Almost all the semiconductor material based component shows the capacitance property between Semiconductors layers, Varactor diode is one of the main components works with the capacitance property.
At MOSFET multiple-layer overlapping is present, due to this reason MOSFET capacitance is also common for MOSFET.
MOSFET capacitance model
The figure shows the capacitance shown by the MOSFET, the capacitance property of MOSFET appears when the voltage is applied.
Technically MOSFET capacitance means the arrangement of charged carriers on both sides of the layers.
- Source – substrate = CSB is the C1 capacitor
- Drain – substrate = CDB is the C2 capacitor
- Gate – substrate = CGB is the C3 capacitor
- Gate – source = CGS is the C4 capacitor
- Gate – drain = CGD is the C5 capacitor
- There is another capacitance between the metal gate layer and the substrate, which is the C6 capacitor.
The whole capacitance at the MOSFET is called the parallel plate capacitance, which works with a simple theory that is anything that contains separated mobile charge is capacitance at the MOSFET.
MOSFET capacitance multiplier
The field-effect transistors like MOSFETs are the main components used to build capacitance multiplier circuits, the transistor component with high DC gain will provide a larger capacitance multiplication at the circuit.
It is an electronic circuit technique used to reduce voltage ripples on the signals at the power supply rectifier circuit, it is the same process that a capacitor filter done at a rectifier circuit.
MOSFET capacitance measurement
The MOSFET capacitance measurement from each section of the MOSFET will be noted as different.
- It is capacitance measured at gate-drain and gain-source terminals of the MOSFET.
- They must be influenced at the delay time of the MOSFET
Reverse transfer capacitance
- It is capacitance measurement between gate-drain terminals of MOSFET
- It must influence the drain current rising and a low capacitance value is needed for high-speed driving.
- It is capacitance measurement on gate-drain and drain-source terminals of the MOSFET
- It will influence the turn-off characteristics of MOSFET and loss with a light load.
MOSFET capacitance calculation
The MOSFET capacitance calculation is not limited between terminals, but also each region of operation had a different calculation equation.
At cut-off region
CGB = Cox WL
CGD = Cox WLD
CGS = Cox WLD
At the active or linear region
CGB = 0
CGD = ½ Cox WL + Cox WLD
CGS = ½ Cox WL + Cox WLD
At saturation region
CGB = 0
CGD = Cox WLD
CGD = 2/3 Cox WL + Cox WLD