A MOSFET gate controls the channel through an electric field.
That is the “field-effect” in field-effect transistor.
The Gate Does Not Need a Door Handle
The gate is insulated from the semiconductor below it. In the ideal model, direct current does not continuously flow through the gate oxide.
Instead:
- voltage is applied to the gate
- charge arrangement changes near the semiconductor surface
- an electric field affects carriers below the gate
- a conductive channel may form between source and drain
This is why a small control condition can affect a separate current path.
Channel Formation
In an n-channel MOSFET, a suitable positive gate voltage can attract electrons near the surface under the gate.
When enough carriers gather there, the region can behave like a conductive channel between source and drain.
When the channel exists, current can flow through the source-drain path if the rest of the circuit provides a voltage difference.
When the channel does not exist, the path is strongly blocked, though not perfectly zero in real devices.
Why This Is Not Mechanical
The MOSFET does not contain a tiny physical bridge that moves into place.
The “path” is an electrical condition inside semiconductor material. The gate changes carrier distribution. The material behavior changes because the field changes the internal conditions.
This is more abstract than a mechanical switch, but it is also what makes MOSFETs tiny, fast, and manufacturable.
Boundary
The channel model hides many details: depletion, inversion, mobility, geometry, and operating regions.
For the developer-facing path, retain this:
A MOSFET gate voltage changes the carrier condition under the gate. When the condition is strong enough, a source-drain channel exists.