According to the different conductivity channels, MOS can be subdivided into two types: NMOS and PMOS. Figure 1-49 shows the schematic diagram of NMOS. From the red box in the figure, it can be seen that a diode is connected in parallel between the D and S terminals of MOS. Some people say that this diode is a parasitic diode, while others say it is a body diode. Many students are curious: why is this diode connected in parallel? Can it be removed? What is its use?
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| Figure 1-49 MOS and parasitic diode/body diode. |
To understand the reason for the diode, we need to start with the technology and structure of MOS. Both MOS and diodes are composed of semiconductor materials. As we know, a diode is composed of a pair of PN junctions as shown in Figure 1-50, where the P-type region corresponds to the anode of the diode, and the N-type region corresponds to the cathode of the diode, with a PN junction in between.
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| Figure 1-50: Diodes and PN Junctions |
Now let's take a closer look at the MOS structure. As shown in Figure 1-51 (1), the oxide O in MOS refers to silicon dioxide SiO2, which is non-conductive. Therefore, the G terminal does not carry current. MOS is a voltage-driven device, so its power consumption is relatively low. As shown in Figure 1-51 (1), in addition to the three terminals D, G, and S, there is also a middle terminal in MOS, which is connected to the S terminal (dashed line). Therefore, in the circuit symbol of MOS (2), the arrow pointing to the N-channel inside MOS is connected to the S terminal (there is always a reason for everything, there is no such thing as love or hate for no reason).
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| Figure 1-51: MOS and PN Junctions |
As shown in Figure 1-51 (3), the N-type region of the D terminal, the P-type region in the middle, and the S terminal all form a diode structure (understanding with Figure 1-50), which is why a diode is connected in parallel in the MOS symbol.
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| Figure 1-52: Forward Current Flow in an MOS Body Diode |
From the above analysis, we can see that this diode is a result of the MOS structure and technology, and it is not quite appropriate to call it a parasitic diode. The term "body diode" is relatively accurate. So what is the use of this body diode? In some scenarios, we do not want this diode to exist because it may cause leakage between the S and D terminals. However, in other scenarios, the conductive characteristics of this diode are utilized to make the system work normally. This is discussed in the later chapter on battery protection principles. Some battery protection boards will activate the protection function after over-discharging a lithium battery, which will 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 works normally, as shown in Figure 1-52. This is the reason for the parallel diode in the MOS symbol and its usage explanation.
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