Why do MOS tubes need to be connected in parallel with a diode?

https://www.youtube.com/watch?v=pql2Ulae7mE

MOS has a body diode connected in parallel between the D and S poles, so why is this diode connected in parallel?

This starts with the process and structure of MOS. The diode is composed of a pair of PN junctions. The P-type region corresponds to the positive pole of the diode, the N-type region corresponds to the negative pole of the diode, and the PN junction is in the middle. SiO2 in the MOS tube itself is not conductive, so the driving pole G basically does not carry current.

In addition to the three poles D, G, and S, there is also an intermediate pole, which is connected to the S pole, so in the circuit symbol of MOS, the arrow pointing to the channel N channel inside the MOS is connected to the S pole.

In addition, the drain of the N-type region is connected to the middle P-type region and then to the source, which just forms a diode structure, so a diode is connected in parallel in the MOS symbol.

What is the use of this body diode?

In some scenarios, such as battery protection, after the lithium battery is over-discharged, the protection function will be turned on: turn off the discharge MOS. When the charger is plugged in, the MOS body diode is used to make the circuit conductive and the system work normally. However, in some scenarios, the existence of this diode is undesirable because it may cause leakage between the S pole and the D pole.