|Year : 2010 | Volume
| Issue : 2 | Page : 179-180
On the Discovery of a Polarity-Dependent Memory Switch and/or Memristor (Memory Resistor)
Department of Physics, Banaras Hindu University, Varanasi - 221 005, India
|Date of Web Publication||27-Feb-2010|
Department of Physics, Banaras Hindu University, Varanasi - 221 005
|How to cite this article:|
Chandra S. On the Discovery of a Polarity-Dependent Memory Switch and/or Memristor (Memory Resistor). IETE Tech Rev 2010;27:179-80
In a recent editorial of IETE Technical Review, Jagadesh Kumar  has rightly pointed out the importance of a novel experimental device (a new passive circuit element apart from R, L, C) identified by Strukov et al. as a memristor (memory resistor). This was predicted by Chua  theoretically long back. The device is characterized by a Figure of eight (8)-type I-V characteristics as a consequence of the device resistance "remembering" the last current (or charge) passed through it. Strukov et al.  emphasized the use of a nano-semiconductor (TiO 2 ) for attaining this characteristic, where the resistance of TiO 2 could be modulated by superimposing O 2- ionic transport which can be stopped as such after the removal of voltage (so as to remember the charge transport). This is the basis of its memristor-type behavior. The purpose of this note is to point out that such an experimental device (termed as polarity-dependent memory switch) had already been reported in mid-90s by our group in India  , which Strukov et al. missed to mention in their paper (private communication with the senior author Dr. R. S. Williams  ). We had clearly reported the basic important features which Strukov et al. reported only recently, namely, (a) a figure of "8"-type I-V characteristics akin to a memory switch; (b) hysteresis at the initial stage of the making of the device; (c) retention of "OFF" as well as "ON" states for many days; and (d) invoking the concept of superposition of ionic transport and electron transport to explain the memory effect. We had given a detailed phenomenological explanation while Strukov et al. have given an analytical model.
The device developed by Strukov et al. used nano-TiO 2 whose conductivity could be modulated by O 2- ion motion. Our device Ti/Cd x Pb 1-x S/Ag used Pb 2+-doped CdS (and Ag as one of the electrodes). Pb 2+ was chosen as the dopant which, apart from modifying the band gap of CdS, was expected to lead to the superposition of Pb 2+ ion motion over the overall electron transport. Interestingly, our device Ti/Cd x Pb 1-x S/Ag showed polarity-dependent memory switching but no such switching was observed by us in CdS without Pb 2+ doping. All the major concepts and experimental findings of Strukov et al.  are strikingly similar, and were unambiguously given in detail in our paper , including the retention of memory for many days. A brief comparison of results of Chandra's group and William's group is given in [Figure 1].
The explanation offered by us involved "ion motion" (metallic cations Pb 2+) while Strukov et al. invoked the vacancy-assisted O 2- ion motion in TiO 2. Strukov et al. called their device as "memristor (memory resistor)" while we called our device as "polarity-dependent memory switch." Strukov et al. emphasized the use of nano-sized TiO 2 for getting such memory devices while we were able to get similar characteristics in "film" devices. In the analytical model  for a "memristor," the crucial contribution for realizing the memristor-type switching behavior comes from the term (µv R on )/D2 whereµv is the dopant ion mobility, Ron is the device resistance in the "ON" state, and D is the semiconductor film thickness. Strukov et al. decreased D to nano-size. The likely higher ion mobilityµv in our case possibly helped in the realization of the switching behavior even in film devices. In view of the above, it is claimed that, in mid-1990s, "polarity-dependent memory switching device in the film form" had already been reported by us with characteristics similar to what was theorized by Chua  for a (till then) hypothetical device "memristor." Further, it is suggested that the use of a nano-semiconductor is not a necessary criterion.
The author thanks National Academy of Sciences, India, for providing grants for the Platinum Jubilee Senior Scientist position.
| Authors|| |
Suresh Chandra is currently working as Platinum Jubilee Senior Scientist of National Academy of Sciences, India and is associated with the Physics Department, Banaras Hindu University, Varanasi, India. He was Lecturer (1958-1964) in Allahabad University, Reader at Gorakhpur University (1972-1979), Professor and Head of Physics Department, Pandit Ravishankar Shukla University, Raipur (1972-1979). Subsequently, he moved to the Department of Physics, Banaras Hindu University as Professor (1979-1998) and held many positions like Head of Department; Co-ordinator, Centre of Advanced Studies and Dean of the Science Faculty. Since superannuation in 1998, he is serving as Emeritus Scientist/Fellow of C.S.I.R./U.G.C. and now of National Academy of Sciences, India. Prof. Chandra worked as Post Doctoral Fellow/Visiting Professor in National Research Council (Ottawa), University of California (Santa Barbara), University of Pennsylvania, University of Buenos Aires, Nagoya University etc.
Prof. Chandra's research interest includes Microwaves, Liquid Crystals, Solid State Ionics, Semiconductor Physics etc. He has published more than 220 research papers and supervised 42 Ph.D. students. He has authored three Books: (i) Superionic Solids: Principles and Applications, North Holland Publication. (ii) Photoelectrochemical Soalr Cells, Gordon and Breach Science Publication. (iii) Handbook of Electrodeposition of Semiconductors, Marcel and Dekker. Apart from these, he has five Edited Books published by Academic Press, World Scientific, Allied Press and MaCmillan. He has won many awards and distinctions for his contribution to teaching and research. He has also served on the Editorial Boards of many journals.
| References|| |
|1.||M.J. Kumar. "Memristor-Why do we have to know about it". IETE Technical Review, vol. 26, pp. 3-6, 2009. |
|2.||L.O. Chua. "Memristor- The Missing Circuit Element". IEEE Transaction on Circuit Theory. vol. 18, pp. 507-19, 1971. |
|3.||D.B. Strukov, G.S. Snider, D.R. Stewart, and R.S. Williams. "The Missing Memristor found". Nature, vol. 453, pp. 80-3, 2008. |
|4.||H.M. Upadhyaya, and S. Chandra. "Polarity dependent memory switching behaviour in Ti/Cd x Pb 1-x S/Ag system" Semiconductor Science and Technology, vol.10, pp. 332-8, 1995. |
|5.||H.M. Upadhyaya, D.P. Singh, and S. Chandra. "Memory Switching in Ti/Cd x Pb 1-x S/Ag due to Pb 2+ motion" In: Chowdari B.V., Chandra S., Singh S., and Srivastava P.C., editors. "Solid State Ionics: Materials and Applications" World Scientific, Singapore; 1992. [PUBMED] |