Physical Aspects of Computation
In this chapter, the authors address perhaps the most practical of obstacles: the constraints that arise from the technology they employ to actually build machines — both from the materials use and from the way in which people arrange the elementary component parts. They discuss that Feynman's unique ability to offer valuable physical insight into complex physical processes still comes through. In actual circuits, the clocks are much slower than the transistors, and so clocking enables people to save a great deal of energy in computations. The single Metal Oxide Semiconductor Field Effect device therefore operates as a NOT gate, since it just flips the input signal. Annihilation solves the problem by letting the electrons in the p-type material fill in holes rather than be stored at increasingly great distances from the junction. The Complementary Metal Oxide Semiconductor inverter can also serve as a useful simple 'laboratory' for investigating some of the energetics of logic gates.