<P> If the gate voltage is below the threshold voltage (top figure), the transistor is turned off and ideally there is no current from the drain to the source of the transistor . In fact, there is a current even for gate biases below the threshold (subthreshold leakage) current, although it is small and varies exponentially with gate bias . </P> <P> If the gate voltage is above the threshold voltage (lower figure), the transistor is turned on, due to there being many electrons in the channel at the oxide - silicon interface, creating a low - resistance channel where charge can flow from drain to source . For voltages significantly above the threshold, this situation is called strong inversion . The channel is tapered when V> 0 because the voltage drop due to the current in the resistive channel reduces the oxide field supporting the channel as the drain is approached . </P> <P> The body effect is the change in the threshold voltage by an amount approximately equal to the change in the source - bulk voltage, V S B (\ displaystyle V_ (SB)), because the body influences the threshold voltage (when it is not tied to the source). It can be thought of as a second gate, and is sometimes referred to as the back gate, and accordingly the body effect is sometimes called the back-gate effect . </P> <P> For an enhancement - mode nMOS MOSFET, the body effect upon threshold voltage is computed according to the Shichman--Hodges model, which is accurate for older process nodes, using the following equation: </P>

Define threshold voltage and what are the main factors that affect threshold voltage of the device