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Electron Devices, IEEE Transactions on

Issue 12 • Date Dec 1989

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Displaying Results 1 - 25 of 40
  • Electrical characteristics and thermally induced metastability in an amorphous-silicon ambipolar transistor

    Page(s): 2928 - 2934
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    Electrical characteristics and thermally induced metastability in a boron-doped hydrogenated amorphous silicon (a-Si:H) thin-film transistor (TFT) deposited at 180°C are discussed. The on/off ratio of the conductance and the field-effect mobility are similar to those for undoped a-Si:H TFTs. The device characteristics exhibit n- and p-channel operations. The enhancement in the drain currents caused by double injections of electrons and holes appears for the p-channel operation of the TFT. Thermal quenching experiments on boron-doped a-Si:H silicon-nitride ambipolar TFTs give clear evidence of the coexistence of two distinct metastable changes: the metastable creation of the active dopants and dangling bonds appear to increase with quenching temperature View full abstract»

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  • Bulk and surface properties of amorphous hydrogenated fluorinated silicon grown from SiF4 and H2

    Page(s): 2853 - 2858
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    A detailed study of the growth of amorphous hydrogenated fluorinated silicon (a-Si:H, F) from a DC glow discharge in SiF4 and H2 is discussed. The electrical properties of the films can be varied over a very wide range. The bulk properties of the best films that were measured included an Urbach energy Eu =43 meV, a deep-level defect density Ns=1.5×1015 cm-3, and a hole drift mobility of 8×10-3 cm2 V-1 s-1, which reflects a characteristic valence band energy of 36 meV. It was found that Eu, N s, and the density of surface states Nss are related to each other. Under the deposition condition of the films with the best bulk properties, Nss reaches its highest value of 1×1014 cm-2. It is suggested that in growth from SiF4/H2, the density of dangling bonds at the growing surface is very sensitive to the deposition conditions View full abstract»

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  • Amorphous-SiC thin-film p-i-n light-emitting diode using amorphous-SiN hot-carrier tunneling injection layers

    Page(s): 2895 - 2902
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    An approach to improve the luminosity of hydrogenated amorphous silicon carbide (a-SiC:H)-based thin-film visible light-emitting diodes is discussed. High bandgap near-stoichiometric hydrogenated amorphous silicon nitride (a-SiN:H) is utilized as a hot-carrier tunneling injection layer. An improvement of both carrier injection efficiency and luminosity is observed. Technical data on the new approach for carrier injection and on the recombination mechanism are presented. Preliminary results are also presented on the photoluminescence and electroluminescence properties of a-SiC:H/a-SiN:H multilayers View full abstract»

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  • Process-controlled staggered ambipolar amorphous-silicon thin-film transistor

    Page(s): 2903 - 2907
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    The results of measurements performed on amorphous-silicon thin-film transistors (TFTs) are presented and interpreted. Both unipolar and ambipolar effect TFTs are obtained in the experiments. Whether they possess unipolar or ambipolar characteristics is controlled by the interface properties between gate insulator and amorphous silicon. With the different compositions of hybrid gate insulators, the devices show different I-V characteristics. A simple method for estimating film parameters of amorphous silicon and device parameters of the TFTs is developed. The characteristic energy of an amorphous-silicon film is obtained from the slope of the ratio of saturation current to transconductance. Threshold voltages and flatband voltages of the TFTs can also be obtained using this method View full abstract»

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  • Amorphous-silicon solar cells

    Page(s): 2775 - 2780
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    The status of a-Si solar cell technology is reviewed. This review includes a discussion of the types of solar cell structure that are being used in commercial products. An overview of the development efforts under way involving new materials, such as alloys and microcrystalline films, and their impact on device performance is given. The status of stability in a-Si solar cells and projections for costs for large-scale manufacturing facilities are reviewed. The development of markets for a-Si photovoltaics is also discussed View full abstract»

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  • Hydrogenated amorphous-silicon photosensor for optically addressed high-speed spatial light modulator

    Page(s): 2959 - 2964
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    A high-speed, optically addressed spatial light modulator (OASLM) which incorporates a hydrogenated amorphous silicon (a-Si:H) photosensor and a ferroelectric liquid-crystal (FLC) modulator is discussed. The device operates with an applied square-wave voltage such that read and write operations occur under reverse bias, while the erase operation occurs under forward bias. The OASLM exhibits a response time of 155 μs and a spatial resolution of >33 1p/mm. The capacitance and resistance of both the a-Si:H and FLC have been measured and are shown to influence the response of the device strongly View full abstract»

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  • Thermal equilibrium electronic properties of a-Si:H

    Page(s): 2770 - 2774
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    Defect and donor states in a-Si:H are in thermal equilibrium above about 100°C, with concentrations that depend on temperature and the position of the Fermi energy. This interaction between the electronic and structural states has a profound effect on all the electronic properties. Some aspects of the thermodynamic model for the equilibrium state are reviewed and shown to explain the properties of doped and compensated a-Si:H and the temperature dependence of the defect density in undoped material. The different conductivity behavior of the low-temperature frozen state and the high-temperatures equilibrium are also explained View full abstract»

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  • The physics of amorphous-silicon thin-film transistors

    Page(s): 2753 - 2763
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    The basic physics underlying the operation and key performance issues of amorphous-silicon thin-film transistors (TFTs) are discussed. The static transistor characteristics are determined by the localized electronic states that occur in the bandgap of the amorphous silicon. The deep states, mostly consisting of Si dangling bonds, determine the threshold voltage, and the conduction band-tail states determine the field-effect mobility. The finite capture and emission times of the deep localized states lead to a dynamic transistor characteristic that can be described by a time-dependent threshold voltage. The transistors also show longer time threshold voltage shifts due to two other distinct mechanisms: charge trapping in the silicon nitride gate insulator and metastable dangling bond state creation in the amorphous silicon. These two mechanisms show characteristically different bias, temperature, and time dependencies of the threshold voltage shift. Illumination of a TFT causes the generation of electron-hole pairs in the space-charge region leading to a steady-state equal flux of electrons and holes and a reduction in the band-bending. In most applications, the photosensitivity should be minimized. The uniformity of large arrays of transistors for display applications is excellent, with variations in the threshold voltage of 0.5-1.0 V View full abstract»

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  • Silicon matrix disorder in amorphous hydrogenated silicon alloys

    Page(s): 2863 - 2867
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    The authors report a detailed investigation of correlations between Urbach energies from photothermal deflection spectroscopy and Raman half-widths of transverse optic (TO)-like Si-Si modes as a measure of silicon matrix disorder in glow-discharge amorphous hydrogenated silicon (a-Si:H) and a-SiGe:H, as well as in glow-discharge and sputtered a-SiC:H and a-SiN:H. A corresponding decrease in TO full width at half-maximum (FWHM) and Urbach energy E0 for soft deposition techniques yields bond angle distributions as narrow as 8.5° for the best a-Si:H films. Even at the lowest levels of nitrogen incorporation, simultaneous increases in E0 and TO-like half-widths indicate that lattice distortions occur due to threefold coordination of nitrogen in the a-Si:H matrix. In contrast, no deviation of silicon TO-FWHM could be detected in a-SiC:H of up to 35 at.% of carbon content, whereas Urbach edges broaden in a well-known manner that is interpreted in terms of -CH3 incorporation into the amorphous network. Diborane doping and sputter deposition, however, give rise to lattice distortions in a-SiC:H, which reflects changes in the carbon coordination View full abstract»

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  • Characteristic lengths for steady-state transport in illuminated, intrinsic a-Si:H

    Page(s): 2789 - 2797
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    The steady-state transport equations are solved for a general small-signal case. This solution leads to the spatial distribution of the carriers in the bulk of undoped hydrogenated amorphous silicon (a-Si:H) under illumination. The general small-signal differential equations for electrons and holes, including a local and/or external electric field, are first established. Subsequently, the analysis is restricted to the case with a negligible external electric field. Two characteristic lengths appear in the analytical solution. They are interpreted for the lifetime regime as the ambipolar diffusion length and the dielectric relaxation length. Depending on the material parameters, one or the other of these lengths dominates the transport. Illustrative numerical examples for typical solar-cell-quality a-Si:H are given View full abstract»

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  • Picosecond photocarrier transport in hydrogenated amorphous-silicon p-i-n diodes

    Page(s): 2781 - 2784
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    Optically detected, picosecond-domain photocarrier transport measurements for hydrogenated amorphous silicon (a-Si:H) p-i-n diodes with uniform and nonuniform electric fields are reported. The transient electroabsorption bleaching mechanism for optical detection is described, and the interpretation of these transients is extended to the case of nonuniform electric fields. A technique for using surface-absorbed light and optical detection for measurement of nonuniform electric fields is presented View full abstract»

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  • Determination of the electric field at interfaces in amorphous-silicon devices using time-of-flight measurements

    Page(s): 2803 - 2809
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    The built-in electric field in hydrogenated amorphous silicon (a-Si:H) Mo-Schottky diodes is estimated using voltage transient measurements generated by a sophisticated time-of-flight or time-resolved photoconductivity experiment with a rather high time resolution of 1 ns. The actual determination of the electric field is done by inverse modeling. The simulator (forward model) for the voltage transient is based on the complete set of transport equations for the charge carriers, including surface recombination. The significance of the diffusion and surface recombination is clearly shown by comparing simulated and measured voltage transients View full abstract»

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  • Short-channel a-Si thin-film MOS transistors

    Page(s): 2940 - 2943
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    Experimental results on planar-type amorphous-silicon MOS transistors with submicrometer-length channels are described. The minimum channel length fabricated by using exposure technology was about 0.4 μm. The 0.55-μm-long device had an on-off current ratio of more than 105 at VD=2 V View full abstract»

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  • a-Si:H,&rlhar2;a-Si, Ge:H, F graded-bandgap structures

    Page(s): 2834 - 2838
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    The optoelectronic properties of graded-bandgap a-Si:H, F&rlhar2;a-Si, Ge:H, F semiconductor structures were studied by the time-of-flight technique and by the voltage-bias dependence of the photocurrent. The mobility-lifetime (μτ) products for both electrons and holes depend strongly on the compositional grading. Values of μτ that are higher than in bulk alloys with the same effective optical bandgap can be obtained. With graded-gap amorphous structures, the electron and hole transport properties can be tuned independently View full abstract»

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  • Field-drifting resonant tunneling through a-Si:H/a-Si1-xCx:H quantum wells at different locations of the i-layer of a p-i-n structure

    Page(s): 2816 - 2820
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    The transport of photogenerated carriers perpendicular to the quantum wells in the p-i-n structure is discussed. The field-drifting resonant tunneling through the a-Si:H/a-Si1-xCx:H double-barrier (or three-barrier) quantum wells, located at different positions of the i-layer, was studied. The room-temperature resonant-tunneling behavior is observed when the quantum well structure is embedded in the middle second of the i-layer. The results provide further evidence of quantum size effects in amorphous-silicon-based superlattice structures View full abstract»

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  • Hydrogenated amorphous-silicon image sensors

    Page(s): 2923 - 2927
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    Recent developments in hydrogenated amorphous-silicon image sensors are reviewed, and their specific problems discussed. One of the most exciting features of amorphous silicon is its large-area capability. Principal development efforts are therefore directed toward contact-type linear image sensors. Contact-type area sensors represent a new challenge and are already being developed. Amorphous silicon is also a good candidate for future high-definition video image sensors. Hybrid technology sensors (direct-readout and matrix-readout sensors), integrated sensors (photodiode and photoconductor types), optical scanning sensors, and other image sensors are reviewed View full abstract»

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  • The emitter-base interface current in silicon bipolar transistors with emitters deposited by plasma-enhanced CVD

    Page(s): 2889 - 2894
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    The DC characteristics of bipolar devices with emitters deposited by the glow discharge of silane are discussed. The emitter material can be amorphous or single crystalline, grown by low-temperature plasma epitaxy. At deposition temperatures as low as 320°C, surface cleaning is the most critical step in the process. Results on different ex-situ and in-situ cleanings are included. A model for the base current of bipolar transistors which is in agreement with the observations is proposed. It is shown that the base-emitter interface limits the transistor performance. After optimization, diode ideality factors approaching unity are obtained View full abstract»

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  • Analysis of distributed threshold voltage transistors

    Page(s): 2965 - 2970
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    The device physics of the distributed threshold voltage thin-film transistor is described. This transistor features a low and constant off-current over a wide range of gate voltages. The origin of this feature is investigated using a gradual channel model. It is pointed out, however, that the gradual channel approximation represents a worst-case analysis; there is room for obtaining even better characteristics by taking into account the limit imposed by the generation rate of carriers. The results are applicable not only to amorphous-silicon thin-film transistors, but also to polysilicon thin-film transistors View full abstract»

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  • Physical models for amorphous-silicon thin-film transistors and their implementation in a circuit simulation program

    Page(s): 2764 - 2769
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    A semianalytic theory to describe both the current-voltage and capacitance-voltage characteristics of amorphous silicon thin-film transistors on the basis of their physics of operation is presented. In this model, the drain current is directly related to the electron concentration at the source side of the channel. This enables one to describe the various regimes of operation of these devices (i.e. subthreshold or above threshold) using only one equation. The output conductance of these devices in saturation is also considered, and it is shown that the finite output impedance is a consequence of the drain voltage modulating the effective channel length by creating a space-charge limited current region of variable length near the drain. The results of this model are in good agreement both with experimental data and the results of comprehensive two-dimensional simulations. These device models have been successfully incorporated into a SPICE circuit simulation program View full abstract»

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  • Damage control at the SnO2/Si interface in optoelectronic amorphous silicon devices: an Auger and electrical study

    Page(s): 2829 - 2833
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    The interface between the transparent conductive oxide (TCO) SnO 2 and the first amorphous-silicon layer in optoelectronic devices (e.g. solar cells) can be damaged as a result of the interaction between the TCO and the plasma used for the glow-discharge deposition of the a-Si:H layers. Electrical barriers at the nominally ohmic contact and diffusion of tin into the active layers may result from chemical reduction of SnO2 and oxidation of Si. Auger depth-profiling techniques are used to measure the TCO damage directly on the devices. A method for quantifying the total amount of reduced tin from the profile data is developed. The extent of the TCO reduction is correlated with the preparation procedures and with the photovoltaic performance of the cells. In particular, the beneficial role of thin protecting metal layers on TCO is investigated by both Auger and electrical measurements View full abstract»

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  • Analysis and design of a-Si TFT/LCD panels with a pixel model

    Page(s): 2953 - 2958
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    The design of a-Si thin-film-transistor/liquid-crystal-display (TFT/LCD) panels using a pixel model which consists of an equivalent electrical circuit and includes parasitic capacitance and the actual structure of deposited layers is discussed. The model's validity is first confirmed by comparing calculations with experimental results on a-Si TFT switching characteristics and TFT/LCD electrooptical characteristics. The gradual-channel approximation of an MOS transistor is applied to the static I-V characteristic of a-Si TFTs. This approximation is successfully used to obtain an analytical expression for pixel electrode voltage during the TFT turn-on time. The calculated dependence of the LCD panel transmittance on signal voltage is shown to reproduce the experimental results. On the basis of the analysis, a TFT-addressed 5-in.-diagonal liquid-crystal color TV has been developed which exhibits excellent display quality View full abstract»

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  • Photoemission spectroscopy of heterojunctions of amorphous hydrogenated silicon with silicon oxide and nitride

    Page(s): 2798 - 2802
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    A study of the growth and electronic structure of a-Si:H/a-SiNx:H and a-Si:H/a-SiOx:H heterojunctions by photoemission spectroscopy is discussed. The interfaces in a-Si:H/a-SiOx are atomically abrupt, except for the SiOx on the Si interface which is graded over ~3 Å due to plasma oxidation. The offset energies between the a-Si:H valence band and those of a-SiNx:H and a-SiOx:H are 1.2 and 4.0 eV, respectively. Extra H(~2×1015) is incorporated in the Si on the SiNx interface region. The hole wave functions in a-Si:H are localized on a scale of 1-2 interatomic distances View full abstract»

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  • Silicon oxynitride and silicon oxynitride-silicon interface: a photoemission study

    Page(s): 2821 - 2824
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    A study of silicon-oxynitride materials and their interfaces with amorphous silicon using synchrotron radiation photoemission is discussed. The effect of the hydrogen atoms as an intralayer at the silicon-oxynitride-a-Si interface is also analyzed. The data relative to the bulk silicon oxynitride show that the valence band edge gradually shifts toward a higher binding energy as the [O]/[N] ratio (A) is increased. For the silicon-oxynitride-a-Si heterojunctions, the valence band discontinuities ΔEv ranged from 1.1±0.15 eV in the case of A=0.08 up to 4.6±0.15 eV in the case of A=2.02. The effect of the hydrogen intralayer was to reduce the ΔEv by 0.5±0.15 eV. The observed intralayer-induced effect is attributed to modification of the interface dipoles. A simple model calculation, based on electronegativity arguments, gives a reduction of 0.4 eV for A=0 and 0.46 for A corresponding to pure silicon dioxide (SiO2) View full abstract»

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  • Interpretation of transient currents in amorphous-silicon hydride p-i-n and n-i-n devices

    Page(s): 2785 - 2788
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    Transient current experiments on amorphous silicon hydride p-i-n and n-i-n devices were performed by M. Silver et al. (1986) as a means of studying the effects of localized states on charge transport in this material. However, some of the features of the observed current transients were not thoroughly understood. Here, simulations of these experiments are used to explain the experimental results and evaluate the parameter set used in the simulation View full abstract»

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  • a-Si1-xCx:H-based transistor performance and the relationship to electrical and optical properties

    Page(s): 2839 - 2843
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    The results of measurements of (1) n- and p-channel a-Si1-x Cx inverted gate field effect transistors, (2) the photoconductivity of those alloys, and (3) the optical absorption spectra, including subband gap absorption, are discussed. Measurements of the transistor characteristics clearly show a monotonic falloff in electronic mobility with carbon addition and a somewhat slower increase in threshold voltage. The electron mobility, in the range of 0.5 cm2/V-s for a-Si:H, is still at usable levels (>0.1 cm2/V-s) even at 40% carbon concentration (gas phase). Optical absorption measurements (photothermal deflection spectroscopy) indicate that the slope of the optical absorption edge decreases with increasing carbon concentration, suggesting an increase in the tail density of states. These data, taken together with the μτ product, can be explained entirely by the decrease in mobility. The drop in the effective mobility is believed to be due to the effective widening of the band-tail state distribution View full abstract»

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IEEE Transactions on Electron Devices publishes original and significant contributions relating to the theory, modeling, design, performance and reliability of electron and ion integrated circuit devices and interconnects.

 

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John D. Cressler
School of Electrical and Computer Engineering
Georgia Institute of Technology