Short Channel Effect and Narrow Width Effect of MOSFET
If we plot the threshold voltage as a function of channel length in MOSFETs, we find that VT decreases with L for very small geometries. This effect is called
the short channel effect (SCE), and is somewhat similar to DIBL.The mechanism is due to something called charge sharing between the source/drain.
Short channel effect:
- The equipotential lines in Fig. shown in given below designating the depletion regions curve around the contours of the source/drain junctions. Onething is that the electric field lines are perpendicular to the equipotential contours.
- we see that the depletion charges that are physically underneath the gate in the approximately triangular regions near the source/drains have their field lines terminate not on the gate, but instead on the source/drains.
- Hence, electrically these depletion charges are "shared" with the source and drain regions and should not be counted in the VT expression.
- We can deal with this effect by replacing the orginal Qd in the rectangular region underneath the gate by a lower Qd in the trapezoidal region in Fig.
- Clearly, for a long channel device, the triangular depletion charge regions near the source and drain are a very small fraction of the total depletion charge underneath the gate.
Narrow width effect:
Another related effect in MOSFETs is the narrow width effect, where the VT goes up as the channel width Z is reduced for very narrow devices. This can be understood from Fig. given below, where some of the depletion charges under the LOCOS isolation regions have field lines electrically terminating on the gate. Unlike the SCE, where the effective depletion charge is reduced due to charge sharing with the source/drain, here the depletion charge belonging to the gate is increased. The effect is not important for very wide devices, but becomes quite important as the widths are reduced below 1μm.