Microelectronics for High Density Integrated Circuits

8.3.1. Microelectronics for High Density Integrated Circuits

Due to size limitation even with scaling down, MOSFET technology cannot continue forever. It will hardly go beyond a few nanometer, even if adequate lithographical technology is available. As a result, the search for the new principles of operation of the small-size devices is becoming more and more important. They possess radically different properties from those of bulk semiconductors. This change in the effective dimensionality offers fascinating changes in electric, magnetic, optical and vi- brational properties. The electron mobility is high in those devices. The researches on the nano devices or quantum devices continue to be both challenging and exciting as novel structures with different materials having different properties are developed. They are useful for millimeter and submillimerer wave applications. They have potential advantages that make them attractive for nonlinear functions. It is possible to realize high frequency, low power consumption and low dimensional devices [30].

The application of soft computing tools on nano device modeling can help in the optimization of system parameter of the quantum (nano) devices to get the desired characteristics. The on-line optimiza-

297 tion during fabrication is also possible with the application of softcomputing tools like ‘Genetic Algo-

OTHER METHODS AND OTHER NANO MATERIALS

rithm’ (GA) [31] or ‘Artificial Neural Networks’ (ANN) [32]. However, the quantum devices have inherent limitations like material and process related limita-

tions, power limitation, wiring limitation, quantum mechanical limitation and system architecture limi- tation. The most likely candidate for future ultra-dense digital circuits is ‘single electronics’, which has links with the motion of a single electron. Single electron device provides a complete and concise representation for most digital functions encountered in logic-design applications.

The generality and robustness of the effect and the relative simplicity of the device structures make the single-electronics the most likely candidate for future ultra dense digital circuits. As a single electron is sufficient to store an information which is not in the case of ‘Transistor or CMOS’ circuits, the circuit has advantages of reducing the power consumption. In single electron devices in which one bit of information is represented by a few electrons, the power consumption is drastically reduced. The single electron devices that allow manipulation of individual electron are ultimate forms of the electron devices. Their potential integration level is obviously extremely high due to its small size. An extremely low ‘operation power’ solves some of the instability and reliability problems. The ‘speed-power’ prod- uct is predicted to lie close to the quantum limit set by the Heisenberg’s uncertainty principle. The processing speed of such device will be nearly equal to electronic speed. The exquisite sensitivity is about five orders of magnitude better than conventional solid-state MOSFET transistors. The integra- tion density is much higher than that available in the existing VLSI / ULSI circuits.

There is, at the current time, a growing interest in the possibilities of designing electronic circuits using evolutionary techniques. During the decade since their discovery, single-electronic tunneling de- vices have received a great deal of attention, both in terms of the physics of the coulomb blockade and for potential devices applications. The single-electron tunneling (SET) devices exploit effects that arise due to the quantized nature of charge. These effects have been observed in systems of small metal structures, in semiconductor structures, and in structures made from conducting polymers. Since these effects are ubiquitous in small structures, they are likely to have an impact on any future nano-scale electronic circuits. These devices can be useful in low power circuits since only a few electrons are needed to be transported. The dense memories where bits are represented by the presence or absence of only a few electrons can also be realized with the use of single electron devices.

Biswas et al [32] investigated the properties and applicability of circuits based on single electron devices. An approach to such new devices was built on the basis of the concept of binary decision diagrams. The unit function of this device was a simple two way switching. A binary decision diagram represented a digital function as a ‘directed cyclic graph’ with each node labeled by a variable. It pro- vided a complete and concise representation for most digital functions encountered in logic-design applications in order to realize some simple and also complicated digital circuits. All logical circuits starting from NOT gate to CPU of a digital computer can be realized with the help of single electron circuits. The ‘timing analysis’ together with ‘propagation delay’ in case of single electron device based digital circuits can also be estimated.

The single electron circuit based multiplication scheme was presented as an efficient application of single electron devices. A comparison-based table was generated to establish that the use of single electron devices can not only increases the density of integration, but also make the execution much faster. Hence, it is seen that the time taken by normal conventional circuit is approximately three times of the ‘single electron’ circuit based multiplication circuit. Hence, the efficiency of ‘single electron’ circuits based multiplication scheme is better than that of conventional multiplication circuit [32].

NANO MATERIALS