In MOSFETs, a voltage on the oxide-insulated gate electrode can induce a conducting channel between the two other contacts called source and drain. The channel can be of n-type or p-type (see article on semiconductor devices), and is accordingly called an nMOSFET or a pMOSFET (also commonly nMOS, pMOS). It is by far the most common transistor in both digital and analog circuits, though the bipolar junction transistor was at one time much more common.
- MOSFET Symbol
The operation of a MOSFET can be separated into three different modes, depending on the voltages at the terminals. For the NMOSFET the modes are:
Cut-off or sub-threshold mode
- is the threshold voltage of the device.
Here the switch is turned off, and there is no conduction between drain and source. While the current between drain and source should ideally be zero since the switch is turned off, there is a weak-inversion current, or subthreshold leakage. With MOSFET scaling subthreshold leakage composes a large percentage of total power consumption.
Triode or linear region
The switch is turned on, and a channel has been created which allows current to flow between the drain and source. The MOSFET operates like a resistor, controlled by the gate voltage. The current from drain to source is:
The switch is turned on, and a channel has been created which allows current to flow between the drain and source. Since the drain voltage is higher than the gate voltage, a portion of the channel is turned off. The onset of this region is also known as pinch-off. In first approximation the drain current is now independent of the drain voltage, and the current is only controlled by the gate voltage:
In digital circuits the transistors are only operated in cut-off and saturation mode. The triode mode is mainly relevant for analog applications.