Dennard scaling
Beginner
The old rule that as transistors got smaller they also used proportionally less power, so chips could get faster without getting hotter.
Novice
The 1974 observation that shrinking a transistor’s dimensions and its voltage by the same factor keeps the power per square millimeter constant while making circuits faster. It held until the mid-2000s.
Expert
Scaling dimensions, voltage and current by gives delay , power per circuit and constant power density. It broke when threshold and supply voltage stopped falling, because of leakage, so energy per transistor no longer falls as fast as transistor count rises.