Showing papers in "Solid-state Electronics in 2001"

Field emission from carbon nanotubes: the first five years

Abstract: Carbon nanotubes, a novel form of carbon discovered in 1991, have been rapidly recognized as one of the most promising electron field emitters ever since the first emission experiments reported in 1995. Their potential as emitters in various devices has been amply demonstrated during the last five years, and recent developments of production techniques are likely to trigger future applications. This report reviews the state of the art of the current research on the electron field emission properties of carbon nanotubes and surveys their ability to provide single or multiple electron sources.

Field emission displays: a critical review

Abstract: The goal of making attractive flat panel displays (FPDs) based on arrays of cold cathodes has now become a reality. Pixtech and Futaba have begun commercial production of low voltage, monochrome field emission displays (FEDs). Moreover, public response to the high voltage, full color, VGA FED prototypes shown by Candescent and Motorola at various technical meetings and exhibits has been extremely positive and encouraging. Yet, the future of the FED industry is uncertain. The tremendous improvements in visual quality and reduction in manufacturing cost of liquid crystal displays, as well as the formidable progress made in other FPD technologies has raised the standard for FEDs. In this article, we first review the status of FEDs based on the Spindt microtip emitter. We focus on the scalability of the Spindt process to large substrates, phosphor selection, high voltage stability, and display lifetime. Second, we discuss in detail the recent advances made in alternate cold cathode technology, including carbon nanotubes and composite materials, and their potential advantages for FPD. This new technology offers a tremendous opportunity to lower capital investment, to cut manufacturing costs and to challenge the existing flat panel industry.

Low-macroscopic-field electron emission from carbon films and other electrically nanostructured heterogeneous materials: hypotheses about emission mechanism ☆

Abstract: Thin flat dielectric films can be low-macroscopic-field (LMF) electron emitters, able to generate electrons when subject to a macroscopic electric field in the range 1–50 V μm −1 . This phenomenon is a known cause of pre-breakdown currents in high-voltage vacuum breakdown, and is now the basis of a broad-area electron-source technology, using carbon-based thin films and other materials. The phenomenon occurs because the dielectric film is, or becomes, an electrically nanostructured heterogeneous (ENH) material, with quasi-filamentary conducting channels between its surfaces. These channels connect to emitting features near or on the film/vacuum surface, or act as electron emitters themselves. The film may contain conducting or semiconducting particles that assist with conductivity and/or act as emitting features. Several forms of thin-film LMF emitter exist: in each case the situation geometry ensures that sufficient field enhancement occurs at the ‘tip’ of the emitting feature for the emission process to be some form of tunnelling field electron emission (probably ‘cold’ in some cases, ‘hot’ in others). This paper explores aspects of the theory of thin-film LMF emission, starting from the need to understand the behaviour of emitters based on amorphous carbon films. A summary review, with extensive references, is given of relevant past work outside the immediate ‘carbon field emission’ context. Relevant aspects of semiconductor field emission theory are noted. Comment is made on the original experiments on diamond field emission, and on theoretical misconceptions in the carbon field emission literature. Analysis of carbon-film emitter behaviour suggests that emission must primarily be due to geometrical field enhancement, that in at least some cases arises from conducting nanostructure inside the film. In one case, published film characteristics can be used to show that sufficient field enhancement should be available. Some problems with an ‘internal field enhancement’ hypothesis are considered and disposed of. Difficulties with Latham’s theory of field-induced emission from ENH materials are pointed out: a new theory, largely qualitative at this stage, can explain longstanding problems: this assumes that dielectric films must be treated as ‘hopping conductors’ not semiconductors. Electron emission takes place via localised surface states: transition to a channel-limited current regime takes place when the surface states no longer have high enough occupation probability to screen the external field, and is accompanied by anomalous band bending at the channel tip. Mathematical theories of band bending and field emission for hopping conductors are required. Some consequences for the design of LMF emitters are noted.

Design considerations in scaled SONOS nonvolatile memory devices

Abstract: Scaling the programming voltage, while still maintaining 10-year data retention time, has always been a big challenge for polysilicon–oxide–nitride–oxide–silicon (SONOS) researchers. We describe progress in the design and scaling of SONOS nonvolatile memory devices. We have realized −9+10 V (1 ms) programmable SONOS devices ensuring 10-year retention time after 107 erase/write cycles at 85°C. Deuterium anneals, applied in SONOS device fabrication for the first time, improves the endurance characteristics when compared with traditional hydrogen or forming gas anneals. We introduce scaling considerations and process optimization along with experiments and SONOS device characterization. A field programmable gate array-based measurement system is described for the dynamic characterization of SONOS nonvolatile memory devices.

Electrical transport characteristics of Au/n-GaAs Schottky diodes on n-Ge at low temperatures

Abstract: The currenti?½voltage characteristics of Au/n-GaAs Schottky diodes grown by metal-organic vapor-phase epitaxy on Ge substrates were determined in the temperature range 80i?½300 K. The zero-bias barrier height for current transport decreases and the ideality factor increases at low temperatures. The ideality factor was found to show the T0 effect and a higher characteristic energy. The excellent matching between the homogeneous barrier height and the effective barrier height was observed and infer good quality of the GaAs film. No generationi?½recombination current due to deep levels arising during the GaAs/Ge heteroepitaxy was observed in this study. The value of the Richardson constant was found to be 7.04 A K?2 cm?2, which is close to the value used for the determination of the zero-bias barrier height.

New procedure for the extraction of basic a-Si:H TFT model parameters in the linear and saturation regions

Abstract: A new procedure is proposed to extract basic parameters for the AIM-Spice amorphous thin film transistor model in the above-threshold region. Our method avoids non-linear optimization, which is mainly the method utilized up to now, when using a program extractor included in AIM-Spice. The present extraction procedure is based on the integration of the experimental data current. The integration method as in known is convenient to decrease the effects of experimental noise. The method is applied to the linear and saturation regions for the above-threshold regime and allows the extraction of all the above-threshold parameters. The accuracy of the simulated curves using the parameters extracted with the new procedure is verified with measured and calculated data using the expressions contained in the model.

A new method for evaluating illuminated solar cell parameters

Abstract: A new method to evaluate the five illuminated solar cell parameters (series resistance Rs, the ideality factor n, the photocurrent Iph, the shunt conductance Gsh and the diode saturation current Is) is described. The method uses the measured current–voltage data and the resulting calculated conductance of the device. By using the conductance, the number of parameters which has to be calculated is reduced and then only four parameters have to be extracted. A nonlinear least squares optimization algorithm based on the Newton model is hence used to evaluate these parameters. To overcome the difficulty in initializing the cell parameters, the simple conductance technique is used. When incorporated into microcomputer-based data acquisition software, the program allows theoretical modeling of solar cells. Results obtained for a commercial solar cell and a module are given. Comparison with other commonly used methods is also discussed.

Very high voltage AlGaN/GaN high electron mobility transistors using a field plate deposited on a stepped insulator

Abstract: We investigate a technique that has the potential to enhance the breakdown voltage ( V br ) of AlGaN/GaN high electron mobility transistors (HEMTs) beyond 1 kV. The technique involves incorporation of a field plate (FP) connected to the gate and placed over a stepped insulator (SI). A comprehensive account of the critical geometrical and material variables controlling the field distribution under the FP is provided. A systematic procedure is given for designing a SIFP device, using 2-D simulation, to obtain the maximum V br with minimum degradation in on-resistance and frequency response. Simulations show that, for a 2-DEG concentration of 1×10 13 cm −2 , the maximum V br achievable with a stepped aluminum nitride (silicon nitride) insulator can be 2.6 (2.3) times higher than that with a uniform insulator; V br ∼1 kV can be obtained using a gate to drain separation as low as ∼7 μm. The methodology of this paper can be extended to the design of SIFP structures in other lateral FETs, such as MESFETs and LD-MOSFETs.

Silicon on insulator technologies and devices: from present to future

Abstract: The context of silicon on insulator (SOI) technologies is reviewed in terms of wafer fabrication, configuration/performance of SOI devices, and typical operation mechanisms in partially and fully depleted SOI MOSFETs. The future of SOI is tentatively explored, by addressing the further scalability of SOI transistors as well as the innovating architectures (double-gate, ground-plane, and extremely thin MOSFETs) proposed for the ultimate generations of SOI transistors.

Ammonia-sensing characteristics of Pt and SiO2 doped SnO2 materials

Abstract: The gas-sensing characteristics of doped SnO2 materials for ammonia were investigated in this paper A good formula of ammonia sensitive materials has been achieved The phase of sensitive material was characterized by X-ray diffraction The gas sensor of indirect heating that was made by this sensitive material had high sensitivity to ammonia gas and stability Infrared was used to study the reacting products of NH3 on the surface of the material The sensitive mechanism of sensor was analyzed and discussed

Analytical approximation for the MOSFET surface potential

Abstract: Surface-potential-based models are among the most accurate and physically based compact MOSFET models available today. However, the need for iterative computations of the surface potential limits their computational efficiency, which is critical in CAD applications. The existing closed-form approximations for the surface potential are based on empirical smoothing functions and have the accuracy of about 2–3 mV which is not always adequate for an accurate modeling of MOSFET characteristics, especially transconductances and transcapacitances. In this work, we present and verify an extremely accurate and computationally efficient closed-form approximation, which can serve as a basis for advanced surface-potential-based MOSFET models.

Electrical and gas-sensing properties of WO3 semiconductor material

Abstract: In this paper, the electrical and gas-sensing properties of calcined tungsten trioxide semiconductor materials were investigated. X-ray diffraction, scan electron microscopy and infrared were used to characterize structure and performance of WO 3 semiconductor material. The average grain size of WO 3 was 22 nm after calcination at less than 800°C and 24–26 nm at more than 900°C for 1 h. The sensors of indirect heating type were fabricated. The effects of calcining temperature and operating temperature on electrical resistance and sensitivity, and sensitivity-gas concentration properties of the WO 3 -based sensors were investigated. The sensor based on WO 3 exhibited high sensitivity and good response characteristics to ethanol gas. The electrical properties of WO 3 were analyzed and the sensitive mechanism was discussed.

Minority carrier transport in GaN and related materials

Abstract: Transport properties of minority carriers in ZnO and related compounds are of critical importance for the functionality of bipolar devices. This review summarizes the available information on the subject of minority carrier transport in ZnO-based semiconductors, focusing on its temperature dependence and the dynamics of nonequilibrium carrier recombination. The influence of deep traps on minority carrier diffusion length and lifetime is discussed. The experimental results, showing the impact of minority carrier transport on the performance of bipolar devices, as well as a discussion of techniques, used for measurements of the minority carrier diffusion length and lifetime, are provided.

Fully-depleted SOI CMOS technology for heterogeneous micropower, high-temperature or RF microsystems

Abstract: Based on an extensive review of research results on the material, process, device and circuit properties of thin-film fully depleted SOI CMOS, our work demonstrates that such a process with channel lengths of about 1 mum may emerge as a most promising and mature contender for integrated microsystems which must operate under low-voltage low-power conditions, at microwave frequencies and/or in the temperature range 200-350 degreesC. (C) 2001 Elsevier Science Ltd. All rights reserved.

High- Q microphotonic electro-optic modulator

Abstract: A microphotonic mm-wave modulator using simultaneous RF and optical resonance in an electro-optic medium is presented. Theory and simulation of modulator operation is discussed, and experimental results demonstrating modulation using simultaneous resonance in the mm-wave range are reported.

Energy-band parameters at the GaAs– and GaN–Ga2O3(Gd2O3) interfaces

Abstract: The valence-band offset (ΔEV ) has been determined to be ∼2.6 eV at the Ga2O3(Gd2O3)–GaAs interface, and ∼1.1 eV at the Ga2O3(Gd2O3)–GaN interface by high-resolution X-ray photoelectron spectroscopy with synchrotron radiation beam. The Pt–Ga2O3(Gd2O3)–GaAs MOS diode exhibits a current–voltage characteristics dominated by Fowler–Nordheim tunneling. From the current–voltage data at forward and reverse biases, we have extracted a conduction-band offset (ΔEC) of ∼1.4 eV at the Ga2O3(Gd2O3)–GaAs interface and an electron effective mass m*∼0.29me of the Ga2O3(Gd2O3) layer. Consequently, the energy-band gap of Ga2O3(Gd2O3) is ∼5.4 eV, while ΔEC for the Ga2O3(Gd2O3)–GaN interface is ∼0.9 eV.

Comparison of MOSFET-threshold-voltage extraction methods

Abstract: The difference in MOSFET threshold voltages caused by the difference in the extraction method is studied, by measuring and analyzing its dependencies on channel length, substrate voltage and drain voltage. It is found that the standard deviation of the difference between threshold voltages caused by the difference in the extraction method is less than that of the threshold voltage itself in a wafer. The dependencies of the threshold voltage on channel length, extracted from the drain current data around the threshold voltage, however, show different behavior from those extracted from the drain current data only in the subthreshold region or only in the ON region. It is considered that “channel-length modulation” causes this different behavior and, therefore, that those extraction methods are not desirable.

Geometry optimization of interdigitated Schottky-barrier metal–semiconductor–metal photodiode structures

Abstract: The optimum geometry of an interdigitated Schottky-barrier metal–semiconductor–metal photodetector (MSM-PD) is discussed. From the calculated MSM-PD capacitance and transit time of optically generated carriers, the response time and quantum efficiency are evaluated and analysed. We propose a simple scaling rule to achieve the best high-speed response of the MSM detector. The optimum interelectrode spacing for interdigitated MSM-PD has been established. The potential of different semiconductor materials for high-speed MSM detectors is examined.

Electron field emission from nanostructured diamond and carbon nanotubes

Abstract: Both diamond and carbon nanotubes are efficient field emitters because of the negative electron affinity associated with the diamond surface and the geometrically nanometer-scale nature of the nanotubes. They offer the important advantage of ease in fabrication and low-cost manufacturing. Both materials have been shown to emit electrons at very low electric fields (3–7 V/μm for a current density of 10 mA/cm2). Moreover, nanotube emitters are found to be able to deliver very high emission currents densities, with current density routinely exceeding 1 A/cm2. The low field operation of these carbon based emitters is attractive for display applications, while the high current capability of nanotube emitters will enable a number of high power, high frequency devices. Further improvements over the emission uniformity hold the key for the realization of the potential of these carbon materials in enabling practically useful cold cathode devices.

Determination of the Fowler–Nordheim tunneling parameters from the Fowler–Nordheim plot

Abstract: A wide range of values have been reported for the Fowler–Nordheim (F–N) parameters A and B for the tunneling emission in MOS capacitor structures. The parameters A and B are respectively the pre-exponential factor and the exponent in the F–N equation. In this study, it is shown that the oxide thickness is the most influential factor in determining the accuracy of the parameter B . The uncertainty of the oxide thickness may be responsible for the discrepancies in the reported values for B . The discrepancy in the value of A was attributed to the difference in the measurement conditions and gate oxide processing.

Organic thin film transistors: from active materials to novel applications

Abstract: In this paper, a bird's eye view of most of the organic materials employed as n-channel and p-channel transistor active layers is given along with the relevant device performances; organic thin film transistors (OTFT) operation regimes are discussed and an interesting perspective application of OTFT as multi-parameter gas sensor is proposed.

Barrier capability of TaNx films deposited by different nitrogen flow rate against Cu diffusion in Cu/TaNx/n(+)-p junction diodes

Abstract: This paper investigates the barrier capability of tantalum nitride (TaNx) layers against Cu diAusion. The TaNx layers were reactively sputtered in contact holes to a thickness of 50 nm by using a diAerent nitrogen flow rate. Results indicate that the TaNx layers fail to be a diAusion barrier due to a relative high resistivity for nitrogen flow ratios exceeding 10%. In addition, we found that the phase of a-Ta(‐N) functions as an eAective barrier against Cu diAusion and that Cu/ TaN(3‐5%)/n a ‐p junction diodes are able to sustain a 30 min furnace anneal up to 500∞C without causing degradation of the electrical characteristics. The high-temperature failure of barrier capability for the TaNx layers is due to interdiAusion of Cu and Si across the TaNx film structure to form Cu3Si. The surface roughness and the film structure of TaNx layers determine the ability of Cu and Si interdiAusion. ” 2001 Elsevier Science Ltd. All rights reserved.

Photo-induced growth of dielectrics with excimer lamps

Abstract: The underlying principles and properties of vacuum ultraviolet (VUV) and ultraviolet (UV) radiation (excimer lamps), generated by a dielectric barrier discharge in a rare-gas (Rg) or a mixture of Rg and halogen, are discussed. Compared with conventional sources, these excimer lamps offer narrow-band radiation at various wavelengths from 108–354 nm and over large areas with high efficiencies and high power densities. The variety of available wavelengths offers an enormous potential for new industrial applications in materials processing. Previously, photo-oxidation of silicon, germanium and silicon–germanium and photo-deposition of single- and multi-layered films of silicon oxide, silicon nitride, and silicon oxynitride have been demonstrated by using excimer lamps. This paper reviews the progress on excimer lamp-assisted growth of high dielectric constant (Ta 2 O 5 , TiO 2 and PZT) and low dielectric constant (polyimide and porous silica) thin films by photo-CVD and sol–gel processing, summarizes the properties of photo-induced growth of Ta 2 O 5 films and discusses the effect and mechanism of low temperature UV annealing with 172 nm radiation. Metal oxide semiconductor capacitors based on the photo-induced Ta 2 O 5 films grown directly on Si at low temperatures exhibit excellent electrical properties. Leakage current densities as low as 5.2×10 −7 A cm −2 and 2.41×10 −7 A cm −2 at 1 MV cm −1 have been achieved for the as-grown Ta 2 O 5 films deposited by photo-induced sol–gel processing and photo-CVD, respectively-several orders of magnitude lower than for any other as-grown films prepared by any other technique. A subsequent low temperature (400°C) UV annealing step improves these to 10 −8 A cm −2 . These values are comparable to those only previously achieved for films annealed at high temperatures between 600°C and 1000°C. These properties make the photo-induced growth of Ta 2 O 5 layers suitable alternative to SiO 2 for high density DRAM application.

Design of single and multiple zone junction termination extension structures for SiC power devices

Abstract: The design of SiC planar and non-planar junction termination extension (JTE) structures are investigated through calibrated quasi-3D numerical simulation. JTE techniques are optimized with respect to breakdown voltage, area consumption, surface fields, and interface charge. Simulated JTE charge profiles predict near-ideal breakdown values with surface fields less than 60% of their bulk values, and show that the critical implant activation percentage can vary as much as 40% with only 10% reduction in breakdown voltage for lightly doped blocking layers. These results are extended to unique multiple-zone JTE and mesa-JTE structures applicable to a wide range of planar and non-planar SiC power devices.

A simple subthreshold swing model for short channel MOSFETs

Abstract: A new approach to calculate the subthreshold swing of short channel bulk and silicon-on-insulator metal oxide semiconductor field effect transistors is presented. The procedure utilizes a channel-potential expression appropriate for submicron dimensions. The final result is similar to that used for long channels except for a factor λ which represents the short channel effects. Comparison with different published results reveals excellent quantitative agreement.

Hall mobility and carrier concentration in free-standing high quality GaN templates grown by hydride vapor phase epitaxy

Abstract: Measured and calculated (without any adjustable material parameter) electron Hall mobility and carrier concentration in the range of 265–273 K are reported for a high-mobility free-standing bulk GaN grown by hydride vapor phase epitaxy The peak electron mobility of 7386 cm 2 /V s at 48 K and a value of 1425 cm 2 /V s at 273 K were measured An iterative solution of the Boltzmann equation was applied to calculate the mobility using the materials parameters either measured on the sample under study or recent values that are just becoming available with only the acceptor concentration being variable Using only one donor and one conducting layer system, the donor and acceptor concentrations of 176×10 16 and 24×10 15 cm −3 , respectively, satisfy simultaneously the charge neutrality and electron mobility at all temperatures within the framework of the iterative method and measurements The donor activation energy was determined to be 252 meV and is consistent with the value of about 30 meV for the hydrogenic ground state in a dilute semiconductor The high electron mobility, low background impurity concentration, low compensation ratio, and negligible dislocation scattering demonstrate the high quality of the material studied

Extraction of Schottky diode parameters with a bias dependent barrier height

Abstract: We describe a technique to extract device parameters of a Schottky barrier diode whose barrier height is bias dependent and which contains a linear series resistance. The extracted parameters include the saturation current (zero bias barrier height), the voltage dependence of the barrier height and of the ideality factor as well as series resistance. The technique makes use of forward biased current‐voltageOI‐VU characteristic and voltage-dependent diAerential slope curve aa dl nI OU = dl nV OU . The method is verified using simulated and experimental I‐V curves of an Al‐pSi structure. The proposed procedure is not limited to Schottky barrier diodes but may be applied to other diode types based on P‐N junction. ” 2001 Elsevier Science Ltd. All rights reserved.

Properties and characterization of chemical vapor deposition diamond field emitters

Abstract: The field emission properties of chemical vapor deposition (CVD) diamond thin films are investigated by measuring the field emission I–V characteristics, the emission site density and the field emitted electron energy distribution. The results are discussed with regard to field emission due to negative electron affinity (NEA) and classical Fowler–Nordheim emission due to geometrical field enhancement. The requirements on the diamond films for NEA mediated field emission are discussed. These requirements are high resistivity, low defect density and few grain boundaries. We show that diamond films matching these requirements are actually bad field emitters. On the other hand we show that the films exhibiting good field emission properties are just opposed to the above mentioned requirements; they exhibit low resistivity, high defect density and many grain boundaries as they are of nanocrystalline nature. The good emitting CVD diamond films are grown on p-type Si(1 0 0) using plasma enhanced CVD at substrate temperatures around 950°C and a gas mixture of 5% CH4 in H2. Using the example of multiwalled carbon nanotube emitter (MWNT) we show how the emitter work function and the local field at the emission site can be determined independently by measuring the field emitted electron energy distribution. For MWNT we find a work function of 4.9 eV and a local field of 2500 V μm−1 (for an emission current of the order of 10 pA). In the case of nanocrystalline CVD diamond emitter we find work function values around 6 eV and local electric fields again of the order of 2500 V μm−1. The electronic structure of the nanocrystalline CVD diamond field emitters is investigated using standard photoemission spectroscopy and simultaneous photoemission and field emission spectroscopy. From the presented measurements we can clearly relate the low field electron emission of the investigated nanocrystalline diamond films to classical Fowler–Nordheim tunneling due to local geometrical field enhancement.

Electrical characterization of ONO triple dielectric in SONOS nonvolatile memory devices

Abstract: Polysilicon-oxide–nitride–oxide–silicon nonvolatile memory devices are characterized by a high concentration of traps in the silicon nitride layer with a well-defined trapping distance, corresponding to the tunnel oxide thickness. A “breakpoint” is observed at a particular frequency with an inverse equivalent to the trap-to-trap tunneling time constant by variable frequency charge pumping technique, from which the tunnel oxide thickness can be decided. By examining the charge transport and trapping properties of nitride films utilizing charge separation technique, an experimental method to extract the silicon nitride storage layer thickness has been developed. Excellent agreement is obtained between the ellipsometry results and extracted values.

The enhancement factor and the characterization of amorphous carbon field emitters

Abstract: We report on the results of a combined experimental and computational investigation on the properties of amorphous carbon nano-tips grown by carbon contamination in a scanning electron microscope and used as electron field emitters. We have calculated the strength of the electric field at the emitter surface Ftip and the dimensionless enhancement factor γ (=F tip /F m ) for various geometries and sizes by means of the finite element method. By using Ftip, experimental data and results from the Fowler–Nordheim plot, we have found consistent values of the work function, the surface emitting area and the current density J. The values of Ftip and J are in reasonable agreement with other experimental deductions.