N-channel enhancement MOSFET construction, working

A MOSFET (metal oxide semiconductor field effect transistor) is a voltage-controlled device in which the gate is insulated from the channel by a thin silicon dioxide layer. The n-channel enhancement MOSFET (E-MOSFET) has no channel at zero gate voltage; the channel is created (enhanced) by applying a positive gate voltage.

Construction

• A lightly doped p-type substrate (body).
• Two heavily doped n⁺ regions diffused into the substrate: the source (S) and drain (D).
• A thin layer of SiO₂ (insulator) over the region between source and drain.
• A metal (or polysilicon) gate (G) over the oxide. There is no physical channel between S and D.
• The substrate is usually connected to the source.

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

Working principle

1. VGS = 0: between the n⁺ source and drain there are two back-to-back PN junctions, so no current flows. ID ≈ 0.
2. Small positive VGS: the gate attracts electrons and repels holes from the substrate surface under the oxide, forming a depletion region.
3. VGS ≥ VT (threshold voltage): enough electrons collect under the oxide to form a thin n-type inversion layer (channel) between source and drain. Increasing VGS further enhances the channel and increases the current.
4. With VDS applied: electrons flow from the source through the channel to the drain; the conventional current ID flows from drain to source.

Operating regions

• Cut-off: VGS < VT, ID = 0.
• Ohmic (triode) region: VDS < VGS − VT; the channel acts as a resistor; ID = k[2(VGS − VT)VDS − VDS²].
• Saturation: VDS ≥ VGS − VT; the channel pinches off near the drain and ID = k(VGS − VT)², nearly independent of VDS.

 ID                       ID
  |     VGS3 ___           |        /
  |     VGS2 ___           |      /
  |     VGS1 ___           |    /
  |   VT<VGS1              |  /
  +---------- VDS          +--+------ VGS
  Output characteristic    VT   Transfer characteristic

Features: very high input resistance (about 10¹² Ω), voltage-controlled, no gate current, easy to make small. It is the basic element of CMOS digital circuits.