In the last chapter, we gave you an introduction of MOS as capacitor. In this chapter, we will see how MOS works as a capacitor with derivation of some related equations.
By Gauss's Law:
Also by thermal equilibrium:
where p and n are hole and electron concentrations of substrate and is hole or electron concentration of the corresponding intrinsic seminconductor.
We see that if we keep making more and more -ve, the charges Qs and Qm keep increasing. Thus, it is acting like a good parallel plate capacitor. Its capacitance can be given as-
Fig 4.3: Gate and Depeletion charge of MOS Capacitor
For +ve bias voltage on gate, increasing will increase Qm and Qs.
is hole or electron concentration of the corresponding intrinsic seminconductor.
more and more -ve, the charges 
