Tag Archives: diode

PCM + ReRAM = OUM as XPoint

The good people at TECHINSIGHTS have reverse-engineered an Intel “Optane” SSD to cross-section the XPoint cells within (http://www.eetimes.com/author.asp?section_id=36&doc_id=1331865&), so we have confirmation that the devices use chalcogenide glasses for both the switching layer and the selector diode. That the latter is labeled “OTS” (for Ovonic Threshold Switch) explains the confusion over the last year as to whether this device is a Phase-Change Memory (PCM) or Resistive Random Access Memory (ReRAM)…it seems to be the special variant of ReRAM using PCM material that has been branded Ovonic Unified Memory or “OUM” (https://www.researchgate.net/publication/260107322_Programming_Speed_in_Ovonic_Unified_Memory).

As a reminder, cross-bar ReRAM devices function by voltage-driven pulses creating resistance changes in some material. The cross-bars allow for reading and writing all the bits in a word-string in a manner similar to Flash arrays.

In complete contrast, Phase Change Memory (PCM) cells—as per the name—rely upon the change between crystalline and amorphous material phases to alter resistance. The standard way to change phases is with thermal energy from an integrated set of heater elements. The standard PCM architecture also requires one transistor for each memory cell in a manner similar to DRAM arrays.

Then we have the OUM variant of PCM as previously branded by Energy Conversion Devices (ECD) and affiliated shell-campanies founded by tap-dancer-extraordinaire Stanford Ovshinsky (https://en.wikipedia.org/wiki/Stanford_R._Ovshinsky). So-called “Ovonic” PCM cells see phase-changes driven by voltage pulses without separate heater elements, such that from a circuit architecture perspective they are cross-bar ReRAMs.

Ovshinsky et al. successfully sold this technology to industry many times. In 2000, it was licensed to STMicroelectronics. Also in 2000, it was used to launch Ovonyx with Intel investment (http://www.eetimes.com/document.asp?doc_id=1176621), at which time Intel said the technology would take a long time to commercialize. In 2005 Intel re-invested (http://www.businesswire.com/news/home/20051019005145/en/Ovonyx-Receives-Additional-Investment-Intel-Capital). Finally in 2009, Intel and Numonyx showed a functional 64Mb XPoint test chip at IEDM (http://www.eetimes.com/document.asp?doc_id=1176621).

In 2007, Ovonxyx licensed it to Hynix (http://www.eetimes.com/document.asp?doc_id=1167173), and Qimonda (https://www.design-reuse.com/news/15022/ovonyx-qimonda-sign-technology-licensing-agreement-phase-change-memory.html), and others. All of those license obligations were absorbed by Micron when acquiring Ovonyx (https://seekingalpha.com/article/3774746-micron-tainted-love). ECD is still in bankruptcy (http://www.kccllc.net/ecd/document/list/3153).

So, years of R&D and JVs are behind the XPoint Optane(TM) SSDs. They are cross-bar architecture ReRAM arrays of PCM materials, and had the term not been ruined by 17-years of over-promising and under-delivering they would likely have been called OUM chips. Many others tried and failed, but Intel/Micron finally figured out how to make commercial gigabit-scale cross-bar NVMs using OUM arrays. Now they just have to yield the profits…


Nakamura Co-Wins Nobel for Blue LEDs

The Nobel Prize in Physics 2014 was awarded jointly to Isamu Akasaki, Hiroshi Amano, and Shuji Nakamura “for the invention of efficient blue light-emitting diodes which has enabled bright and energy-saving white light sources”. In the late 1980s red and green LEDs had been around for decades, but despite large programs in both academia and industry there had been almost no R&D progress in blue LEDs (this editor did process R&D in an LED fab in that era). Then Akasaki and Amano at the University of Nagoya showed work on improved p-doping in GaN due to electron irradiance, leading to p-n junctions to make diodes.

Structure of a blue LED with a InGaN/AlGaN double heterojunction [Source: S. Nakamura, T. Mukai & M. Senoh, Appl. Phys. Lett. 64, 1687 (1994)].

Structure of a blue LED with a InGaN/AlGaN double heterojunction (Source: S. Nakamura, T. Mukai & M. Senoh, Appl. Phys. Lett. 64, V1687, 1994).

From 1989 to 1994, Shuji Nakamura worked at Nichia Chemicals in Tokushima, Japan where he led a small team of co-workers to achieve a quantum efficiency of 2.7% using a double heterojunction InGaN/AlGaN (see Figure). With these important first steps, the path was cleared towards the development of efficient blue LEDs and solid-state white lighting. Nakamura-sensei is now a Professor of Physics at the University of California, Santa Barbara, and co-founder of Soora Corp. where GaN-on-GaN technology is used to increase efficiency through the elimination of the buffer-layers needed with saphhire substrates. The “Tales of Nakamura” article at IEEE Spectrum provides an excellent summary of this extraordinary man’s life story, including the US$600M payout from Nichia that was reduced to US$8M by a higher court.
Incandescent light bulbs lit the 20th century; the 21st century will be lit by LED lamps with high lm/W efficiency. The most recent record is just over 300 lm/W, which can be compared to 16 for regular light bulbs and close to 70 for fluorescent lamps. As about one fourth of world electricity consumption is used for lighting purposes, the LEDs contribute to saving the Earth’s resources.
Shine on!