Types and Structure of MOS Tubes

MOSFET is a type of FET (the other is JFET), which can be manufactured as enhancement type or depletion type, P-channel or N-channel, and 

there are 4 types, but the actual application is only enhancement-mode N-channel MOS. Tube and enhanced P-channel MOS tube, so N-MOS is 

usually mentioned, or PMOS refers to these two. The picture on the right is the symbol of these two MOS transistors.

For these two enhanced MOS tubes, NMOS is more commonly used. The reason is that the on-resistance is small and easy to manufacture. 

Therefore, NMOS is generally used in switching power supply and motor drive applications. In the following introduction, NMOS is also the 

main focus.

It can be seen on the MOS tube schematic that there is a parasitic diode between the drain and the source. This is called the body diode. 

This diode is very important when driving an inductive load (such as a motor). By the way, the body diode only exists in a single MOS tube, 

and it is usually not found inside an integrated circuit SMD. The figure below is the structure diagram of the MOS tube, and the usual 

schematic diagrams are drawn as shown on the right. (Diodes for gate protection are sometimes not drawn)

There are parasitic capacitances between the three pins of the MOS tube, as shown in the figure on the right. This is not what we need, but 

due to manufacturing process limitations. The existence of parasitic capacitance makes it more troublesome when designing or selecting a 

driving circuit, but there is no way to avoid it.

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