Jeffery L. Gray

Transport equations for the analysis of heavily doped semiconductor devices

Abstract Transport equations for use in analyzing heavily doped semiconductor devices are considered. These transport equations describe the effects of the nonuniform band structure and the influence of Fermi-Dirac statistics, which are important in heavily doped semiconductors. Previous workers [1, 2] have derived transport equations in terms of the nonuniform band structure. These equations, however, are not convenient for use in semiconductor device analysis because the band structure of heavily doped semiconductors is not well known. In this paper, the transport equations of Marshak and van Vliet [1, 3] are recast into a simple, Boltzmann-like form in which the effects associated with the nonuniform band structure and degenerate carrier concentrations are described by two parameters, the effective gap shrinkage, Δ G , and the effective asymmetry factor, γ. The experimental determination of both of these parameters is also discussed. Finally, Adlers contention [4], that some important features of semiconductor device operation can be modeled accurately by using an electrically measured Δ G with an arbitrarily chosen γ, is considered. The validity of this procedure, under certain simplifying assumptions, is established.

Design of GaAs Solar Cells Operating Close to the Shockley–Queisser Limit

With recent advances in device design, single-junction GaAs solar cells are approaching their theoretical efficiency limits. Accurate numerical simulation may offer insights that can help close the remaining gap between the practical and theoretical limits. Significant care must be taken, however, to ensure that the simulation is self-consistent and properly comprehends thermodynamic limits. In this paper, we use rigorous photon recycling simulation coupled with carrier transport simulation to identify the dominant loss mechanisms that limit the performance of thin-film GaAs solar cells.

A computer model for the simulation of thin-film silicon-hydrogen alloy solar cells

A 1-D computer simulation code, Thin-Film Semiconductor Simulation Program (TFSSP), for the modeling of TF Si:H solar cells is discussed. The code incorporates a variety of physical models, such as a position-dependent bandgap, electron affinity, dielectric constant, density of states, mobility, exponential band tails, and dangling-bond defect states. This flexibility allows different model assumptions to be analyzed and compared. TFSSP and the physical models used are described. An example simulation of a typical TF Si:H solar cell is presented. >

Contactless nondestructive measurement of bulk and surface recombination using frequency-modulated free carrier absorption

Abstract A measurement procedure is described which allows the contactless measurement of bulk lifetime and surface recombination. The procedure uses the the free-carrier absorption of a long-wavelength laser beam by a modulated free-carrier wave to measure and separate the bulk recombination from the surface recombination. The dependence of the absorption on the modulation frequency is used to accomplish the separation. Limitations of the technique are also discussed.

Numerical modeling of photon recycling in solar cells

The incorporation of a detailed model for the photon recycling effect into an exact numerical solar cell simulation code is described. The commonly encountered radiative lifetime multiplication factor is shown to have both a spatial and bias point dependence. Example simulations show that photon recycling can significantly reduce the effective recombination current and thereby have a large effect on the open circuit voltage of a solar cell. Possible ways to capitalize upon this effect are discussed. >

ADEPT: a general purpose numerical device simulator for modeling solar cells in one-, two-, and three-dimensions

ADEPT (a device emulation program and toolbox), a numerical simulation code for modeling solar cells composed of a variety of semiconductor materials in one, two, and three spatial dimensions, is described. Material models for silicon, GaAs, AlGaAs, CuInSe/sub 2/, CdTe, CdS, and thin film Si:H have been implemented. One-dimensional simulations can be run easily in a personal computer environment, and recent advances in sparse matrix solvers make it possible to run 2D simulations on small workstations.<<ETX>>

Multi-terminal dual junction InGaP 2 /GaAs solar cells for hybrid system

The performance of two and three-terminal solar cells under 13-sun AM1.5G illumination is reported. The solar cells are comprised of monolithically grown InGaP2 and GaAs single junction cells. The two-terminal device consists of single junctions in series. The three-terminal device structure comprises the single junctions independently interconnected by an InGaP2 layer serving as the middle contact. Device optimization was based on modeling of the InGaP2 top junction band gap for various spectral irradiance profiles. It was found that the optimal band gap combination for a 2.6eV filtered AM1.5G spectrum is achieved with 1.84eV and 1.43 eV for the top and bottom junctions, respectively. External quantum efficiency and illuminated current-voltage (I–V) measurements of the component two and three-terminal tandem cells are discussed.

Correlated Nonideal Effects of Dark and Light I–V Characteristics in a-Si/c-Si Heterojunction Solar Cells

a-Si/c-Si (amorphous Silcon/crystalline Silicon) heterojunction solar cells exhibit several distinctive dark and light I-V nonideal features. The dark I-V of these cells exhibits unusually high ideality factors at low forward-bias and the occurrence of a “knee” at medium forward-bias. Nonidealities under illumination, such as the failure of superposition and the occurrence of an “S-type” curve, are also reported in these cells. However, the origin of these nonidealities and how the dark I-V nonidealities manifest themselves under illumination, and vice versa, have not been clearly and consistently explained in the current literature. In this study, a numerical framework is used to interpret the origin of the dark I-V nonidealities, and a novel simulation technique is developed to separate the photo-current and the contact injection current components of the light I-V. Using this technique, the voltage dependence of photo-current is studied to explain the failure of the superposition principle and the origin of the S-type light I-V characteristics. The analysis provides a number of insights into the correlations between the dark I-V and the light I-V. Finally, using the experimental results from this study and from the current literature, it is shown that these nonideal effects indeed affect the dark I-V and the light I-V in a predictable manner.

A simple parametric study of TPV system efficiency and output power density including a comparison of several TPV materials

This paper presents a parametric study of thermophotovoltaic (TPV) system efficiency and output power density based upon a simple model of the TPV system. The efficiencies presented here are based on thermodynamic limits. Some issues relating to the choice of TPV materials are considered. It is shown that the optimum TPV cell band gap depends not only on the emitter spectrum, but on the type and effectiveness of the spectral selection. Trade‐offs between efficiency and output power are also illustrated. In addition, issues associated with creating a more detailed TPV system model are discussed.

Wafer bonding for use in mechanically stacked multi-bandgap cells

Two and three junction monolithic two-terminal solar cells have been developed that have 1-Sun, AM0 efficiencies of greater than 25%. In order to reach 1-Sun efficiencies of 30% and greater, solar cells with more junctions are required. Mechanically stacking junctions with different band gaps provides a means of developing such a cell. The authors propose a four-junction, mechanically stacked cascade solar cell structure that projects to a 34.8% AM0 efficiency. Wafer bonding provides a means of mechanically joining semiconductor materials with different lattice constants. They present optical, electrical and mechanical data on wafer bonding GaAs and InP substrates. The data indicate that wafer bonding can be used to develop a four-junction device.