This post answers the question: “What is gate capacitance of MOSFET?”. The same type of capacitance can exist in a variety of integrated circuits.

Let’s consider the MOSFET structure depicted below.

What is gate capacitance of MOSFET
MOSFET layer structure

Gate and channel here are separated from each other with the gate oxide layer. As soon as positive voltage is applied to the gate, electrons flows from the source into the channel and aggregates there. As soon as the gate voltage becomes higher than threshold voltage, electrons are travelling from the source to drain.

What is gate capacitance of MOSFET
Electric charge and field in MOSFET

During the above described process of channel inverting there is a capacitor form between gate and the channel as depicted below. There is an electric field forms between positive charges of the gate and negative charges of the channel. Capacitance of this capacitor is CGS=εOXLWd, and called gate-to source capacitance. We can also note the gate-to-channel capacitance COX=εOXd.

From these considerations it is easy to show the SRC model on the figure below.

What is gate capacitance of MOSFET
Switch-resistor-capacitor model of the MOSFET

Here vGS is a gate-source voltage and iG=COXLWdvGSdt.

In reality the MOSFET operation is much more complex with involving many internal capacitances, including gate-substrate, drain-source, gate-drain, drain-substrate and other capacitances. All these capacitances are depend on the drain-source vDS and gate-source voltages vGS.

More educational content can be found at Reddit community r/ElectronicsEasy.