with 16 nm FinFET devices, the nature of the VTH variation changes. On planar CMOS, random dopant fluctuations (RDFs) had the biggest influence.
|Possible VTH Variations|
Ideal FinFETs have no doping, and therefore no RDFs. However, this only works for low voltage operation. We also need high-speed devices, which requires multiple threshold voltages (Vth's). For FinFETs, this requires doping leading to RDF variation.