n-channel E-MOSFET construction and working

The n-channel enhancement MOSFET (E-MOSFET) has no channel when VGS = 0; the channel is created ("enhanced") by applying a positive gate voltage above the threshold voltage Vt.

Construction

• A lightly doped p-type substrate (body).
• Two heavily doped n⁺ regions are diffused into it: the source (S) and the drain (D).
• A thin layer of SiO₂ insulator covers the surface between them, and a metal (or polysilicon) gate (G) is deposited on the oxide. No channel is built in.
• The substrate is usually connected to the source.

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

Working principle

1. VGS = 0: the drain-substrate and source-substrate p–n junctions are back-to-back, so no current flows even if VDS is applied (ID = 0): the device is OFF.
2. VGS > 0 (below Vt): the positive gate repels holes from under the oxide, forming a depletion region of negative ions; there are still no free electrons.
3. VGS ≥ Vt: electrons from the n⁺ regions (and the substrate) are attracted to the surface, forming an n-type inversion layer (channel) between source and drain. A larger VGS puts more electrons in the channel and so makes it more conductive.
4. Small VDS (triode/ohmic region): ID rises almost linearly with VDS, so the device acts like a voltage-controlled resistor: ID = k[2(VGS - Vt)VDS - VDS²].
5. Large VDS ≥ VGS - Vt: the channel is pinched off at the drain end; ID saturates and depends on VGS only: ID = k (VGS - Vt)².

Characteristics

Drain characteristics (ID vs VDS for different VGS):

 ID
  |        ____ VGS = 6 V
  |      ./____ VGS = 5 V
  |     / ____  VGS = 4 V
  |    / /
  |   / /_____  VGS = 3 V
  +--+--+------ VDS          (VGS ≤ Vt gives ID = 0)
   ohmic | saturation (VDS > VGS - Vt)

Transfer characteristic: ID = 0 for VGS < Vt, and a parabola ID = k (VGS - Vt)² beyond Vt.

Features

• Normally OFF, so it is used as a switch (CMOS logic) as well as an amplifier.
• Very high input resistance (~10¹⁴ Ω) because of the insulated gate.
• The transconductance is gm = 2k (VGS - Vt).