4.4.4 Gate Length Scaling

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 [180].

Short channel effects can be controlled if $W_{\mathrm{fin}} < 0.7\,L_{\mathrm{gate}}$ [168,180,181]. When the fin width $W_{\mathrm{fin}}$ 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 $W_{\mathrm{fin}} = 20\,{\mathrm{nm}}$ and a printed gate length of approximately $L_{\mathrm{gate}} = 42\,{\mathrm{nm}}$.

The device is very sensitive to the gate underlap ${\Delta}L$ and the lateral source/drain doping gradient [180] of the doping profile at the junctions. For the FinFET simulations a source/drain doping concentration of $N_{\mathrm{D}}=10^{20}\,{\mathrm{cm}}^{-3}$ has been assumed. The doping profile at the junctions have a Gaussian shape. The source/drain gradient defined in Fig. 4.32 according to [180] was set to $1\,{\mathrm{nm}}/{\mathrm{dec}}$. For all simulations a gate underlap of ${\Delta}L = 4\,{\mathrm{nm}}$ was used. Therefore the effective gate length is assumed to be fixed at $L_{\mathrm{eff}}=L_{\mathrm{gate}}-2\,{\Delta}L=52\,{\mathrm{nm}}$.

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 [180].

Figure 4.31: Impact of the gate length and body thickness on the off-state current at $V_{{\textrm {DS}}}=1.0{\hspace {.35ex}}{\textrm {V}}$.

Figure 4.32: Source/drain gradient.

Figure 4.33: Impact of the source/drain gradient on the off-state current. $V_{{\textrm {DS}}}=1.0{\hspace {.35ex}}{\textrm {V}}$, ${\Delta }L = 4{\hspace {.35ex}}{\textrm {nm}}$.