9.6. IMPORTANT ISSUES IN REAL MOSFETS 475
n+ n+n+ n+Inversion charge
S(a)p-typeVGS
L
VDS
VDS = VDS(sat)Inversion charge
S(b)p-typeVGS
L
VDS
VDS > VDS(sat)L
Figure 9.27: (a) A schematic of the MOSFET channel whenVDS=VDS(sat). (b) A schematic
showing the decrease in the effective channel length forVDS>VDS(sat).
9.6.3 Important Effects in Short-Channel MOSFETs ............
Advances in lithographic techniques are allowing MOSFET channel lengths to shrink to sizes
below 1.0μm. Experimental devices with channel lengths smaller than 0.1μm have been fabri-
cated. The force for miniaturization is coming from the need for dense circuits for high-density
memory and logic applications as well as from the need for high-frequency microwave devices.
In short-channel devices the simple models we developed for the current-voltage characteristics
become quite invalid for quantitative description. In addition to the effects discussed in the pre-
vious subsection for long-channel devices, specific issues relating to short-channel devices also
play an important role. Important issues that need to be considered areVTlowering; surface
scattering, velocity saturation and overshoot; hot carrier generation, impact ionization and drain
induced barrier lowering and punch through. Some of these are now discussed.
Gate Leakage
As gate lengths are reduced, gate oxide thickness must also be reduced to maintain a constant
aspect ratio. Currently, the SiO 2 gates in MOSFETs are only a few monolayers thick. Future de-
vices will require high-K dielectric gates, as shown in figure 9.30, which allows for the physical
thickness to be large while maintaining a small equivalent oxide thickness,d ̃ox