A MOSFET is a transistor whose gate voltage controls a channel between source and drain.
MOSFET stands for metal-oxide-semiconductor field-effect transistor.
The Main Terminals
A simplified MOSFET has four important terminals:
| Terminal | First Role |
|---|---|
| Gate | Control electrode |
| Source | Carrier source side of the channel |
| Drain | Carrier drain side of the channel |
| Body | Semiconductor substrate or well |
Many beginner diagrams show only gate, source, and drain. The body still matters physically, even when it is tied to a fixed voltage in many circuits.
The Controlled Channel
For an n-channel MOSFET, the first model is:
flowchart LR source["Source"] --> channel["Controlled channel"] channel --> drain["Drain"] gate["Gate voltage"] --> channel body["Body"] -. influences .-> channel
The gate does not work by sending steady current through itself in the ideal model. It works by creating an electric field across an insulating oxide layer.
That electric field changes whether a channel can form between source and drain.
Why the Gate Is Special
The gate is separated from the channel by an insulator.
That means the gate behaves partly like one plate of a capacitor:
- charge on the gate creates an electric field
- the field affects the semiconductor below
- the channel condition changes
This is why MOSFETs are often described as voltage-controlled devices.
Boundary
Real MOSFETs have gate leakage, body effects, capacitance, threshold variation, and limits.
The durable first model is:
A MOSFET uses gate voltage to control whether source and drain are connected by a conductive channel.