6.2 Channel length modulation and body bias effect
The observed current IDS does not saturate, but has a small finite slope as shown in fig 6.31. This is attributed as channel
length modulation. This in MOSFET is caused by the increase in depletion layer width at the drain as the drain voltage is increased. This leads to a shorter channel length (reduced by ) and increased drain current. When the channel length of MOSFET is decreased and MOSFET is operated beyond channel pinch-off, the relative importance of pinch-off length with respect to physical length is increased. This effect can be included in saturation current as :
Hereis called channel length modulation coefficient.
Fig 6.31: Actual vs Ideal IDS-VDS graph
Till now we assumed that the body of MOSFET is to be grounded. We will now take effect of body bias into account i.e. body being applied a negative voltage in case of n-MOSFET. Application of VSB > 0 increases the potential build up across the semiconductor. Depletion region widens in order to compensate for the extra required field, which implies higher VT. Viewing it from the point of energy band diagram, a higher potential needs to be applied to the gate in order to bend the bands by the same ammount in order to create the same electron concentration in the channel. With the application to the body bias, it modulates to the threshold voltage governed by the threshold voltage governed by the following equations :
) and increased drain current. When the channel length of MOSFET is decreased and MOSFET is operated beyond channel pinch-off, the relative importance of pinch-off length 
is called channel length modulation coefficient.
is known as the body coefficient. 
