Where does the body diode in a MOSFET come from? I understand that its built in and an intrinsic part of the MOSFET. All illustrations of a MOSFET that I've seen depict the structure as two diodes back to back but non hint at where the diode comes from.
simulate this circuit – Schematic created using CircuitLab
To be more accurate, in a typical power U-mosfet structure in use today [aka trench mosfet], this body diode is a PIN diode:
Image from this textbook.
It's a result of tying the body terminal to the source terminal during manufacture. Note that the gate has no physical connection to the body as is implied in the diode schematic.
The FET needs a semiconductor for the current to flow in. For an n-channel enhancement device, this needs to be p-type (so that an n-channel can be induced in it with a + gate voltage). Since the S & D are necessarily n-type, and need to be constructed in the body, a N-P diode is necessarily formed.
