Given the current limitations with CMOS designs, such as low temperature thresholds and efficiency in power consumption, there is a vast need to expand into superconducting computers in order to manage consumers’ need for power and performance. Although supercomputers require extremely low temperatures, they are capable of considerably higher Floating Point Operations per Second (FLOPS) with similar power requirements. The current goal within superconductive computing is to reach an exaFLOPS, however petaFLOPS are the best presently attainable. In order to keep the superconductive metal oxides running at optimal temperatures, cryogenics were introduced.
IARPA has contracted IBM, Raytheon-BBN and Northrop Grumman to develop a small, yet scalable, superconducting computer in the cryogenic computing complexity (C3) program in order to expand the current capacities of high performance computing focusing on cryogenic memory and logic, communications, and systems (Keller, 2014).
Operating electronics at cryogenic temperature improves their performance, as well as lowering noise, allowing them to run at higher speeds, and increases efficiency (Kirschman, 2009). In superconducting computers, the use of super-cooled copper wire has the ability to allow current flow almost indefinitely. Therefore, the switching voltage is significantly reduced (Pop, 2014). The threshold voltage is the minimum gate source voltage that is needed to create a conducting path between the source and drain terminals. Superconducting computers operating at low temperatures require significantly lower threshold voltage. If the design begins to heat up, the supply voltage will not reach threshold voltage which results in sub-threshold leakage. In a device, when high temperatures are reached, leakage in p–n junctions become excessive which may render the device useless (Krischman, 2003).
Reducing the threshold voltage also reduces the heat of resistivity and capacitance which allows for smaller designs without the risk of leakage. Thus, engineers introduced the idea of cryogenic memory. In the beginning stage of the C3 program, the three teams will develop components for the memory and subsystems (Keller, 2014). Cryogenic memory is still in its infancy; however, this program is designed to flesh out its varying possibilities for high performance computing. While not much is currently known, cryogenic memory allows for memory and caches capable of supporting the processing power by the CPU of a superconducting computer (Anthony, 2014). With the use of recent ideas of energy efficient cryogenic memory and superconducting logic without static dissipation, the teams will use these ideas to meet the energy demands of today’s high-performance computers (Keller, 2014).
Liquid nitrogen, as a means of cooling both components and memory, is most readily available and is capable of temperatures below -196 C; however, it is not quite cold enough to reduce resistivity to zero (Kross). While liquid nitrogen is effective, there are other alternate sources for cooling. Liquid helium offers the coldest but is finite and not as abundant as nitrogen. Liquid hydrogen can also reach colder temperatures than liquid nitrogen; however, it is combustible at one thousand and sixty five degree Fahrenheit (NOAA). With these draw backs, liquid nitrogen proves to be the cheapest and easiest to obtain material to cool these super computers (Kross). Nonetheless, to progress further there needs to be some better way to cool superconducting metals more efficiently without risking an explosion.
Apart from expanding cryogenic memory, the second main focus of this program is the logic, communications, and systems desired to build superconducting logic circuits that demonstrate the potential of the technology for high-performance computing (Keller, 2014).
As designs are getting smaller the supply voltage was lowered in order to reduce electric fields within the design (Stockinger, 2001). Reducing electric fields mitigates the impact of wires in close proximity. Compared to semiconducting computers, such as CMOS designs, getting temperatures low enough to reduce resistivity would also cause the design to break down (Krischman, 2003). One reason for using Silicon is because of its durability compared to other semiconductors. For example, compared to Germanium, Silicon is used more in electronics because it can withstand higher temperatures (GSU, 2000). However, as temperatures decrease towards approximately 40 K for Silicon, an effect called freeze-out begins to occur. Freeze-out is where dopants are not sufficiently ionized which causes a defined lack of carriers. Dopants usually require some thermal energy in order to ionize and produce carriers in semiconductors; therefore, significantly cooling the superconductive metal oxides becomes the major challenge (Krishcman, 2003).
Electronics has come a long way since the first computers, but we still have not met our goals yet. Right now engineers are trying to achieve speeds that process at the rate of the human brain. Without proper cooling methods and power distribution, achieving a supercomputer of this caliber will be difficult.
By Aaron Carnahan and Thomas Garner
The University of Mississippi Electrical Engineering Department introduced a Digital CMOS/VLSI Design course this semester. As part of this course, students researched a contemporary issue and wrote a blog article about their findings for presentation on SemiWiki. Your feedback is greatly appreciated.
Georgia State University. (2000). Silicon and Germanium. Retrieved from http://hyperphysics.phy-astr.gsu.edu/hbase/solids/sili.html
Pop, Sebastian. (2014). Cryogenic Memory and Superconductors Allow Supercomputer to Reach 1 Exaflop. Retrieved from http://news.softpedia.com/news/Mysterious-Supercomputer-Will-Use- Cryogenic-Memory-and-Superconductors-to-Reach-1-Exaflop-466602.shtml
Kross, Brian. Is there anything colder than liquid nitrogen?. Retrieved From http://education.jlab.org/qa/liquidnitrogen_02.html
Keller, John. (2014). Beyond CMOS: three industry teams aim at next generation of high- performance computing (HPC). Retrieved from http://www.militaryaerospace.com/ articles/2014/12/iarpa-c3-contracts.html
Kirschman, Randall. (2009). Cryogenic Electronics. Retrieved from http://www.cryogenicsociety.org/resources/cryo_central/cryogenic_electronics/
NOAA. HYDROGEN, REFRIGERATED LIQUID (CRYOGENIC LIQUID). Retrieved from http://cameochemicals.noaa.gov/chemical/3606