n-channel depletion MOSFET operation

An n-channel depletion MOSFET (D-MOSFET) has a channel built in during manufacture, so drain current flows even at V_GS = 0. It can work in two modes: depletion mode (V_GS negative) and enhancement mode (V_GS positive).

Construction

        S        G        D
        |      __|__      |
     ___|___  |metal|  ___|___
    |  n+   ||=SiO2=||  n+   |
    |_______|________|_______|
    |   diffused n-channel     |
    |                          |
    |    p-type substrate      |
    |__________________________|
                 |
             SS (body, tied to S)

• A lightly doped p-type substrate forms the body.
• Two heavily doped n+ regions form the source and drain.
• A thin n-type channel is diffused between them, linking source and drain.
• A thin SiO₂ layer insulates the metal gate from the channel, so gate current ≈ 0 and input resistance is very high.

Operation

1. V_GS = 0: with a positive V_DS, electrons flow from source to drain through the existing n-channel. The current at V_GS = 0 with the channel pinched off is I_DSS.

2. V_GS negative (depletion mode): the negative gate repels electrons from the channel and attracts holes from the p-substrate, which recombine with electrons. The number of free electrons in the channel falls (the channel is depleted), so I_D decreases. At V_GS = V_P (pinch-off voltage, e.g. −4 V) the channel is fully depleted and I_D = 0.

3. V_GS positive (enhancement mode): the positive gate attracts more electrons (minority carriers) from the substrate into the channel. The channel becomes richer in carriers and I_D rises above I_DSS. This is possible because the gate is insulated, so no gate current flows. (A JFET cannot do this.)

4. Effect of V_DS: as V_DS rises, the channel narrows near the drain because the gate-to-drain voltage becomes more negative. At V_DS = V_GS − V_P the channel pinches off at the drain end, and I_D becomes nearly constant (saturation).

Characteristics

In saturation the transfer characteristic follows Shockley's equation: I_D = I_DSS (1 − V_GS/V_P)².

 Transfer             Drain
 I_D                  I_D |   ______ +1 V (enh.)
  |       /               |  /______ 0 V  (I_DSS)
  |      /                | /_______ −1 V
 I_DSS  *                 |/________ −2 V (depl.)
  |   /                   +------------- V_DS
  |_/______ V_GS
 V_P   0   +

 Symbol (n-channel D-MOSFET):
           D
           |
     G ||--+   solid line =
       ||<-+-- SS   built-in channel
       ||--+
           S

Summary

D-MOSFETs are used in RF amplifiers and as constant-current loads because they conduct at zero bias and accept both polarities of gate voltage.