Until the early 90's, the standard supply voltage of digital circuits was kept at 5V, and constant-voltage scaling was the common strategy for developing new, faster technology generations [89]. Although a reduction of the feature size at constant supply voltage would not only increase the speed of the device but also reduce the energy per logic operation, the total power dissipation per chip area is raised as the number of devices on the chip and the clock frequencies increase. While prower consumption was not a primary concern in the early years of integrated circuits a moderate reduction of standard supply voltages to 3.3V, 2.5V, and most recently 1.8V has brought some improvement. Yet there is still ample room for improvement and a fast growing market for portable electronics with a long battery life has kept the question of power consumption open
Other critical issues associated with high supply voltages and thermal power dissipation are various severe reliability problems which force technological compromises that also have a negative effect on the performance. This exacerbates the problems associated with thermal power dissipation and battery life in portable equipment even more.