Digital systems and design

# Gate capacitance of MOSFET 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.

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.

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 ${C}_{GS}=\frac{{\epsilon }_{OX}LW}{d}$, and called gate-to source capacitance. We can also note the gate-to-channel capacitance ${C}_{OX}=\frac{{\epsilon }_{OX}}{d}$.

From these considerations it is easy to show the SRC model on the figure below. Here ${v}_{GS}$ is a gate-source voltage and ${i}_{G}={C}_{OX}LW\frac{d{v}_{GS}}{dt}$.
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 ${v}_{DS}$ and gate-source voltages ${v}_{GS}$.