MOSFET: Substrate Bias Effects
Body Effect:
we assumed that source S was connected to the substrate B. In fact, it is possible to apply a voltage between S and B that is shown in figure. With a reverse bias between the substrate and the source (VB negative for an n-channel device), the depletion region is widened and the threshold gate voltage required to
achieve inversion must be increased to accommodate the larger Qd. A simplified view of the result is that W is widened uniformly along the channel, so that equ (1) should be changed to equ (2)
equ (1)
equ (2)
The change in threshold voltage due to the substrate bias is
If the substrate bias VB is much larger than 2ΦF (typically -0.6 V), the threshold voltage is dominated by VB and
where VB will be negative for the n-channel case. As the substrate bias is increased, the threshold voltage becomes more positive. The effect of this bias becomes more dramatic as the substrate doping is increased, since ΔVT is also proportional to . For a p-channel device the bulk-to-source voltage VB is positive to achieve a reverse bias, and the approximate change ΔVT for VB > 2Φf is
Thus the p-channel threshold voltage becomes more negative with substrate bias.
The substrate bias effect (also called the body effect) increases VT for either type of device. This effect can be used to raise the threshold voltage of a marginally enhancement device (VT = 0) to a somewhat larger and more manageable value.