Recent advances in VLSI memories have enabled integration of 10 to 30 million devices on prototype chips for 16 Mbit DRAMs, 16 Mbit EPROMs, and 4 Mbit SRAMs. An experimental 64 Mbit DRAM recently reported clearly shows that an integration density of more than 100 million devices on a chip will be feasible in the near future. These advances have been made not only by progress in fine processing technology, but also by the development of three-dimensional memory cells such as trench capacitor cells and stacked capacitor cells for DRAMs and polysilicon PMOS load cells for SRAMs. Various circuit innovations to increase the signal-to-noise ratio and circuit speed have been, and will continue to be, essential. Future circuits will be required to operate at very low voltages, and the prototype 64 Mbit DRAM has shown that an operating voltage as low as 1.5 volts is feasible. Improvements in packaging technology for reducing package volume and footprint area, as well as for production of multipackage modules, are also becoming more and more important.

The advent of personal data processing and communication, coupled with device miniaturization, has been pushing ULSI's toward lower voltage operation. However, as long as the silicon ULSI circuits are used around room temperature, there will be a limit. Various approaches to reducing the operating voltage will be discussed, with special emphasis placed on device physics, and analog and digital circuit configurations. The low-energy limit is discussed with a comparison to a biological system in order to develop enlightening implications to the future directions.