (VDD).
  • Because the p-substrate (the main silicon layer) and the P-well are both conductive, the body potentials of NMOS devices are shared among all transistors.
  • However, for PMOS devices, since the N-well is isolated from the P-well and p-substrate, the body potential can either be VDD or some other bias potential.

Triple-Well or Deep N-Well (DNW) Structure (Figure b):

  • In this structure, a deep N-well (DNW) is introduced on top of the p-type substrate.
  • The P-well and N-well, where transistors are formed, are then placed on top of this deep N-well.
  • This separation of the P-well allows more control over its voltage.
  • Moreover, because the circuit within the deep N-well is isolated from the p-substrate, it is less susceptible to noise that can travel through the p-substrate.
  • This technique is often used to protect analog circuitry (which is sensitive to noise) from noise generated by digital circuits.
  • In the diagram, “STI” stands for shallow trench isolation, which is an oxide film used to separate individual transistors.

In summary, the choice of well structure in semiconductor devices, whether it’s a double-well or triple-well structure, plays a crucial role in controlling the body potential of transistors and addressing noise-related concerns in integrated circuits.

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