the channel length becomes very short, these equations become quite inaccurate. New physical effects arise. For example, carrier transport in the active mode may become limited by velocity saturation. When velocity saturation dominates, the saturation drain current is more nearly linear than quadratic in VGS. At even shorter lengths, carriers transport with near zero scattering, known as quasi-ballistic transport. In the ballistic regime, the carriers travel at an injection velocity that may exceed the saturation velocity and approaches the Fermi velocity at high inversion charge density. In addition, drain-induced barrier lowering increases off-state (cutoff) current and requires an increase in threshold voltage to compensate, which in turn reduces the saturation current. Body effect The occupancy of the energy bands in a semiconductor is set by the position of the Fermi level relative to the semiconductor energy-band edges. Application of a source-to-substrate reverse bias of the source-body pn-junction introduces a split between the Fermi levels for electrons and holes, moving the Fermi level for the channel further from the band edge, lowering the occupancy of the channel. The effect is to increase the gate voltage necessary to establish the channel, as seen in the figure. This change in channel strength by application of reverse bias is called the 'body effect'. Simply put, using an nMOS example, the gate-to-body bias VGB positions the conduction-band energy levels, while the source-to-body bias VSB positions the electron Fermi level near the interface, deciding occupancy of these levels near the interface, and hence the strength of the inversion layer or channel. The body effect upon the channel can be described using a modification of the threshold voltage, approximated by the following equation: where VTB is the threshold voltage with substrate bias present, and VT0 is the zero-VSB value of threshold voltage, is the body effect parameter, and 2φB is the approximate potential drop between surface and bulk across the depletion layer when and gate bias is sufficient to ensure that a channel is present. As this equation shows, a reverse bias causes an increase in threshold voltage VTB and therefore demands a larger gate voltage before the channel populates. The body can be operated as a second gate, and is sometimes referred to as the "back gate"; the body effect is sometimes called the "back-gate effect". Circuit symbols A variety of symbols are used for the MOSFET. The basic design is generally a line for the