Micro Crystal Semiconductor saw this question online: Generally, when introducing the operation of MOS transistors, it seems that it is said that a voltage is applied to the gate, and when the potential difference between the gate and the substrate is greater than VTH (threshold voltage), a channel is formed, and the transistor can conduct without any concern about the source and drain. However, in actual calculations and use, why is it necessary to use the gate-source voltage VGS (gate-source voltage) greater than VTH as the judgment criterion?

In fact, this is easy to understand. We can start from the relationship between VGS and VTH:

The relationship between VTH and VGS can be intuitively represented by the transfer characteristic curve of the MOS transistor. When VGS is greater than VTH, the MOS transistor conducts, and current can pass through; when VGS is less than VTH, the MOS transistor is cut off, and almost no current will pass through. Therefore, to achieve the switching function of the MOS transistor, it is necessary to ensure that VGS is greater than VTH.

So, when it is mentioned that the difference between the gate voltage and the substrate voltage is greater than the threshold voltage (VTH) so that a channel can be formed, leading to the conduction of the MOS transistor, why do we focus more on the relationship between the gate-source voltage (VGS) and the threshold voltage (VTH) in actual calculations and use?

This can be expressed by a simple formula: VGS = VTH + Vds.

Where, Vds is the voltage between the source and drain. From this formula, it can be seen that as long as Vds is large enough, even if VGS is slightly less than VTH, the MOS transistor can conduct. Therefore, in practical applications, we usually pay attention to the magnitude of Vds, rather than the difference between VGS and VTH.

We focus more on the relationship between the gate-source voltage (VGS) and the threshold voltage (VTH) because this directly determines the conduction and cut-off states of the MOS transistor. When the MOS transistor is conducting, the gate-source voltage (VGS) is greater than the threshold voltage (VTH), the channel is formed, and current can pass through the device. When the MOS transistor is cut off, the gate-source voltage (VGS) is less than the threshold voltage (VTH), the channel is blocked, and current cannot pass through.

The difference between the gate voltage and the substrate voltage being greater than the threshold voltage (VTH) is a necessary condition to ensure the normal operation of the MOS transistor, but it is not sufficient to determine the conduction and cut-off of the MOS transistor. The relationship between the gate-source voltage (VGS) and the threshold voltage (VTH) directly affects the conduction and cut-off of the MOS transistor.

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