Current status of one- and two-dimensional numerical models: Successes and limitations

The capabilities of one and two-dimensional numerical solar cell modeling programs (SCAP1D and SCAP2D) are described. The occasions when a two-dimensional model is required are discussed. The application of the models to design, analysis, and prediction are presented along with a discussion of problem areas for solar cell modeling

Simulation of phonon-assisted band-to-band tunneling in carbon nanotube field-effect transistors

Electronic transport in a carbon nanotube (CNT) metal-oxide-semiconductor field effect transistor (MOSFET) is simulated using the non-equilibrium Green's functions method with the account of electron-phonon scattering. For MOSFETs, ambipolar conduction is explained via phonon-assisted band-to-band (Landau-Zener) tunneling. In comparison to the ballistic case, we show that the phonon scattering shifts the onset of ambipolar conduction to more positive gate voltage (thereby increasing the off current). It is found that the subthreshold swing in ambipolar conduction can be made as steep as 40mV/decade despite the effect of phonon scattering.Comment: 13 pages, 4 figure

Multiple timescale encoding of slowly varying whisker stimulus envelope in cortical and thalamic neurons in vivo

7 p., 5 figures and referencesAdaptive processes over many timescales endow neurons with sensitivity to stimulus changes over a similarly wide range of scales. Although spike timing of single neurons can precisely signal rapid fluctuations in their inputs, the mean firing rate can convey information about slower-varying properties of the stimulus. Here, we investigate the firing rate response to a slowly varying envelope of whisker motion in two processing stages of the rat vibrissa pathway. The whiskers of anesthetized rats were moved through a noise trajectory with an amplitude that was sinusoidally modulated at one of several frequencies. In thalamic neurons, we found that the rate response to the stimulus envelope was also sinusoidal, with an approximately frequency-independent phase advance with respect to the input. Responses in cortex were similar but with a phase shift that was about three times larger, consistent with a larger amount of rate adaptation. These response properties can be described as a linear transformation of the input for which a single parameter quantifies the phase shift as well as the degree of adaptation. These results are reproduced by a model of adapting neurons connected by synapses with short-term plasticity, showing that the observed linear response and phase lead can be built up from a network that includes a sequence of nonlinear adapting elements. Our study elucidates how slowly varying envelope information under passive stimulation is preserved and transformed through the vibrissa processing pathway.This work was supported by the following: International Human Frontier Science Program Organization shortterm fellowship (B.N.L.); Spanish Ministry of Science and Innovation Grant BFU2008-03017/BFI (M.M.), cofunded by the European Regional Development Fund; CONSOLIDER Grant CSD2007-00023; European Commission Coordination Action ENINET, Contract LSHM-CT-2005-19063; and a McKnight Scholar Award in the Neurosciences (A.L.F.)Peer reviewe

Report on High Intensity Solar Cells. Period Covered: June 1, 1983 to November 4, 1984

The purpose of this program is to provide general analytic support to Sandia National Laboratory’s effort to develop high efficiency, high concentration solar cells. This support has taken the following forms: 1) Implementation of the two-dimensional silicon code on Purdue’s Cyber 205. 2) The release of both the one- and two-dimensional silicon codes to Sandia National Laboratory. 3) Continued enhancement of the codes and updating of the physical models used by the codes. 4) Use of the two-dimensional code to investigate the performance and design of high concentration solar cells.speed over conventional sequential multi-mode systems. The multi-mode system which uses Golay codes is shown to provide the best overall performanc

Effect of impurity trapping on the capacitance‐voltage characteristics of n‐GaAs/N‐AlGaAs heterojunctions

We have studied the capacitance-voltage (C- V) characteristics of Schottky barriers on inverted nGaAs/ N-AIGaAs and normal N-AIGaAs/n-GaAs heterojunctions. Impurities introduced during film growth produced a negative sheet charge of 6.0 X 10 II cm -2 at the interface of the inverted n-GaAs/N-AIGaAs heterojunction. The effectiveness of GaAs quantum wells in trapping these impurities was investigated. GaAs quantum wells 20 A wide were placed in intervals of 2500 A for the first 0.75 pm of the AIGaAs layer; in the last 0.25 pm, the periodicity of the quantum wells was progressively decreased by half with the last quantum well placed at about 160 A from the GaAs/ AIGaAs interface. The resulting measured interface charge concentration of 4.4 X 1010 cm -2 is more than a magnitude lower than measured before the use of the quantum wells and is essentially at the limit of the accuracy of the C-V technique for this structure

Temperature dependence of minority and majority carrier mobilities in degenerately doped GaAs

Measured minority and majority carrier mobility temperature dependencies in heavily doped n- and p-GaAs are compared. Majority carrier mobilities in heavily doped GaAs are essentially temperature ~T! independent while minority carrier mobilities exhibit a roughly 1/T dependence. Majority carrier freezeout, which reduces both majority–minority carrier and ionized impurity scattering, is shown not to be responsible for the 1/T minority carrier mobility dependence. The difference in minority and majority carrier mobility T dependencies is explained in terms of the increased degree of degeneracy of majority carriers with decreased temperature, which decreases majority–minority carrier scattering

Simulations of nanowire transistors: Atomistic vs. Effective Mass Models

As device sizes shrink towards the nanoscale, CMOS development investigates alternative structures and devices. Existing CMOS devices will evolve to 3D non-planar devices at nanometer sizes. They will operate under strong confinement and strain, regimes where atomistic effects are important. This work investigates atomistic effects in the transport properties of nanowire devices by using a nearest-neighbor tight binding (TB) model (sp3s*d5-SO) [1] for electronic structure calculation, coupled to a 2D Poisson solver for electrostatics. The 2D cross section of a 3D device is described with an arbitrary geometrical shape such as rectangular, cylindrical and tri-gate/FinFET type of structures (Fig. 1(a-d)) using a finite element mesh. Upon convergence, the ballistic transport characteristics are calculated with a semi-classical ballistic model [2]. Comparisons to the effective mass approach (EM) are discussed. Finally, the nonequilibrium Greens’ function (NEGF) approach is used to obtain the transmission coefficients for nanowires in different orientations. This approach will be deployed on nanoHUB.org as an enhancement of the existing Bandstructure Lab [3]

Experimental determination of the effects of degenerate Fermi statistics on heavily p‐doped GaAs

The effects of degenerate Fermi statistics on electron injection currents for p+‐GaAs grown by molecular beam epitaxy are presented. To achieve Be dopant concentrations of greater than 8×1019 cm−3, the substrate temperature during growth was reduced to approximately 450 °C from the usual 600 °C. In this heavily doped material, we measure unexpectedly large electron injectioncurrents which are interpreted in terms of an effective narrowing of the band gap. At extremely heavy doping densities, the Fermi level pushes into the valence band and degenerate Fermi statistics must be taken into account. For doping concentrations greater than 1×1020 cm−3, effects due to degenerate Fermi statistics oppose the band‐gap shrinkage effects; consequently, a reduction in the electron injection currents is observed. The result is a substantial reduction in gain for AlGaAs/GaAs heterostructure bipolar transistors when the base is doped above 1020 cm−3

Minority Hole Mobility in n+ GaAs

The minority hole diffusivity, or equivalently the hole mobility, was measured in n+GaAs with the zero‐field time‐of‐flight technique. The minority hole mobility was measured for the donor doping range of 1.3×1017 cm−3 to 1.8×1018 cm−3 and was found to vary from 235 to 295 cm2/V s. At the lower doping level, the minority hole mobility is comparable to the corresponding majority hole mobility, but at 1.8×1018 cm−3 the minority hole mobility was 30% higher than the majority carrier hole mobility. These results have important implications for the design of devices such as solar cells and pnp‐heterojunction bipolar transistors