On “Effective Channel Length” in 0.1-µm MOSFET's

The conventional, 1-D definition of “effective channel length” (Leff) is examined in light of the spatial dependence of channel sheet resistance in 0.1-µm MOSFET's calculated from a 2-D device model. For short-channel devices, the sheet resistance deviates significantly from the uniform, long-channel behavior that Leff in general is different from the “metallurgical channel length,” Lmet. While geometrical (charge-sharing) effects tend to make Leff slightly shorter than Lmet, lateral source-drain doping gradients, especially when coupled with retrograde channel doping, can make Leff substantially longer than Lmet. The latter might help explain the apparent “excess” short channel effect often observed in 0.1-µm CMOS devices. © 1995 IEEE