N-channel enhancement-mode MOSFET

An N-channel enhancement-mode MOSFET is a voltage-controlled device: a voltage applied to its gate controls whether current can flow between its drain and source.

In its simplified structure, the source and drain are N-type regions within a P-type semiconductor. An insulating oxide layer separates the gate from the semiconductor. Because this layer prevents a steady conducting path into the gate, the gate has a very high input resistance. Negligible steady current flows into it, so a voltage source can control the MOSFET with little loading.

Formation of the N-channel

The controlling voltage is the gate–source voltage, $$V_{\mathrm{GS}}$$: the gate voltage measured relative to the source. With $$V_{\mathrm{GS}}=0$$, the ideal enhancement-mode MOSFET has no conducting channel between source and drain.

A sufficiently positive gate voltage attracts electrons towards the semiconductor surface beneath the gate. This forms an N-type channel connecting the source and drain. The threshold voltage, $$V_{\mathrm{th}}$$, is the gate–source voltage at which the channel begins to conduct appreciably. Increasing $$V_{\mathrm{GS}}$$ above threshold strengthens the channel and allows a greater drain current.

Drain current and drain–source voltage

The drain–source voltage, $$V_{\mathrm{DS}}$$, is the drain voltage measured relative to the source. When $$V_{\mathrm{DS}}$$ is positive and $$V_{\mathrm{GS}}$$ is above threshold, conventional drain current $$I_{\mathrm{D}}$$ flows from drain to source.

For a fixed $$V_{\mathrm{GS}}$$ above threshold, $$I_{\mathrm{D}}$$ rises as $$V_{\mathrm{DS}}$$ increases, then tends towards a saturation region, where further increases in $$V_{\mathrm{DS}}$$ produce much smaller increases in current. At a given $$V_{\mathrm{DS}}$$, a higher $$V_{\mathrm{GS}}$$ strengthens the channel and allows a greater $$I_{\mathrm{D}}$$.

$$I_{\mathrm{DSS}}$$ is the drain current specified at zero gate–source voltage and a stated drain–source voltage. It is ideally zero for an enhancement-mode MOSFET, although a small leakage current can flow.

MOSFET switching

An N-channel enhancement-mode MOSFET can use a voltage to switch current through a load. Its three terminals are the gate, source and drain. The voltage between the gate and source controls the path between the drain and source; the load is connected in the circuit so that its current passes through this path.

When the gate–source voltage, $$V_{GS}$$, is below the threshold voltage, the MOSFET is off. Its drain–source path has a very high resistance and ideally carries no current. When $$V_{GS}$$ is sufficiently positive, the MOSFET turns on. Its drain–source path then has a low resistance, allowing current through the load.

Gate–source voltage and switching

The controlling voltage is the gate voltage relative to the source, not simply relative to another point in the circuit. A gate voltage can therefore turn the MOSFET on only if it is sufficiently positive compared with the source voltage.

The threshold voltage marks the point at which the MOSFET begins to conduct under specified conditions. Exceeding it does not necessarily make the drain–source resistance low enough for the intended load. To use the MOSFET as an effective switch, choose a gate–source voltage at which its specified on-state resistance is suitably low for that load.