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Online ISSN 1751-8598 Print ISSN 1751-858X

IET Circuits, Devices & Systems

Volume 1, Issue 3, June 2007

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Volume 1, Issue 3

June 2007

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  • Quantum dots and nanowires

    • Editorial: Quantum dots and nanowires
      Arithmetic logic unit of a computer based on spin-polarised single electrons
      • Author(s): T. Basu ; S.K. Sarkar ; S. Bandyopadhyay
      • Source: IET Circuits, Devices & Systems, Volume 1, Issue 3, p. 194 –199
      • DOI:  10.1049/iet-cds:20060036
      • Type: Article
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      • p. 194 –199 (6)
        The design of an arithmetic logic unit (ALU) is presented using ‘single spin logic’ where classical binary bits 0 and 1 are encoded in orthogonal spin polarisations of single electrons hosted in semiconductor quantum dots. The logic signal (spin state) is transmitted from one spin to the next via nearest-neighbour exchange interaction. The ALU circuits are implemented by placing the quantum dots in specific geometric patterns on a wafer so as to realise the desired relationships between the input and output spin states.
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      Photocurrent response of the carbon nanotube–silicon heterojunction array
      • Author(s): D.A. Straus ; M. Tzolov ; T.-F. Kuo ; A. Yin ; J.M. Xu
      • Source: IET Circuits, Devices & Systems, Volume 1, Issue 3, p. 200 –204
      • DOI:  10.1049/iet-cds:20060105
      • Type: Article
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      • p. 200 –204 (5)
        A highly ordered array of parallel, identical carbon nanotubes is grown non-lithographically in a bottom-up fabrication approach to form a heterojunction with a silicon substrate. Evidence of a space-charge separated region at the nanotube–silicon interface is present in the form of diode rectification and a closed-circuit zero-bias photocurrent in response to infrared light. Because carbon nanotubes are narrow bandgap semiconductors, their heterojunction with silicon was analysed spectrally via Fourier transform infrared photocurrent spectroscopy with the aim of investigating the suitability of this structure for infrared (IR) detector applications. IR photoresponse shows signs of temperature-dependent activation that is complex but consistent with estimates of the heterojunction barrier height. Considering the many interesting benefits and properties of carbon nanotubes, these results despite their earliness suggest that nanotube–silicon heterojunction systems could form the foundation for a new kind of infrared detection device.
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      Fabrication of highly ordered anodic aluminium oxide templates on silicon substrates
      • Author(s): A. Yin ; M. Tzolov ; D. Cardimona ; L. Guo ; J. Xu
      • Source: IET Circuits, Devices & Systems, Volume 1, Issue 3, p. 205 –209
      • DOI:  10.1049/iet-cds:20060101
      • Type: Article
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      • p. 205 –209 (5)
        The controlled fabrication of highly ordered anodic aluminium oxide (AAO) templates of unprecedented pore uniformity directly on Si, enabled by new advances on two fronts – direct and timed anodisation of a high-purity Al film of unprecedented thickness (50 µm) on Si, and anodising a thin but pre-textured Al film on Si, has been reported. To deposit high-quality and ultra-thick Al on a non-compliant substrate, a prerequisite for obtaining highly ordered pore arrays on Si by self-organisation while retaining a good adhesion, a specially designed process of e-beam evaporation followed by in situ annealing has been deployed. To obtain an AAO template with the same high degree of ordering and uniformity but from a thin Al film, which is not achievable by the self-organisation alone, pre-patterning of the thin Al surface by reactive ion etching using a free-standing AAO mask that was formed in a separate process was performed. The resultant AAO/Si template provides a good platform for integrated growth of nanotube, nanowire or nanodot arrays on Si. Template-assisted growth of carbon nanotubes (CNTs) directly on Si was demonstrated via a chemical vapour deposition method. By controllably removing the AAO barrier layer at the bottom of the pores and partially etching back the AAO top surface, new CNT/Si structures were obtained with potential applications in field emitters, sensors, oscillators and photodetectors.
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      Modified emission of semiconductor nano-dots in three-dimensional photonic crystals
      • Author(s): A.Z. Khokhar ; R.M. De La Rue ; N.P. Johnson
      • Source: IET Circuits, Devices & Systems, Volume 1, Issue 3, p. 210 –214
      • DOI:  10.1049/iet-cds:20060295
      • Type: Article
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      • p. 210 –214 (5)
        Mono-dispersed polystyrene spheres were used to grow synthetic opal photonic crystals on glass substrates using controlled evaporation. Commercially available cadmium telluride (CdTe) semiconductor nano-dots with emission in the visible were infiltrated into a slice of opal voids by capillary action. The optical properties of CdTe dot loaded opal were studied by using a white light source and showed a red shift of the opal stop-band due to an increase in the effective refractive index. The emission of the CdTe dots was matched with the edge of the 〈111〉 direction stop-band of bare opal. Stop-band confined emission from CdTe dots was observed by pumping with an argon-ion laser. The full width at half maximum of the CdTe emission from an infiltrated section of the opal was significantly reduced due to the stop-band effect of bare opal.
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      Stoichiometric effects on optical properties of cadmium sulphide quantum dots
      • Author(s): S. Chaure ; N.B. Chaure ; R.K. Pandey ; A.K. Ray
      • Source: IET Circuits, Devices & Systems, Volume 1, Issue 3, p. 215 –219
      • DOI:  10.1049/iet-cds:20070048
      • Type: Article
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      • p. 215 –219 (5)
        Size quantised cadmium sulphide (CdS) nanocrystalline thin films with different particle sizes and stoichiometric ratios were successfully grown on indium tin oxide substrates using an aqueous synthetic route. The effect of cadmium (Cd) to sulphur (S) ratio on the optical properties of CdS nanocrystalline films was investigated using EDAX, UV–vis spectroscopy, photoluminescence and Raman spectroscopy. A satisfactory stoichiometric condition was achieved for 0.13 M concentration of thiourea whereas concentrations in the range of 1–1.2% of mercaptoethanol capping agents produced values much smaller than Wannier exciton diameter for CdS grain sizes, leading to quantum confinement. Photoluminescence emission bands and Raman peaks were analysed for the physical understanding of optimum growth of CdS quantum dots.
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  • General articles

    • Dynamic models for substrate coupling in mixed-mode systems
      • Author(s): J.M.S. Silva  and  L. Miguel Silveira
      • Source: IET Circuits, Devices & Systems, Volume 1, Issue 3, p. 221 –232
      • DOI:  10.1049/iet-cds:20060206
      • Type: Article
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      • p. 221 –232 (12)
        In modern monolithic integrated circuits, substrate coupling is a major concern in mixed-mode systems design. Verification of such systems implies the availability of accurate and simulation-efficient substrate coupling models. Traditionally, for frequencies up to a few gigahertz, pure resistive models have been considered sufficient. However, with increasing frequencies of operation, dynamic models become mandatory. The authors motivate the use of dynamic resistive-capacitive (RC) models of substrate coupling as a natural extension to the standard purely resistive models. They propose an extraction methodology that starts with information about the process parameters and circuit's contact layout, and leads to a contact-to-contact RC element model. The underlying algorithm is based on a finite difference discretisation of the substrate, leading to a large tridimensional mesh which is reduced by means of a fast multigrid algorithm. Unlike standard model order reduction algorithms which can produce models of similar accuracy to state-space descriptions, the proposed method leads to a realisable RC model that can trivially be incorporated into circuit simulation tools. As a first approximation, such a model is shown to correspond to a single time-constant system. Furthermore, it is shown that this time constant can be computed from knowledge of the conductivity and permittivity of a single dominant layer. It is verified that this formulation can accurately model substrate coupling effects for frequencies up to several tens of gigahertz.
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      Supply current spectrum estimation of digital cores at early design
      • Author(s): G. Blakiewicz  and  M. Chrzanowska-Jeske
      • Source: IET Circuits, Devices & Systems, Volume 1, Issue 3, p. 233 –240
      • DOI:  10.1049/iet-cds:20060040
      • Type: Article
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      • p. 233 –240 (8)
        A new, approximate method is presented to calculate a digital core supply current spectrum. The method is based on characterisation of supply current pulses in terms of the rise and fall times and pulse width. The upper limits (an envelope) for the supply current spectrum are derived using logic signal transition densities at digital core internal nodes. Contrary to the known methods, the proposed one uses limited data and generates much needed supply current information strongly desired by a system designer at a very early system planning. By using the probabilistic and statistical techniques the method is only weakly dependent on input sequence pattern and gives reliable final results.
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      Design of on-chip error correction systems for multilevel NOR and NAND flash memories
      • Author(s): F. Sun ; S. Devarajan ; K. Rose ; T. Zhang
      • Source: IET Circuits, Devices & Systems, Volume 1, Issue 3, p. 241 –249
      • DOI:  10.1049/iet-cds:20060275
      • Type: Article
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      • p. 241 –249 (9)
        The design of on-chip error correction systems for multilevel code-storage NOR flash and data-storage NAND flash memories is concerned. The concept of trellis coded modulation (TCM) has been used to design on-chip error correction system for NOR flash. This is motivated by the non-trivial modulation process in multilevel memory storage and the effectiveness of TCM in integrating coding with modulation to provide better performance at relatively short block length. The effectiveness of TCM-based systems, in terms of error-correcting performance, coding redundancy, silicon cost and operational latency, has been successfully demonstrated. Meanwhile, the potential of using strong Bose–Chaudhiri–Hocquenghem (BCH) codes to improve multilevel data-storage NAND flash memory capacity is investigated. Current multilevel flash memories store 2 bits in each cell. Further storage capacity may be achieved by increasing the number of storage levels per cell, which nevertheless will correspondingly degrade the raw storage reliability. It is demonstrated that strong BCH codes can effectively enable the use of a larger number of storage levels per cell and hence improve the effective NAND flash memory storage capacity up to 59.1% without degradation of cell programming time. Furthermore, a scheme to leverage strong BCH codes to improve memory defect tolerance at the cost of increased NAND flash cell programming time is proposed.
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      Noise optimisation analysis of an active pixel sensor for low-noise real-time X-ray fluoroscopy
      • Author(s): M.H. Izadi  and  K.S. Karim
      • Source: IET Circuits, Devices & Systems, Volume 1, Issue 3, p. 251 –256
      • DOI:  10.1049/iet-cds:20070028
      • Type: Article
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      • p. 251 –256 (6)
        Theoretical calculations and simulation results for the noise of a hybrid active pixel sensor designed for real-time digital fluoroscopy are presented. Noise performance is given as a function of transistor dimensions, allowing the designer to choose appropriate device dimensions when designing flat-panel imaging circuits. Optimal device dimensions are derived for minimising the input referred noise of the active pixel to meet the stringent requirements for low-noise digital X-ray fluoroscopy (<1000 noise electrons).
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      Single OTRA-based current-mode monostable multivibrator with two triggering modes and a reduced recovery time
      • Author(s): Y.-K. Lo  and  H.-C. Chien
      • Source: IET Circuits, Devices & Systems, Volume 1, Issue 3, p. 257 –261
      • DOI:  10.1049/iet-cds:20060359
      • Type: Article
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      • p. 257 –261 (5)
        A simple current-mode monostable multivibrator is introduced. The proposed topology consists of a single operational transresistance amplifier (OTRA) as a main active building block with a few external passive elements. The proposed circuit is simpler compared to counterparts composed of op amps and commercially available ICs. The presented topology can be triggered under positive- and negative-triggering modes at different input terminals. Moreover, the proposed topology provides an advantageous feature of shortening the recovery time required for applying the consecutive triggering signal. The operations of the topology previously presented are reviewed. Then, the proposed circuit is described and analysed in detail. The theoretical analysis is verified through experimental results on a prototype circuit. The improvement on reducing the recovery time is also investigated.
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