for a p-channel "depletion-mode" MOS transistor a positive gate-source voltage will deplete the channel of its free holes, turning it “OFF”. In wide planar transistors the threshold voltage is essentially independent of the drain–source voltage and is therefore a well defined characteristic, however it is less clear in modern nanometer-sized MOSFETs due to drain-induced barrier lowering. In the figures, the source (left side) and drain (right side) are labeled n+ to indicate heavily doped (blue) n-regions. The depletion layer dopant is labeled NA− to indicate that the ions in the (pink) depletion layer are negatively charged and there are very few holes. In the (red) bulk the number of holes p = NA making the bulk charge neutral. If the gate voltage is below the threshold voltage (left figure), the "enhancement-mode" 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. Therefore, datasheets will specify threshold voltage according to a specified measurable amount of current (commonly 250 μA or 1 mA). If the gate voltage is above the threshold voltage (right figure), the "enhancement-mode" 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 because the voltage drop due to the current in the resistive channel reduces the oxide field supporting the channel as the drain is approached. Body effect The body effect is the change in the threshold voltage by an amount approximately equal to the change in the source-bulk voltage, , 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. 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: where; is the threshold voltage when substrate bias is present, is the source-to-body substrate bias, is the surface potential, is threshold voltage for