CURRENT TRENDS ON NANO ELECTRONICS

Authors: Yaram Narasimhareddy,V.Chandarasekhara Raju,M.V.SubbaRao

ABSTRACT: Nanoelectronics is potentially one of the branches of Nanotechnology with the most significant commercial impact and covers a very wide range of interdisciplinary areas of research and development such as telecommunications, automotive, multimedia, consumer goods and medical systems. The emergence of new research directions such as Hybrid molecular electronics, One dimensional structures such as nanowires, Nano-electromechanical-systems (NEMS) or Carbon Nanotubes (CNT) will strategically impact on future developments in the nanoelectronics domain and their long-term applications. Nanotubes, Nanocapsules, Nanotextiles, Stretchable silicon, Nanoelectronic displays and difficult problems in nanotechnology are office without the need for wires. The computers connect to the network using radio signals and computers can be up to 100 feet or so apart. Utilizing the well understood chemical properties of atoms & molecules, nanotechnology proposes the construction of novel molecular devices possessing extraordinary properties. The single electron transistor or SET is a new type of switching device that uses controlled electron tunneling to amplify current. At last, the SET presents that it is the different construction is which is based on helical logic, atomic scale motion of electrons in an applied rotating electric field.

INTRODUCTION: Nanoelectronics refer to the use of nanotechnology on electronic components, especially transistors. Although the term nanotechnology is generally defined as utilizing technology less than 100 nm in size, nanoelectronics often refer to transistor devices that are so small that inter-atomic interactions and quantum mechanical properties need to be studied extensively. As a result, present transistors do not fall under this category, even though these devices are manufactured with 45 nm, 32 nm or 22 nm technology. Nanoelectronics are sometimes considered as disruptive technology because present candidates are significantly different from traditional transistors. Some of these candidates include: hybrid molecular/semiconductor electronics, one dimensional nanotubes/nanowires, or advanced molecular electronics.

CARBON NANOTUBES: 100 amps of electricity crackle in a vacuum chamber, creating a spark that transforms carbon vapor into tiny structures. Depending on the conditions, these structures can be shaped like little, 60-atom soccer balls, or like rolled-up tubes of atoms, arranged in a chicken-wire pattern, with rounded ends. These tiny, carbon nanotubes, discovered by Sumio Iijima at NEC labs in 1991, have amazing properties. They are 100 times stronger than steel, but weigh only one-sixth as much! They are incredibly resilient under physical stress; even when kinked to a 120-degree angle, they will bounce back to their original form, undamaged. And they can carry electrical current at levels that would vaporize ordinary copper wires.Moore's Law states that the number of transistors that can be placed on a silicon chip will double every 18 months. Scientists have been predicting the breakdown of this law, however, because the physical limits of techniques used to create silicon chips are being pushed to the extreme. Soon, the wires needed to build such densely populated chips will have to be finer than those capable through photolithographic technology. One contender for new electronics materials is carbon nanotubes. They can be manufactured to be only 1or 2 nm in diameter and several micrometers in length. Structural relatives to fullerenes, carbon nanotubes have been the object of theoretical work for the past few years. One of the fascinating characteristics of this class of carbon compounds is their dual nature depending on their diameter, they can function either as conductors or semiconductors.Another problem with using carbon nanotubes is placing them on a surface so that they go in a particular direction. Several alignment methods of nanotubes have been tried, such as chemical vapor deposition, epoxy resin clipping, film rubbing, and carbon arc discharge. M. Fujiwara and colleagues at Hiroshima University have reported a method for using magnetic fields to orient nanotubes in an April 12 ASAP.

NANO CAPSULES: the ideal case for optimal activity would be to entrap the ODNs within the internal core of polymeric nanocapsules in order to mask them and to prevent them from any interaction with proteins. In the state of the art, all the methodologies available to prepare nanocapsules involve the preparation of emulsions, either O/W emulsions which lead to nanocapsules with an oily core suspended in water (Al Khouri process) or W/O emulsions which lead to nanocapsules with an aqueous core suspended in oil (Vranckx). Oily nanocapsules are unable to encapsulate the water soluble ODNs and aqueous nanocapsules in an oily phase are not compatible with the conditions for i.v. administration. This is the reason why, in this study, we have developed a new process of preparation of aqueous nanocapsules containing ODNs which were successfully suspended in a water medium. We have localized the ODNs in the core of these nanocapsules which explains why this carrier is providing a high protection of ODNs against enzymatic degradation.

STRETCHABLE SILICON COULD BE NEXT WAVE IN ELECTRONICS: Researchers at the University of Illinois at Urbana-Champaign have developed a fully stretchable form of single-crystal silicon with micron-sized, wave-like geometries that can be used to build high-performance electronic devices on rubber substrates. "Stretchable silicon offers different capabilities than can be achieved with standard silicon chips,", stretchable and bendable electronics could be used in applications such as sensors and drive electronics for integration into artificial muscles or biological tissues, structural monitors wrapped around aircraft wings, and conformable skins for integrated robotic sensors, said Rogers To create their stretchable silicon, the researchers begin by fabricating devices in the geometry of ultrathin ribbons on a silicon wafer using procedures similar to those used in conventional electronics. Then they use specialized etching techniques to undercut the devices. The resulting ribbons of silicon are about 100 nanometers thick - 1,000 times smaller than the diameter of a human hair. In the next step, a flat rubber substrate is stretched and placed on top of the ribbons. Peeling the rubber away lifts the ribbons off the wafer and leaves them adhered to the rubber surface. Releasing the stress in the rubber causes the silicon ribbons and the rubber to buckle into a series of well-defined waves that resemble an accordion. "The resulting system of wavy integrated device elements on rubber represents a new form of stretchable, high-performance electronics,- "The amplitude and frequency of the waves change, in a physical mechanism similar to an accordion bellows, as the system is stretched or compressed. -œAs a proof of concept, the researchers fabricated wavy diodes and transistors and compared their performance with traditional devices. Not only did the wavy devices perform as well as the rigid devices, they could be repeatedly stretched and compressed without damage, and without significantly altering their electrical properties.- These stretchable silicon diodes and transistors represent only two of the many classes of wavy electronic devices that can be formed "In addition to individual devices,complete circuit sheets can also be structured into wavy geometries to enable stretchability.

SINGLE ELECTRON TRANSISTOR: The SET transistor can be viewed as an electron box that has two separate junctions for the entrance and exit of single electrons (as in figure ). It can also be viewed as a field-effect transistor in which the channel is replaced by two tunnel junctions forming a metallic island. The voltage applied to the gate electrode affects the amount of energy needed to change the number of electrons on the island. The SET transistor comes in two versions that have been nicknamed "metallic" and "semiconducting". These names are slightly misleading, however, since the principle of both devices is based on the use of insulating tunnel barriers to separate conducting electrodes. In the original metallic version, a metallic material such as a thin aluminium film is used to make all of the electrodes. The metal is first evaporated through a shadow mask to form the source, drain and gate electrodes. The tunnel junctions are then formed by introducing oxygen into the chamber so that the metal becomes coated by a thin layer of its natural oxide. Finally, a second layer of the metal - shifted from the first by rotating the sample - is evaporated to form the island. In the semiconducting versions, the source, drain and island are usually obtained by "cutting" regions in a two-dimensional electron gas formed at the interface between two layers of semiconductors such as gallium aluminium arsenide and gallium arsenide. In this case the conducting regions are defined by metallic electrodes patterned on the top semiconducting layer. Negative voltages applied to these electrodes deplete the electron gas just beneath them, and the depleted regions can be made sufficiently narrow to allow tunneling between the source, island and drain. Moreover, the electrode that shapes the island can be used as the gate electrode. In this semiconducting version of the SET, the island is often referred to as a quantum dot, since the electrons in the dot are confined in all three directions. Indeed, it has been possible to construct a new periodic table that describes dots containing different numbers of electrons.

OPERATION OF SET TRANSISTOR: The key point is that charge passes through the island in quantized units. For an electron to hop onto the island, its energy must equal the Coulomb energy e2/2C. When both the gate and bias voltages are zero, electrons do not have enough energy to enter the island and current does not flow. As the bias voltage between the source and drain is increased, an electron can pass through the island when the energy in the system reaches the Coulomb energy. This effect is known as the Coulomb blockade, and the critical voltage needed to transfer an electron onto the island, equal to e/C, is called the Coulomb gap voltage. Now imagine that the bias voltage is kept below the Coulomb gap voltage. If the gate voltage is increased, the energy of the initial system (with no electrons on the island) gradually increases, while the energy of the system with one excess electron on the island gradually decreases. At the gate voltage corresponding to the point of maximum slope on the Coulomb staircase, both of these configurations equally qualify as the lowest energy states of the system. This lifts the Coulomb blockade, allowing electrons to tunnel into and out of the island. The Coulomb blockade is lifted when the gate capacitance is charged with exactly minus half an electron, which is not as surprising as it may seem. The island is surrounded by insulators, which means that the charge on it must be quantized in units of e, but the gate is a metallic electrode connected to a plentiful supply of electrons. The charge on the gate capacitor merely represents a displacement of electrons relative to a background of positive ions.

COUNTING ELECTRONS WITH SET:If we further increase the gate voltage so that the gate capacitor becomes charged with -e, the island again has only one stable configuration separated from the next-lowest-energy states by the Coulomb energy. The Coulomb blockade is set up again, but the island now contains a single excess electron. The conductance of the SET transistor therefore oscillates between minima for gate charges that are integer multiples of e, and maxima for half-integer multiples of e (figure 3).Accurate measures of charge Such a rapid variation in conductance makes the single-electron transistor an ideal device for high-precision electrometry. In this type of application the SET has two gate electrodes, and the bias voltage is kept close to the Coulomb blockade voltage to enhance the sensitivity of the current to changes in the gate voltage. The voltage of the first gate is initially tuned to a point where the variation in current reaches a maximum. By adjusting the gate voltage around this point, the device can measure the charge of a capacitor-like system connected to the second gate electrode. A fraction of this measured charge is shared by the second gate capacitor, and a variation in charge of ¼e is enough to change the current by about half the maximum current that can flow through the transistor at the Coulomb blockade voltage. The variation in current can be as large as 10 billion electrons per second, which means that these devices can achieve a charge sensitivity that outperforms other instruments by several orders of magnitude.

The precision with which electrons can be counted is ultimately limited by the quantum delocalization of charge that occurs when the tunnel-junction conductance becomes comparable with the conductance quantum, 2e2/h. However, the current through a SET transistor increases with the conductance of the junctions, so it is important to understand how the single-electron effects and Coulomb blockade disappear when the tunnel conductance is increased beyond 2e2/h.

Towards room temperatureUntil recently single-electron transistors had to be kept at temperatures of a few hundred millikelvin to maintain the thermal energy of the electrons below the Coulomb energy of the device. Most early devices had Coulomb energies of a few hundred microelectronvolts because they were fabricated using conventional electron-beam lithography, and the size and capacitance of the island were relatively large. For a SET transistor to work at room temperature the capacitance of the island must be less than 10-17 F and therefore its size must be smaller than 10 nm.Researchers have long considered whether SET transistors could be used for digital electronics. Although the current varies periodically with gate voltage - in contrast to the threshold behaviour of the field-effect transistor - a SET could still form a compact and efficient memory device. However, even the latest SET transistors suffer from "offset charges", which means that the gate voltage needed to achieve maximum current varies randomly from device to device. Such fluctuations make it impossible to build complex circuits. One way to overcome this problem might be to combine the island, two tunnel junctions and the gate capacitor that comprise a single-electron transistor in a single molecule - after all, the intrinsically quantum behaviour of a SET transistor should not be affected at the molecular scale. In principle, the reproducibility of such futuristic transistors would be determined by chemistry, and not by the accuracy of the fabrication process. Only one thing is certain: if the pace of miniaturization continues unabated, the quantum properties of electrons will become crucial in determining the design of electronic devices before the end of the next decade.

MAKING NANO ELECTRONICS FOR DISPLAYS: Today's flat-screen LCD televisions are made in enormous, expensive chambers in which the electronics that control individual pixels in the display are formed on large slabs of glass.Improving LCDs is only the first step. the technique could make it feasible to build televisions using bright and colorful light emitting diodes (LEDs) of the type used in the enormous screens at sports arenas. Because the printing method would make it easier to integrate the materials needed,the LEDs could be much smaller and more tightly packed than these large-format displays. And since the printing technique can make high-performance devices on flexible substrates, it could pave the way to roll-up LED displays.The ability to print onto a curved surface could also make it possible to mimic the compact structure of the human eye, which could lead to smaller night-vision equipment.

DIFFICULT PROBLEMS IN NANOTECHNOLOGY: 1. Precise and arbitrary manipulation and positioning of large numbers of nanostructures. 2. The development of improved techniques for multi-scale modeling (i.e., unified approaches for the modeling across multiple scales of extended systems of nanostructures). 3. The design, fabrication, and demonstration of an extended nanocomputer system that is integrated on the molecular scale (i.e., the nanometer scale), including both an ultra-dense nanoprocessor and an ultra-dense nanomemory array. 4. Development of techniques for imaging atomic-scale features in real time under a wide range of conditions. 5. Better understanding of the health issues related to nanoparticles and other nanostructures. 6. Bulk synthesis or bulk separation of carbon nanotubes with controlled chirality's. 7. Improved theories for understanding and predicting physical processes (chemical reactions, atomic transport, crystal structures, etc.) at nanometer length scales. 8. Although nanoelectronic technology holds promise for the future, it is still under development and practical applications are unlikely to emerge in the near future.

CONCLUSION: In summary, research in the field of bio-molecular nanoelectronics bears a huge potential for both fundamental understanding and technological exploitation in several aspects related to human development, from medical diagnostics/therapy tocomputation and a variety of (opto)electronic devices.It is in our hands to make the best utilization of nano technology in the present and upcoming days. A common thread between Stone Age, medieval, industrial and molecular nanotechnology is the exponential curve. This ever-accelerating curve representing human knowledge, science and technology will be driven a new way by what will probably become the first crude, pre-assembler nanotech products.By treating atoms as discrete, bit like objects, molecular manufacturing will bring a digital revolution to the production of material objects. Working at the resolution limit of matter, it will enable the ultimate in miniaturization and performance. Research programs in chemistry, molecular biology and scanning probe microscopy are laying the foundations for a technology of molecular machine systems. The motion of electrons in a transistor has been described as a complex dance. Switching action in one property of a transistor that has been demonstrated. Bardeen, Brattain and Shockley were concerned about the amplification properties of transistors they had invented. It remains to see whether amplification can be achieved to any experimentally observable extent in such a single atom transistor.

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