n-channel enhancement MOSFET

An n-channel enhancement MOSFET (E-MOSFET) is a field-effect transistor in which no channel exists at V_GS = 0; a channel is created ("enhanced") only when a positive gate voltage above the threshold V_T is applied.

Construction

        S        G        D
        |      __|__      |
     ___|___  |metal|  ___|___
    |  n+   ||=SiO2=||  n+   |
    |_______|  ~~~~  |_______|
    |      induced n-channel   |
    |      p-type substrate    |
    |__________________________|
                 |
             SS (body)

• A lightly doped p-type substrate forms the body.
• Two heavily doped n+ regions are diffused into it to form the source and drain.
• A thin layer of silicon dioxide (SiO₂) insulates the substrate surface; a metal (or polysilicon) gate is placed on top. The gate is insulated, so gate current is practically zero (very high input resistance, about 10¹⁰–10¹⁵ Ω).
• There is no built-in channel between source and drain. The substrate is usually connected to the source.

Working principle

1. V_GS = 0: source and drain are separated by p-type material, forming two back-to-back pn junctions. Only a tiny leakage current flows, so I_D ≈ 0 (device OFF).
2. 0 < V_GS < V_T: the positive gate repels holes from the region under the oxide, leaving a depletion region of negative ions. Still no conduction.
3. V_GS ≥ V_T (threshold, typically 1–3 V): enough electrons (minority carriers of the p-substrate) are attracted under the gate to form a thin n-type inversion layer. This channel connects source and drain, and with V_DS > 0, electrons flow from source to drain: I_D flows.
4. Larger V_GS → thicker channel → larger I_D. Hence the name "enhancement".
5. Effect of V_DS: as V_DS increases, the channel near the drain narrows. When V_DS ≥ V_GS − V_T, the channel is pinched off at the drain end and I_D saturates.

In saturation: I_D = k (V_GS − V_T)², valid for V_GS > V_T.

 I_D |            ______ V_GS = 6 V
     |          _/______ V_GS = 5 V
     |        _/________ V_GS = 4 V
     |_______/__________ V_GS ≤ V_T
     0               V_DS

E-MOSFETs are used as switches in digital ICs (CMOS) because they are normally OFF.