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Short Channel Effects

When channel lengths are the same order of magnitude as depletion-layer widths of the source and drain, they cause a number of issues that affect design.
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Short-channel effects occur when the channel length is the same order of magnitude as the depletion-layer widths of the source and drain junction. In MOSFETs, channel lengths must be greater than the sum of the drain and source depletion widths to avoid edge effects. Otherwise, a number of effects appear.
Among the reported effects cited by a number of researchers at universities around the globe are:
1. “Off-state” leakage current.
2. Impact ionization, in which a charge carrier can be affected by other charge carriers;
3. Velocity saturation/mobility degradation;
4. Drain-induced barrier lowering (DIBL), which is caused by encroachment of the drain depletion region into the channel;
5. Drain punch through, whereby current flows regardless of gate voltage-a phenomenon that can occur if the drain is at high enough voltage compared to the source and the depletion region around the drain extends to the source;
6. . Surface scattering;
7. Channel length modulation;
8. Threshold voltage roll-off.