MOSFET - MOSFET Operation - Body Effect

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 V_GB positions the conduction-band energy levels, while the source-to-body bias V_SB 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 V_TB is the threshold voltage with substrate bias present, and V_T0 is the zero-V_SB value of threshold voltage, is the body effect parameter, and 2φ_B is the approximate potential drop between surface and bulk across the depletion layer when V_SB = 0 and gate bias is sufficient to insure that a channel is present. As this equation shows, a reverse bias V_SB > 0 causes an increase in threshold voltage V_TB 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".