Verified answer

it has a source, drain, gate and substrate source.

The substrate is internally tied to the source.

The reference has a lot more information.

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4 Terminal Mosfet

fourth terminal is bulk

if we're talking about the simplest mosfets formed in p-substrate, then for nmos bulk is just the psub especially under the channel, and for pmos bulk is the nwell the pmos is formed in. in the process flow, the psub exists from whatever starting material (wafer) is used, and nwell would be formed (implanted) before any other parts of the pmos.

MOSFET = 4 terminal active device Turns voltage to current....ie you put an input voltage get an output current. used to build larger circuits that perform complex functions... either analog or digital essentially it is the basic building block for most ICs today for some applications

Most of the answers are good. It could also be if you refer to a schematic software, the TAB such as D packs is the 4th terminal, it is the same electrical connection as the center contact, whatever the device.

I hope this answers your question

some MOSFETs have two gates. That may be the case of the one you are learning about...Or just the substrate.