MOS capacitor-semiconductor physical device characteristics and detailed explanation

MOS capacitor

The MOS system composed of metal-oxide-semiconductor can be regarded as a parallel plate capacitor, metal and semiconductor are regarded as two parallel plates, and the SiO2 in the middle is the insulating medium, as shown in Figure 1-11(a). MOS capacitor. According to the definition of capacitance, it must meet:

Where VG is the gate voltage added to the MOS capacitor, and Qm is the amount of charge on the metal surface. According to the electrical neutrality condition, Qm should be equal to the charge quantity Qb of the entire space charge region in value.

Now take the reciprocal of formula (1-11) and substitute , Namely:

In this way, the total MOS capacitance is decomposed into SiO2 layer capacitance 1.jpg. And semiconductor space charge area capacitor 2.jpg two capacitors, as shown in Figure 1-11 (b) shown in series. then:

Figure 1-11 MOS capacitor and equivalent circuit

The oxide layer capacitance is equivalent to a plate capacitor, and its capacitance per unit area is:

Where VG is the thickness of the oxide layer, Qm is SiO2’s dielectric constant, also known as permittivity, its value is，It can be seen that it is a parameter related to materials and geometric dimensions. MOS capacitor. When the thickness of the SiO2 layer is constant,That is to say, it does not change with the external voltage. The capacitance of the space charge zone can also be regarded as a plate capacitor, but its spacingIs variable, and its capacitance per unit area can be expressed as:

Where is the dielectric constant of Si, and its value is Is the thickness of the space charge region. MOS capacitor. As mentioned earlier, the width of the space charge region changes with the applied voltage. therefore,It changes with 4 changes. If the space charge is accumulating, since χ4 is very small, soVery large, the total MOS capacitance is approximately equal to the oxide layer capacitanceIn the strong inversion type, χ4 reaches the maximum，Reached a minimum. Therefore, the total MOS capacitance is also extremely small.

You can derive more detailed content of MOS capacitor with voltage change (ie, capacitor voltage characteristic), you can derive it by yourself according to the two formulas (1-12) and (1-9), or refer to related materials, which will not be described here.

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