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ScienceWeek
APPLIED PHYSICS: AN END TO TRANSISTORS?
The following points are made by R.P. Cowburn (Science 2006 311 183:):
1) Today's digital microelectronic circuits are constructed from transistors that switch currents on and off to process the code and data associated with modern information technology. Transistors may not always take center stage, however, as new work [1] reports. As integrated circuits become ever more dense, the problems in building good transistors multiply. Most researchers attack this problem by advanced optimization of the materials and design of transistors, but Imre et al[1] are part of a group of researchers with a more radical solution: Get rid of the transistors. Imre et al[1] have experimentally demonstrated a universal logic gate, from which all of the logic functions needed in digital microelectronics can be constructed, that is based on magnetic nanostructures and uses no transistors.
2) Electrons possess the properties of both charge and spin. Charge is responsible for electricity and is the quantity sensed by the transistors in an integrated circuit. Spin, on the other hand, is responsible for magnetism and is not used in most integrated circuits. The blossoming field of "spintronics" seeks to make use of the spin of the electron in digital microelectronics [2]. Such a dramatic change at the microscopic level may necessitate an equally dramatic change in the top-level architecture of devices. This will be particularly the case for devices based upon the quantum mechanical interaction of single spins, but may well also be true even for spintronic devices built on classical ferromagnets, such as that proposed by Imre et al[1].
3) The architecture chosen by Imre et al[1] is based on the concept of cellular automata. Cellular automata are networks of cells with rules that describe how neighboring cells interact; they can, when correctly arranged, perform computations, as previously demonstrated by Amlani et al. using single-electron devices [3]. Although these devices were operational only at cryogenic temperatures, the results opened the tantalizing possibility of computation without conventional transistors, and hence a new approach to the continuation of scaling of microelectronics far into the future.
4) Five years ago, it was demonstrated that magnetic nanostructures could allow a physical implementation of a cellular automata architecture that would work at room temperature [4]: Quantum mechanical exchange within the nanostructure locks all of the spins together, forming a single giant macrospin of enormous moment and hence much greater thermal stability. As Imre et al[1] now show, not only can information propagate across a cellular automata device formed from magnetic nanostructures, but complete logic functions can also be implemented.[5]
References (abridged):
1. A. Imre et al., Science 311, 205 (2006)
2. S. A. Wolf et al., Science 294, [1488] (2001)
3. I. Amlani et al., Science 284, [289] (1999)
4. R. P. Cowburn, M. E. Welland, Science 287, [1466] (2000)
5. R. P. Feynman, Feynman Lectures on Computation, A. J. G. Hey, R. W. Allen, Eds. (Addison-Wesley, Reading, MA, 1996)
Science http://www.sciencemag.org
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APPLIED PHYSICS: ON FIELD-EFFECT TRANSISTORS
The following points are made by C.H. Ahn et al (Nature 2003 424:1015):
1) Our daily life is permeated by semiconducting field-effect transistors (FETs). About 10^(18) of these microscopic electronic switches are produced every year to run the tools that are indispensable to us, including our cars, computers, cellular phones and kitchen appliances. These devices work by modulating, through the application of an electric field, the electrical charge carrier density, and hence electrical resistance, of a thin semiconducting channel. This remarkably simple and very successful principle provides new opportunities for basic science and innovative device applications when applied to novel correlated electron systems whose properties depend strongly on the carrier concentration.
2) Excited by the opportunities of the field effect in such materials -- including organic conductors, high-temperature superconductors, and colossal magnetoresistance compounds --dozens of groups have worked on this approach, yielding a panoply of striking results. The burgeoning field of plastic electronics is a notable example that has reached technological fruition.
3) These opportunities probably served as a motivation for the research directions of the group at Bell Laboratories. Several of this group's articles in Nature and Science, for example, addressed electric-field-induced superconductivity in new materials. The public debate about the scientific misconduct that underlies a large part of this group's claims, however, may unfortunately and wrongly give the impression that the electric field effect in such materials does not exist.
4) In materials ranging from ionic insulators and elemental semiconductors to simple metals, the band structure model and associated Fermi liquid description provide a deep understanding of their fundamental physical properties. In this framework, insulators and intrinsic semiconductors have only filled and empty bands, whereas metals have at least one partially filled band. These concepts reach their limits, however, as the interactions between electrons become strong or the dimensionality of the system is reduced. When the electrostatic Coulomb interactions become dominant, localization occurs for commensurate filling (for example, 1/4 or 1/2 filling), resulting in a special type of insulator, frequently called the Mott insulator. One-dimensional (1D) GaAs quantum wires, organic Bechgaard salts, and undoped two-dimensional (2D) high-temperature superconductors are notable examples of such insulators.
5) In summary: Semiconducting field-effect transistors are the workhorses of the modern electronics era. Recently, application of the field-effect approach to compounds other than semiconductors has created opportunities to electrostatically modulate types of correlated electron behavior -- including high-temperature superconductivity and colossal magnetoresistance --and potentially tune the phase transitions in such systems.
Nature http://www.nature.com/nature
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SELF-ALIGNED, VERTICAL-CHANNEL, POLYMER FIELD-EFFECT TRANSISTORS
The following points are made by N. Stutzmann et al (Science 2003 299:1881):
1) Many advanced electronic device configurations, such as vertical transistors (1) and vertical-cavity surface-emitting lasers, require the formation of well-defined vertical sidewalls in functional multilayer structures. Most conventional techniques for fabrication of such sidewalls are based on photolithographic patterning followed by either reactive ion etching or anisotropic wet chemical etching. Application of these techniques to polymer multilayer structures is difficult because of plasma-induced degradation of electroactive polymers and the lack of anisotropic etching techniques for polymers.
2) Different patterning techniques for low-cost fabrication of solution-processible polymer field-effect transistors (FETs) have been demonstrated, including photolithographic patterning (2), screen printing (3), soft lithographic stamping (4), micromolding in capillaries (5), and high-resolution inkjet printing. These techniques allow accurate definition of polymer patterns with micrometer resolution, but they do not permit the formation of vertical sidewalls and, with some exceptions such as near-field photolithography, their extension to submicron resolution patterning is complex and becomes more expensive the higher the required resolutions.
3) Embossing is a nonlithographic patterning technique that has found widespread industrial use in the manufacture of diffraction gratings, compact disks, and security features such as holograms, but that is also capable of imprinting nanoscale patterns into single, sacrificial polymer layers that can be transferred subsequently into a functional layer by conventional etching. The LIGA technique (a German abbreviation for lithographic galvanic deposition) that is widely used for fabrication of micro-electrical-mechanical structures is based on the embossing of high-aspect-ratio structures in poly(methyl methacrylate) (PMMA). Direct laser-assisted imprinting of silicon surface layers has been demonstrated.
4) In summary: The manufacture of high-performance, conjugated polymer transistor circuits on flexible plastic substrates requires patterning techniques that are capable of defining critical features with submicrometer resolution. The authors used solid-state embossing to produce polymer field-effect transistors with submicrometer critical features in planar and vertical configurations. Embossing is used for the controlled microcutting of vertical sidewalls into polymer multilayer structures without smearing. Vertical-channel polymer field-effect transistors on flexible poly(ethylene terephthalate) substrates were fabricated, in which the critical channel length of 0.7 to 0.9 micrometers was defined by the thickness of a spin-coated insulator layer. Gate electrodes were self-aligned to minimize overlap capacitance by inkjet printing that used the embossed grooves to define a surface-energy pattern.
References (abridged):
1. S. M. Sze, Physics of Semiconductor Devices (Wiley, New York, ed. 2, 1981)
2. C. J. Drury, C. M. J. Mutsaers, C. M. Hart, M. Matters, D. M. d. Leeuw, Appl. Phys. Lett. 73, 108 (1998)
3. Z. Bao, Y. Feng, A. Dodabalapur, V. R. Raju, A. J. Lovinger, Chem. Mater. 9, 1299 (1997)
4. J. A. Rogers, Z. Bao, A. Makhija, P. Braun, Adv. Mater. 11, 741 (1999)
5. J. A. Rogers, Z. Bao, V. R. Raju, Appl. Phys. Lett. 72, 2716 (1998)
Science http://www.sciencemag.org
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