N-channel depletion MOSFET construction, working

The n-channel depletion MOSFET (D-MOSFET) has a physical n-channel built between source and drain, so current flows even at VGS = 0. A gate voltage can either deplete the channel (negative VGS) or enhance it (positive VGS). So it can work in both depletion and enhancement modes.

Construction

• A p-type substrate.
• Two heavily doped n⁺ regions (source and drain) diffused in the substrate.
• A thin lightly doped n-type channel diffused between the source and drain, below the oxide.
• A thin SiO₂ layer over the channel, with a metal gate on top (insulated from the channel).
• Substrate (body) usually joined to the source.

     S       G       D
     |    +-----+    |
  ---+    |metal|    +---
 | n+|    +-----+    | n+|
 |   |~~ n-channel ~~|   |
 |       p substrate     |
 +----------B------------+

Working principle

1. VGS = 0: with VDS applied, electrons flow through the n-channel from source to drain. Drain current is IDSS.
2. Negative VGS (depletion mode): the negative gate repels electrons from the channel (holes are induced), so the channel becomes narrower and ID decreases. At VGS = VP (pinch-off, a negative value), the channel is fully depleted and ID ≈ 0.
3. Positive VGS (enhancement mode): the positive gate attracts more electrons into the channel, so the channel widens and ID rises above IDSS.

Characteristics

Shockley equation (saturation region): ID = IDSS (1 − VGS/VP)²

 ID                          ID
  |      VGS=+1 ___           |            /
  |      VGS= 0 ___ IDSS      |  IDSS _.--'
  |      VGS=-1 ___           | .-'
  |      VGS=-2 ___           |/
  +----------------- VDS      +---+--------- VGS
  Drain characteristic      VP   0

• VGS < VP: cut-off.
• Ohmic region at low VDS, saturation at higher VDS.

Comparison with E-MOSFET: D-MOSFET has a built-in channel, conducts at VGS = 0 (normally-ON), and works with both polarities of VGS; E-MOSFET has no channel at VGS = 0 and needs VGS > VT.

Applications: RF amplifiers, input stages of high-impedance amplifiers, and constant current sources.