Stacked CMOS marks the end of the roadmap (for now)

Digital circuitry hasn’t always been synonymous with CMOS. Early MOSFET-based logic used either n-type or p-type transistors exclusively to create NMOS and PMOS, respectively. NMOS technology, in particular, proved a popular choice due to its relative simplicity and fast switching. ‘Always-on’ high power consumption weighed it down, however, and it was replaced by CMOS in the 1980s. The latter technology creates complementary pairs of n and p-type transistors, which (ideally) only draw power during a switching operation.

And so, for roughly four decades now, n and p-type switches have been intimately placed together. Mind you, not too closely together, as to prevent capacitance issues from decreasing performance and increasing power consumption. Soon, however, there will be no way around it: in the ever-continuing efforts to increase component density, n en p-type transistors must come closer. Ultimately, they’ll even be stacked on top of each other.