Why is a MOS tube connected in parallel with a diode? What is the purpose?

MOS tube, an abbreviation for MOSFET, the full spelling is Metal-Oxide-SemiconductorField-Effect Transistor, translated as Metal-Oxide-Semiconductor Field-Effect Transistor, depending on the conductive channel, MOS can be subdivided into two types of NMOS and PMOS.

The diagram below shows a diagram of an NMOS. As can be seen from the red box in the diagram, the MOS has a diode connected in parallel between the D and S poles, some say this diode is a parasitic diode, others say it is a body diode, which is accurate? Many students are also very curious: why is this diode connected in parallel? Is it possible to remove it?

This starts with the MOS process and structure. Whether it is MOS or a diode, both are made of semiconductor material and we all know that a diode is made of a pair of PN structures, see the diagram below, the P-type area corresponds to the anode of the diode and the N-type area corresponds to the negative pole of the diode.

We go back to the MOS structure, from the following figure (1) can be seen, MOS in the oxide O refers to silicon dioxide SiO2, SiO2 does not conduct, so the drive pole G basically does not go current, so MOS power consumption is relatively low, is a voltage driver device.

From (1) it can also be seen that MOS has a Sub pole in addition to the three poles D, G and S. Sub and S poles are connected, so (2) the circuit symbol for MOS will have the arrow pointing to the N-channel of the trench inside the MOS and the S pole connected together (there is no love nor hate in the world without a reason).

As can be seen in (3), the N-type region from the D-pole -> middle P-type region -> Sub-pole -> S-pole just forms a diode structure and is in reverse bias, which is the reason for the parallel connection of a diode in the MOS symbol.

From the above analysis we can see that this diode is a result of the MOS structure and process, it is not appropriate to call it a parasitic diode, the name body-diode is still relatively accurate.

So what is the use of this body-diode? In some scenarios, the presence of this diode is not desired, as it would leave the possibility of leakage between the S and D poles.

There is love and there is hate, in other scenarios it is this diode that is used to conduct electricity and allow the system to work properly. For example, previously wrote an article: “Battery Protection 1: Lithium Battery Overdischarge Protection Principle UVP”, there are battery protection boards, after the lithium battery overdischarge, will turn on the protection function: turn off the discharge MOS. when plugged into the charger, it uses the MOS body diode, making the circuit conductive, the system works normally.

This is the reason for the MOS symbol in parallel with the diode, and an introduction to its use.

Leave a Reply

Your email address will not be published. Required fields are marked *