MOSFET construction and working principle

A MOSFET is a field-effect transistor in which an insulated gate (metal-oxide-semiconductor structure) controls the conducting channel between source and drain by an electric field. The most common type is the n-channel enhancement MOSFET.

Construction

• A p-type silicon substrate (body).
• Two heavily doped n⁺ regions, the source and the drain.
• A thin SiO₂ insulating layer over the surface between them.
• A metal or polysilicon gate on the oxide.
• Terminals: source (S), gate (G), drain (D), body (B, normally tied to the source).

        S      G      D
        |   +------+  |
      --+   | gate |  +--
     | n+   +------+   n+ |
     |    ==== SiO2 ====  |
     |     p-substrate    |
     +---------B----------+

Working principle (n-channel enhancement type)

1. VGS = 0: there are two back-to-back PN junctions between source and drain, so ID = 0.
2. Positive VGS: the gate and the substrate form a capacitor. The positive gate repels holes and attracts electrons toward the oxide interface.
3. VGS ≥ VT (threshold): an inversion layer of electrons (the n-channel) forms under the oxide, connecting source and drain.
4. With VDS > 0: electrons drift from source to drain, so the current ID flows from drain to source.
5. As VDS rises, the channel narrows near the drain. At VDS = VGS − VT, the channel pinches off and ID saturates.

Equations

• Ohmic (triode): ID = k[2(VGS − VT)VDS − VDS²]
• Saturation: ID = k(VGS − VT)²

where k = μnCox W/(2L).

 ID                        ID
  |   VGS4 ____            |       /
  |   VGS3 ____            |     /
  |   VGS2 ____            |   /
  +-------- VDS            +--+---- VGS
  Output characteristic    VT   Transfer

Types: n-channel and p-channel, each as enhancement or depletion type. The depletion type has a built-in channel and conducts at VGS = 0.

Features: very high input impedance, very low gate current, small size and low power; used in digital ICs (CMOS), switches and amplifiers.