Journal of Scientific & Industrial Research Vol. 63, October 2004, pp 795-806 Emerging trends in ultra-miniaturized CMOS (Complementary metal-oxide- semiconductor) transistors, single-electron and molecular-scale devices: A comparative analysis for high-performance computational nanoelectronics V K Khanna Solid State Devices Division, Central Electronics Engineering Research Institute, Pilani 333 031 The current status and trends of the three ultra-small scale integrated circuit technologies, namely nanoscale CMOS, single- and molecular electronics are comprehensively reviewed. A comparative study is made, pointing out their relative pros and cons for nanoelectronic computing. Crucial aspects of MOS downscaling include, from the physical viewpoint, the infamous short-channel effect caused by drain induced barrier lowering (DIBL), the narrow width effect associated with small channel width, the combined small-geometry effect, and hot-carrier degradation; together with the conflicting requirements of shallow silicided junctions and low junction leakage; random doping fluctuations; ultrathin gate oxide reliability; polysilicon depletion effect; atomic scale roughness at the Si/SiO 2 interface; and high lithographic expenses, on the technological side. For sustaining growth in device density, a possible route for the microelectronics industry is to shift from the traditional field-effect transistor-based paradigm to one based on nanostructures. Single electronics has not been able to bear the envisaged fruits. While prospects of solo single-electron logic are murky, the concept of a mixed single- electron device/FET multi-valued logic and memory appears to be beneficial. But to achieve the ultimate performance, it may be expedient to transform our philosophy fundamentally to start from the molecular level, instead of scaling down old technologies to nanometer level. Molecular electronics appears to be the appropriate approach because the development cost of scaled technologies, and cost-effectiveness of resulting devices is not encouraging. The review seeks to provoke keen interest in these futuristic nanotechnologies. Keywords : Nanotechnology, Computers, Single-electron transistor, Molecular electronics, Nanocells, Quantum dots IPC Code: Int. Cl. 7: H 01 L 29/00, H 01 L 21/336, H 01 L 27/00 1 Introduction faced by the nanoMOSFET technology, and sheds The computer industry has progressed by leaps and light on the upcoming technologies to maintain the bounds. This phenomenal success is ascribed to the constant pace of progress in this vital sector. constant downsizing of contemporary CMOS devices Investment in terms of development effort and cost and circuitry resulting in lower cost, faster, and denser together with relative economic and technological computers with reduced power consumption and gains expected are the prime factors under enhanced functionality. Consumer’s demand for deliberation. portable battery-operated products has stimulated considerable efforts for exploration of low-voltage 2 Prelude to Switching and Amplifying Devices devices. MOSFETs (Metal-oxide-semiconductor for Nanocomputers field-effect transistors) having decananometer channel To enable the reader to understand the subject and lengths are already mass fabricated while those less appreciate the importance of the issues raised, we than 10 nm have been demonstrated in research begin with an introduction to the switching and environments. Below 10 nm, MOSFETs are close to amplifying devices that constitute the hardware of their basic limits of operation 1, and at this scale, their present-day electronic digital computers. All digital realization is confronted with gigantic physical and computers contain a basic structural unit, ‘the financial constraints. Information technology needs transistor’ which is a device capable of performing high-speed devices. Naturally, alternative approaches two functions: switching and amplification. The state like, single electronics and molecular electronics have of the transistor can be employed to adjust the voltage attracted the attention of researchers all over the on a wire as high or low, designated as binary zero and globe 2. This paper discusses critically the obstacles one on the computer. Further, using a small input ____________ signal the transistor can control an output signal that is Email: [email protected] several-fold larger than this signal. The switching 796 J SCI IND RES VOL 63 OCTOBER 2004 prevented by limitations of fabrication technology and quantum mechanical laws governing the device operation. As a solution to this problem, two categories of alternative devices to the MOSFET have appeared viz., single-electron transistors and molecular devices. In the next section, the problems encountered in shrinking MOSFETs are addressed. Then the novel nanoelectronic switching and amplifying devices are described. Although the operating principles of these devices are radically different from those of the MOSFET, these devices retain the terminology of source, drain and gate in the same conceptual roles as the MOSFETs. To acquaint the reader with the terminology of these devices the single-electron transistor 3-10 contains a small island of semiconductor or metal, ranging in size from 5-100 nm. This island is embedded between two narrow walls of some other material or an insulating oxide of the island material. It is said that the island is enclosed between two potential energy barriers. Electrons confined to islands exhibit two essential quantum mechanical effects: energy quantization and tunneling. These effects control the electron transport in a nanoelectronic device. Quantum mechanics allows the energy of each electron to be one of a finite number of one-electron energy levels. Moreover, when the potential barriers are very thin, ~ 5-10 nm, there exists Fig. 1—(a) Simplified structure of N-channel MOSFET. (b) P- a finite probability for ‘tunneling’ of electrons to move channel and N-channel MOSFETs in an N-well CMOS technology to or from the island, provided there is a vacant state of the same energy on the opposite side. function allows the implementation of logical and The other approach of molecular electronics 11-19 is arithmetic functions in a computer while amplification based on devising molecular structures that can act as permits the transmission of signals within the computer switching elements, and assembling these molecules without attenuation. into the accurate extended structures for computation. The transistor most commonly used in digital The ability of a single molecule to conduct current computers is the metal-oxide-semiconductor field- was not recognized easily because very constricted effect transistor (MOSFET)[Fig. 1(a)]. In this device, wire structures offer high resistance even if they are current flows from the source terminal to the drain made from good electrical conductors. Chain terminal when the voltage applied to the gate terminal molecules composed of repeating units of aromatic is sufficient to invert the underlying silicon to form a groups with acetylene linkage are known to conduct conducting channel from source to drain. A pair of electric current. They are called molecular wires and complementary devices, one P-channel MOSFET and can be made appreciably long. Incorporating quantum one N-channel MOSFET constituting the wells into molecular structures for confinement of complementary metal-oxide-semiconductor (CMOS) mobile electrons, switching devices can be made. A structure, [Fig. 1(b)], is widely used in digital circuits. quantum well can be embedded in a molecular wire To augment the capabilities of computers, their by inserting pairs of barrier groups that break the fundamental structural unit, namely the MOSFET, has sequence of conjugated π-orbitals. been made progressively smaller in size resulting in With this brief tutorial, we begin to investigate the ultradense electronic circuits. The MOSFET has now problems encountered in shrinking the MOSFET reached a stage where its further miniaturization will be devices. KHANNA: COMPARATIVE ANALYSIS FOR HIGH-PERFORMANCE COMPUTATIONAL NANOELECTRONICS 797 3 Downscaling of MOS Devices, Significant Probl- when the channel length becomes comparable to the ems and Recourse to Other Approaches source-substrate or drain-substrate depletion depth, Ever since J S Kilby invented and demonstrated the and is caused by the overlapping of the depletion integrated circuit in 1958, the manufacturing of region due to the gate field with the depletion regions semiconductor ICs has continued to grow near the source and drain junctions, terminating the exponentially. In 1965, Gordon Moore, the co-founder built-in fields from source and drain 22 . This of Intel, observed that the number of transistors per unit overlapping decreases the total amount of depleted area in an integrated circuit doubles every 18 months. charge available in the P-substrate to compensate the This observation referred to as the Moore’s law has field applied to the gate. The net result of this charge been corroborated during the past four decades as sharing is that the depletion region below the gate is a integrated circuit complexity has advanced from small- trapezoidal-shaped region instead of the rectangular- scale integration (SSI: active device count 1-100 per shaped volume visualized for easy calculation. chip), through medium-scale integration (MSI: 100- Therefore the charge near the non-parallel sides of the 10 3), large-scale integration (LSI: 10 3-10