Drain-induced barrier lowering

The origin of the threshold decrease can be understood as a consequence of charge neutrality: the Yau charge-sharing model.

Barrier lowering increases as channel length is reduced, even at zero applied drain bias, because the source and drain form p–n junctions with the body, and so have associated built-in depletion layers associated with them that become significant partners in charge balance at short channel lengths, even with no reverse bias applied to increase depletion widths.

The term DIBL has expanded beyond the notion of simple threshold adjustment, however, and refers to a number of drain-voltage effects upon MOSFET I-V curves that go beyond description in terms of simple threshold voltage changes, as described below.

As channel length is reduced, the effects of DIBL in the subthreshold region (weak inversion) show up initially as a simple translation of the subthreshold current vs. gate bias curve with change in drain-voltage, which can be modeled as a simple change in threshold voltage with drain bias.

However, at shorter lengths the slope of the current vs. gate bias curve is reduced, that is, it requires a larger change in gate bias to effect the same change in drain current.

At extremely short lengths, the gate entirely fails to turn the device off.

[2] DIBL also affects the current vs. drain bias curve in the active mode, causing the current to increase with drain bias, lowering the MOSFET output resistance.

This increase is additional to the normal channel length modulation effect on output resistance, and cannot always be modeled as a threshold adjustment.

is the low drain voltage (for a linear part of device I-V characteristics).