The body effect describes the changes in the threshold voltage by the change in, the source-bulk voltage. Since 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"; the body effect is sometimes called the "back-gate effect".
For an enhancement mode, n-mos MOSFET body effect upon threshold voltage is computed according to the Shichman-Hodges model (accurate for very old technology) 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, and is threshold voltage for zero substrate bias, is the body effect parameter, is oxide thickness, is oxide permitivity, is the permitivity of silicon, is a doping concentration, is the charge of an electron.
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