Overview of Nanoelectronic Devices <s> • Sensitivity to Input Voltage and Current Fluctuations: <s> Abstract A coherent overview of the exciting field and challenging areas of research in mesoscopic physics and nanoelectronics is given. The unifying role of the S -matrix theory or “ input-quantum process-output ” language of quantum mechanics in the formulation of quantum transport in mesoscopic physics and nanoelectronics is clarified, with time-independent S -matrix theory applied to mesoscopic systems and double-time-axis time-dependent S -matrix thoory, coupled with the lattice Weyl-Wigner formulation of quantum dynamics of electrons in solids, applied to the highly nonlinear, far-from-equilibrium and high-speed nanoelectronic devices. At steady state and near equilibrium, the two independent formulations yield equivalent results. The technological impact of the many-body quantum distribution-function (QDF) (typified by the Wigner distribution function) approach is demonstrated by a complete transition from the level of basic nonlinear quantum transport physics to an engineering computer-aided design tool for integrated circuits (ICs). This is done in the derivation of the equivalent circuit model for resonant tunneling devices which incorporates the quantum inductance. In this review, emphasis is given to major developments in quantum transport, computational nanoelectronics, new device concepts, and novel transport physical phenomena found in small structures, with potential device and IC applications and deemed most likely to have a significant impact on the future developments of nanoelectronics. <s> BIB001 </s> Overview of Nanoelectronic Devices <s> • Sensitivity to Input Voltage and Current Fluctuations: <s> Properties of synthetic electroactive and photochromic molecular systems are described, with particular emphasis on microscopic processes controlling these properties. Both low-molecular weight and polymeric materials can be (and many have already been) used as conducting systems. The chemical flexibility of many molecular systems also allows for using them as information-processing materials, whose action is based on their photochromic properties. A relation between properties of individual molecules and those of macroscopic molecular systems allows one to envisage construction of molecular-scale devices. However, due to limitations associated with technological problems (construction of molecular-scale elements, their addressing, quantum-mechanical (size) limitations, etc.), the construction of such elements is still a problem that awaits solution. > <s> BIB002 </s> Overview of Nanoelectronic Devices <s> • Sensitivity to Input Voltage and Current Fluctuations: <s> We review the current status of Si/SiGe n- and p-type MODFETs with an emphasis on their microwave performance. A comparison with state-of-the-art Si technology is given, and the potential use of Si/SiGe devices in complementary logic is pointed out. <s> BIB003
Landauer  has pointed out that, unlike in FET's, switching in RTD's or RTT's can be very sensitive to fluctuations in the input voltages, which could accidentally drive devices off-resonance. • Cryogenic Operation: It has been possible to build circuits with hybrid RTD-FET's operating at room temperature  . Individual 30 nm silicon SET's show strong oscillations of current versus gate voltage at 150 K (half of room temperature on an absolute scale)  ,  . However, most nanoelectronic devices built to date are functional only at cryogenic temperatures-the boiling point of liquid nitrogen and well below. At high temperatures, random thermal motion often provides electrons with the small additional energy they need to get onto the island. Making islands smaller, however, would increase the relevant energy separation or for states on the island, thus reducing the possibility of the device switching on when it should be off. Extrapolating, a device based on an island the size of a single atom (approximately 0.1 nm in radius) could operate at 100 times room temperature (if it would not melt!), but this limit only can be approached with molecular electronics. • Materials: III-V Semiconductors Are Less Than Satisfactory and Si Nanoelectronics Are Needed: Further progress must be made toward fabricating solid-state nanoelectronic devices in silicon rather than in III-V semiconductors. Different III-V semiconductors can be grown in atomically perfect crystalline sandwiches. By contrast, producing clean junctions and barriers in Si-based semiconductors is difficult, because the natural insulator in Si, SiO , is amorphous rather than crystalline, and also tends to have many more impurities than III-V layers do. But SiO is still the best insulator and barrier we have: as little as 5 nm can be grown uniformly and will prevent electrons from crossing despite several volts applied across it (less than this can be grown as a tunnel barrier). III-V equivalents are far inferior in the electric fields they can withstand. Moreover, despite III-V materials' ultraclean buried interfaces, their exposed top surface will always become contaminated with stray, perhaps mobile, charges, which degrade the stability of devices. In Si applications, thick oxides are always grown to protect devices from such problems. Heterostructures made from Si-based materials, particularly sandwiches of Si/SiGe, are emerging as a promising alternative offering the best of both worlds BIB003 . • Background Charge Problem: Likharev  ,  and other investigators have noted that random background charges tend to accumulate in semiconductors in the vicinity of a small operating quantum-effect or singleelectron device. These can render the device inoperable. Improved materials are needed to reduce the impact of this effect. Silicon-on-insulator (SOI) technology  might be of assistance, as might the relatively nonconductive, nonpolarizable organic compounds that could be used with molecular electronics. BIB001 ,  : This is a difficulty intrinsic to the quantum mechanical tunneling effects employed in all the solid-state nanodevices we describe. Its impact can only be mitigated by ensuring that all nanometer-scale barriers are made with extreme precision and uniformity in width  .  : Sufficient precision and uniformity to ensure reliable, predictable behavior of large numbers of devices are very difficult to achieve on a severalnanometer scale in solids. Even the aforementioned heterostructure sandwiches can only achieve such precision in one of the three dimensions BIB003 . On the other hand, devices made from individual electrically active molecules supported on organic substrates may offer solutions to some of the more serious issues just enumerated, including accumulation of background charge, imprecision or irreproducibility of lengths, and the inability to make small enough quantum confinement structures to achieve room-temperature operation BIB002 .