Improved off-current and subthreshold slope in aggressively scaled poly-Si TFTs with a single grain boundary in the channel

A polycrystalline-silicon thin-film transistor (TFT), with a single grain boundary (GB) present in the channel, is simulated using two-dimensional numerical simulation, which includes a model of deep trap states at GBs. It is observed that the potential barrier resulting from a GB in the channel acts to suppress current flowing through the channel when the barrier height is greater than the thermal voltage. The conduction mechanism in the subthreshold regime is clarified. The turn-on characteristics of the device are controlled primarily by gate-induced grain barrier lowering as opposed to modulation of carriers in the channel by the gate voltage. In the negative bias region it is found that suppression of the off current is aided by the GB potential barrier. Scaling of the various geometrical parameters of the device are investigated. Improved subthreshold characteristics, compared to an equivalent silicon-on-insulator (SOI) structure, are found for aggressively scaled devices, due to the presence of a GB in the channel.     

Nonequilibrium population of charge carriers in structures with InGaN deep quantum dots

Electronic and optical properties of ensembles of quantum dots with various energies of activation from the ground-state level to the continuous-spectrum region were studied theoretically and experimentally with the InGaN quantum dots as an example. It is shown that, depending on the activation energy, both the quasi-equilibrium statistic of charge carriers at the levels of quantum dots and nonequilibrium statistic at room temperature are possible. In the latter case, the position of the maximum in the emission spectrum is governed by the value of the demarcation transition: the quantum dots with the transition energy higher than this value feature the quasi-equilibrium population of charge carriers, while the quantum dots with the transition energy lower than the demarcation-transition energy feature the nonequilibrium population. A model based on kinetic equations was used in the theoretical analysis. The key parameters determining the statistic are the parameters of thermal ejection of charge carriers; these parameters depend exponentially on the activation energy. It is shown experimentally that the use of stimulated phase decomposition makes it possible to appreciably increase the activation energy. In this case, the thermal-activation time is found to be much longer than the recombination time for an electron-hole pair, which suppresses the redistribution of charge carriers between the quantum dots and gives rise to the nonequilibrium population. The effect of nonequilibrium population on the luminescent properties of the structures with quantum dots is studied in detail.     

Current conduction and reliability implications under currentramping stress of VLSI contacts

The current conduction mechanisms of VLSI contact structures have been studied using the current ramping stress method and two-dimensional device simulations. Because of Joule heating effect and the subsequent emergence of intrinsic carriers in the connecting diffusion region, reduction in electrical resistance is observed as the current ramps beyond a transition point. The transition temperature, experimentally determined I-V curve and current spread-out across the diffusion/well junction are confirmed by two-dimensional device simulations. The reliability implications and current-carrying capability of the contact/diffusion structure for VLSI applications are also discussed     

An Analytical Expression for the Transfer Characteristics of a Polycrystalline Silicon Thin-Film Transistor With an Undoped Channel

An analytical model for the transfer characteristics of a polycrystalline silicon thin-film transistor (TFT) with an undoped channel is developed. While the modeling methodology is general, the analytical form is based on the observation that the trap states in a grain boundary are exponentially dispersed over the energy space defined by the energy gap of the adjacent grains. The dispersion relationship is parameterized by an energy EA. The complex mechanisms governing carrier transport in a TFT are modeled in terms of an effective ldquodriftrdquo mobility mu_eff that also accounts for the thermionic emission of charge carriers across the grain boundaries. A gate bias V _pt can be identified that roughly locates the transition from the ldquopseudo-subthresholdrdquo and the ldquoturn-onrdquo regimes of operations. Techniques for the extraction of EA and the transition voltage V _pt are proposed. A particularly simple expression of mu_eff can be obtained in terms of these and other parameters.     

A phenomenological approach to estimating transit times in GaAsHBTs

A method is proposed for estimating transit times in heterojunction bipolar transistors (HBTs) which are fabricated from semiconductor materials in which the conduction band can be represented by a two-valley model. The transition times for exchange of electrons between the conduction band valleys are treated as phenomenological parameters and are shown to be linked by the electric field in the device. Incorporation of the transition rates into the continuity equations for upper and lower valley electrons yields a set of equations which can be solved under transient conditions to yield directly the transit times of carriers across the base and the base-collector space-charge region. Results from this approach are compared with Monte Carlo calculations and shown to exhibit good agreement     

Evolution of exciton states near the percolation threshold in two-phase systems with II–VI semiconductor quantum dots

From studies of two-phase systems (borosilicate matrices containing ZnSe or CdS quantum dots), it was found that the systems exhibit a specific feature associated with the percolation phase transition of charge carriers (excitons). The transition manifests itself as radical changes in the optical spectra of both ZnSe and CdS quantum dot systems and by fluctuations of the emission band intensities near the percolation threshold. These effects are due to microscopic fluctuations of the density of quantum dots. The average spacing between quantum dots is calculated taking into account their finite dimensions and the volume fraction occupied by the quantum dots at the percolation threshold. It is shown that clustering of quantum dots occurs via tunneling of charge carriers between the dots. A physical mechanism responsible for the percolation threshold for charge carriers is suggested. In the mechanism, the permittivity mismatch of the materials of the matrix and quantum dots plays an important role in delocalization of charge carriers (excitons): due to the mismatch, “a dielectric trap” is formed at the external surface of the interface between the matrix and a quantum dot and, thus, surface exciton states are formed there. The critical concentrations of quantum dots are determined, such that the spatial overlapping of such surface states provides the percolation transition in both systems.     

Excited-State-Mediated Capture of Carriers Into the Ground State and the Saturation of Optical Power in Quantum-Dot Lasers

Excited-state-mediated capture of carriers from the waveguide into the lasing ground-state in quantum dots (QDs) is studied. Such a two-step capture places a fundamental limitation on ground-state lasing-the output power saturates at high injection currents. The saturation power is controlled by the transition time between the excited- and ground-state in a QD. The longest, cut-off transition time exists, beyond which no ground-state lasing is possible     

A class of call-fail-safe and distribution-effective multi-band multi-carrier traffic allocation methods for 3GB wireless systems

For better capacity and higher availability, present day third generation (3G) wireless systems based on the code division multiple access(CDMA) technology are evolving to operate on multiple carriers (frequencies) spread over multiple bands. In order to provide better quality of service, the 3GB (3rd generation and beyond) systems need to distribute calls equitably to different carriers on different base-stations accessible to the mobiles irrespective of the bands or carriers on which those mobiles initiated their calls. However, there is a risk of call failure when a call originated on a carrier in a band is migrated to another carrier in a different band, particularly because of the differences in the radio coverage of the base-stations operating in different bands. This paper presents a class of methods that offer equal robustness against call failures and varying degrees of call distribution effectiveness. For call distribution, these methods employ an enhanced carrier capacity measure (ECM) proposed in this paper. ECM augments the gross capacities of the carriers (to house calls) with pre-configured biases specific to the mobile users. We develop here an intuitively appealing distribution-effectiveness measure based on the ECM for comparing the methods. Relative performances of the proposed methods with respect to call failure rate and distribution effectiveness are established by means of simulation results for calls originating anywhere in the cell coverage area as well as calls originating exclusively near the cell boundaries. The latter results help to study the effect of mobility on the performances of the algorithms.     

Current gain in polysilicon emitter transistors

The common-emitter current gain β in a shallow polysilicon emitter transistor is derived by solving the minority-carrier transport equation in the silicon-polysilicon structure. Coupled With the majority-carrier transport, an equation for the current gain is obtained which depends on the physical properties of the polysilicon as well as the silicon emitter and base doping profiles. The calculations are based on the carrier trapping model proposed in the literature and ignore any minority-carrier recombinations. The model predicts the current gain of a polysilicon emitter transistor increases at low current and high temperature and approaches that of a conventional metal contact transistor at high current and low temperature. This is due to the potential barrier across the grain boundaries, while impeding the majority-carrier flow, inject minority carriers as a bias is developed across the grain boundaries. It also predicts a decrease in cutoff frequency fTdue to minority carriers stored in the polysilicon.     

Understanding the Beneficial Role of Grain Boundaries in Polycrystalline Solar Cells from Single‐Grain‐Boundary Scanning Probe Microscopy

The superior performance of certain polycrystalline (PX) solar cells compared to that of corresponding single‐crystal ones has been an enigma until recently. Conventional knowledge predicted that grain boundaries serve as traps and recombination centers for the photogenerated carriers, which should decrease cell performance. To understand if cell performance is limited by grain bulk, grain surface, and/or grain boundaries (GBs), we performed high‐resolution mapping of electronic properties of single GBs and grain surfaces in PX p‐CdTe/n‐CdS solar cells. Combining results from scanning electron and scanning probe microscopies, viz., capacitance, Kelvin probe, and conductive probe atomic force microscopies, and comparing images taken under varying conditions, allowed elimination of topography‐related artifacts and verification of the measured properties. Our experimental results led to several interesting conclusions: 1) current is depleted near GBs, while photocurrents are enhanced along the GB cores; 2) GB cores are inverted, which explains GB core conduction. Conclusions (1) and (2) imply that the regions around the GBs function as an extension of the carrier‐collection volume, i.e., they participate actively in the photovoltaic conversion process, while conclusion (2) implies minimal recombination at the GB cores; 3) the surface potential is diminished near the GBs; and 4) the photovoltaic and metallurgical junction in the n‐CdS/p‐CdTe devices coincide. These conclusions, taken together with gettering of defects and impurities from the bulk into the GBs, explain the good photovoltaic performance of these PX cells (at the expense of some voltage loss, as is indeed observed). We show that these CdTe GB features are induced by the CdCl₂ heat treatment used to optimize these cells in the production process.     

Investigation about the hot-carrier repelling effect in semiconductor devices, using an analytical solution of the hydrodynamic model

In this paper, the author discusses the consequences of the back-thermal diffusion of hot carriers, using an analytical solution of the hydrodynamic model (HDM). He observed a total deformation in the carrier distribution, around the reverse-biased semiconductor junctions, due to the back-thermal diffusion mechanism. At sufficient reverse bias, the diffusion of minority carriers (generation in neutral regions) is found to be influenced by the carrier-temperature gradient. Therefore, charge carriers are pushed forth (by electric-field effect) and back (by the thermal gradients) and the balance between these forces are controlled by the applied bias value. This may explain the origin of the terahertz oscillations in semiconductors after excitation by ultrafast laser pulses, which is currently an issue of debate. In order to confirm the validity of the proposed theory, the transport of hot carriers across a reverse-biased p-i-n diode is simulated using the HDM, and the conventional drift-diffusion model. He shows that the number of hot minority carriers is not at the junction boundary, but has an appreciable value, as if they are repelled back from the junction boundary to the cold neutral-region side, by thermal diffusion. He also proposes a simple analytical solution of the set of hydrodynamic equations, to demonstrate his theory of hot-carrier transport in semiconductor devices in general and across p-n junctions in particular. On the basis of his analysis, he discusses the terahertz generation from a reverse-biased p-i-n diode, which is subjected to femtosecond laser pulses.     

建设核心网,为3G作好准备

介绍在同一数据核心网中如何支持不同代的网络，以实现２Ｇ向３Ｇ的过渡；指出发展基于分组／信元的核心交换网，为３Ｇ设备和服务的展开做好准备的时机已经到来；说明如何提高数据与语音业务容量，使运营商能够向３Ｇ基础用户群提供服务。     

Characterization of an individual grain boundary in metal-inducedlaterally crystallized polycrystalline silicon thin-film devices

A distinct grain boundary (GB) is formed when two crystallization fronts collide in metal-induced laterally crystallized (MILC) polycrystalline silicon (poly-Si) thin films. This has been used to study carrier transport across a single dominant GB. The average number of traps per unit area is found to be about 1.6× 10¹²/cm² in this GB, significantly higher than that associated with the regular GBs in the bulk of MILC poly-Si. Though this single GB occupies a negligible fraction of the total device volume, it has been found to significantly affect both the resistance of MILC resistors and the leakage current of MILC thin-film transistors     

Computed transient solution for the m.s.m. diode with traps

A numerical technique has been developed to simulate a 1-carrier model of the m.s.m. diode with traps. The distributions of free and trapped carriers, and consequently the terminal current, are predicted as functions of time following a step change in the voltage across the device.     

Quasi-ray Gaussian beam algorithm for time-harmonic two-dimensionalscattering by moderately rough interfaces

Gabor-based Gaussian beam (GB) algorithms, in conjunction with the complex source point (CSP) method for generating beam-like wave objects, have found application in a variety of high-frequency wave propagation and diffraction scenarios. Of special interest for efficient numerical implementation is the noncollimated narrow-waisted species of GB, which reduces the computationally intensive complex ray tracing for collimated GB propagation and scattering to quasi-real ray tracing, without the failure of strictly real ray field algorithms in caustic and other transition regions. The Gabor-based narrow-waisted CSP-GB method has been applied previously to two-dimensional (2-D) propagation from extended nonfocused and focused aperture distributions through arbitrarily curved 2-D layered environments. In this 2-D study the method is applied to aperture-excited field scattering from, and transmission through, a moderately rough interface between two dielectric media. It is shown that the algorithm produces accurate and computationally efficient solutions for this complex propagation environment, over a range of calibrated combinations of the problem parameters. One of the potential uses of the algorithm is as an efficient forward solver for inverse problems concerned with profile and object reconstruction     

The insulated gate tunnel junction triode

The gate modulated voltage breakdown of the drain diode in the MOS transistor is considered and shown to be direct electric field control of a reverse biased surfacep+-njunction. A structure designed to isolate this effect has been suggested by Atalla and experimentally evaluated by Nathanson, et al., and by the authors. The mechanism of operation discussed involves the application of an external electric field normal to the surface of the highly doped side of the junction to produce direct field emission of carriers. The reverse biased low doped side of the junction then acts as a collector of the field-emitted carriers resulting in a net current flow across the junction. Using the Atalla structure, a model is presented and a quantitative theory is developed to explain and predict the device performance. It is found that the actual device may be represented as an MOS transistor in series with an "ideal" field-controlled tunnel junction, and that the performance of the actual device can never be better than that of the limiting MOS transistor. The theoretical characteristics of the ideal field-controlled tunnel junction are derived and found to agree closely with the experimental results. It is shown that, at the present time, the device is limited by the "ideal" tunnel junction region and not by the series MOS transistor.     

Questionability of drift-diffusion transport in the analysis of small semiconductor devices

Analysis and design of such semiconductor devices as bipolar and field-effect transistors now relies, in part, on the assumption that current flows by drift and diffusion. The minute size of existing devices raises questions about the appropriateness of this assumption, however, because an internal region critical to device performance may become so small that carriers crossing it fail to experience many collisions. To demonstrate possible consequences of continuing to base analysis on assumed drift and diffusion, we contrast the predictions of two models for the electrical base region of a bipolar transistor. The first model assumes that carriers in transit across the base will experience many collisions, which requires a base region of sufficiently large dimensions. The second model assumes that a carrier in transit will suffer no collisions, displaying what tends to occur in the limiting case of small dimensions. A comparison of saturation current, temperature dependence, and base transit time shows that significant differences exist.     

Thermally Induced Transient Absorption of Light by Poly(3,4‐ethylenedioxythiophene):Poly(styrene sulfonic acid) (PEDOT:PSS) Films: A Way to Probe Charge‐Carrier Thermalization Processes

Infrared‐induced transient absorptions in the millisecond and sub‐picosecond time domains have been used to study the dynamics of charge carriers of a conducting polymer, poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS). On the millisecond timescale, the transient absorption is ascribed to a thermal effect induced by absorbed infrared light. The decay of the transient absorption is limited by the transport of heat from the polymer film to the substrate and corresponds to the decay kinetics of infrared‐induced changes in the resistivity of the material. Near 1.5 eV, the infrared‐induced absorption can be modeled in terms of an interband transition. The assignment of the optical transients in terms of carrier heating opens the possibility to study charge carrier thermalization processes using short laser pulses. Pump‐probe spectroscopy on a sub‐picosecond timescale shows that the initial thermalization of the excited charge carriers occurs with a time constant of less than 500 fs, i.e., faster than for noble metals.     

Energy Saving Scheme for Multicarrier HSPA + Under Realistic Traffic Fluctuation

In the near future, an increase in cellular network density is expected to be one of the main enablers to boost the system capacity. This development will lead to an increase in the network energy consumption. In this context, we propose an energy efficient dynamic scheme for HSDPA + (High Speed Downlink Packet Access-Advanced) systems aggregating several carriers and which adapts dynamically to the network traffic. The scheme evaluates whether node-B deactivation is feasible without compromising the user flow throughput. Furthermore, instead of progressive de-activation of carriers and/or node-B switch-off, we evaluate the approach where feasible combination of inter-site distance and number of carriers is searched to obtain best savings. This is done by also considering the effect of transition delays between network configuration changes. The solution exploits the fact that re-activation of carriers might permit turning off other BSs earlier at relatively higher load than existing policies. Remote electrical downtilt is also considered as a means to maximize the utilization of higher modulation and coding schemes in the extended cells. This approach promises significant energy savings when compared with existing policies - not only for low traffic hours but also for medium load scenarios.