If a very thin fin is used hard limits of gate length scaling are reached. The off-state current will be comprised by terms such as quantum mechanical tunneling or band-to-band tunneling .
Short channel effects can be controlled if [168,180,181]. When the fin width is reduced, leakage currents in the middle of the fin are reduced. Thus, the off-state current is reduced and short channel effects are minimized as shown in Fig. 4.31. The fat point denotes the device analyzed in Section 4.4.2. To obtain a lower off-state current, the device width must be reduced. E.g., a smaller device with a comparable off-state current must have a fin width of and a printed gate length of approximately .
The device is very sensitive to the gate underlap and the lateral source/drain doping gradient  of the doping profile at the junctions. For the FinFET simulations a source/drain doping concentration of has been assumed. The doping profile at the junctions have a Gaussian shape. The source/drain gradient defined in Fig. 4.32 according to  was set to . For all simulations a gate underlap of was used. Therefore the effective gate length is assumed to be fixed at .
Fig. 4.33 shows the dependency of the off-state current on the lateral source/drain gradient. A higher source/drain gradient reduces the drain induced barrier lowering effect [182,183] and therefore the off-state current. For a fixed off-state current smaller gate lengths are possible in combination with a higher source/drain gradient. Nevertheless, a higher source/drain gradient introduces a series resistance which may severely degrade the on-state current .
Robert Klima 2003-02-06