Channel length modulation

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One of several short channel effects in CMOS scaling, Channel Length Modulation (CLM) is the effect of a pinchoff region forming before the drain under large drain bias. This shortens the channel region, and leaves a gap of uninverted silicon between the end of the formed inversion layer.

Normally, it could be expected that a velocity saturation would be reached, where phonon scattering would prevent the electrons moving through the channel from moving faster than a certain velocity. However, in the pinched off region, an extremely high electric field is present. This is due to the normal long channel voltage being applied to a scaled device. Since the electric field is measured as V/cm, the size of the device greatly affects the electric field. Thus, as this field increases the current density through the region must increase to satisfy Ohm's Law. Therefore, even though the transistor is in its saturation region, its current does continue to increase with the drain voltage.