Minority-carrier Transport Parameters Research Articles

Photogenerated Carrier Transport Properties in Silicon Photovoltaics

Electrical transport parameters for active layers in silicon (Si) wafer solar cells are determined from free carrier optical absorption using non-contacting optical Hall effect measurements. Majority carrier transport parameters [carrier concentration (N), mobility (μ), and conductivity effective mass (m*)] are determined for both the n-type emitter and p-type bulk wafer Si of an industrially produced aluminum back surface field (Al-BSF) photovoltaic device. From measurements under 0 and ±1.48 T external magnetic fields and nominally “dark” conditions, the following respective [n, p]-type Si parameters are obtained: N = [(3.6 ± 0.1) × 1018 cm−3, (7.6 ± 0.1) × 1015 cm−3]; μ = [166 ± 6 cm2/Vs, 532 ± 12 cm2/Vs]; and m* = [(0.28 ± 0.03) × me, (0.36 ± 0.02) × me]. All values are within expectations for this device design. Contributions from photogenerated carriers in both regions of the p-n junction are obtained from measurements of the solar cell under “light” 1 sun illumination (AM1.5 solar irradiance spectrum). From analysis of combined dark and light optical Hall effect measurements, photogenerated minority carrier transport parameters [minority carrier concentration (Δp or Δn) and minority carrier mobility (μh or μe)] under 1 sun illumination for both n- and p-type Si components of the solar cell are determined. Photogenerated minority carrier concentrations are [(7.8 ± 0.2) × 1016 cm−3, (2.2 ± 0.2) × 1014 cm−3], and minority carrier mobilities are [331 ± 191 cm2/Vs, 766 ± 331 cm2/Vs], for the [n, p]-type Si, respectively, values that are within expectations from literature. Using the dark majority carrier concentration and the effective equilibrium minority carrier concentration under 1 sun illumination, minority carrier effective lifetime and diffusion length are calculated in the n-type emitter and p-type wafer Si with the results also being consistent with literature. Solar cell device performance parameters including photovoltaic device efficiency, open circuit voltage, fill factor, and short circuit current density are also calculated from these transport parameters obtained via optical Hall effect using the diode equation and PC1D solar cell simulations. The calculated device performance parameters are found to be consistent with direct current-voltage measurement demonstrating the validity of this technique for electrical transport property measurements of the semiconducting layers in complete Si solar cells. To the best of our knowledge, this is the first method that enables determination of both minority and majority carrier transport parameters in both active layers of the p-n junction in a complete solar cell.

Carrier Transport Parameter Imaging in a Multicrystalline Silicon Solar Cell by Laser-Induced Photocarrier Radiometry

The distribution of minority carrier transport parameters (i.e., minority carrier lifetime, diffusion coefficient, and two surfaces recombination velocities) of multicrystalline silicon (mc-Si) solar cell was investigated using laser-induced photocarrier radiometry (PCR). Images of the amplitude and phase of the PCR measurements clearly showed the crystal boundaries of the mc-Si solar cell. The distribution of minority carrier transport parameters was obtained by best fitting the experimental results to the theoretical two-layer carrier density wave model of solar cells. The best-fitting results demonstrated that there was great difference in the minority carrier transport parameters among various grains.

Experimental study on scan imaging of silicon wafer by laser-induced photocarrier radiometry

In this paper, the modulated laser-induced excess minority carrier density wave (CDW) of bipolar semiconductor is developed. An analytical expression of CDW in frequency domain is introduced. The numerical simulations are carried out to analyze the effects of minority carrier transport parameters (minority carrier lifetime, diffusion coefficient, and two surfaces recombination velocities) on response of laser-induced photocarrier radiometry (PCR) signal to frequency for semiconductor silicon wafer. The PCR amplitude increases with increasing the minority carrier lifetime, and decreased with increasing the carrier diffusion coefficient and surface recombination velocity. In contrast, the PCR phase lag decreases with increasing the minority carrier lifetime, and increases with increasing the carrier diffusion coefficient and surface recombination velocity. The silicon (Si) wafer with an artificial defect (mechanical scratch) on the surface is experimentally investigated by PCR scanning image system. The distribution maps of the minority carrier transport parameters are obtained by best-fitting method which is based on the carrier density wave analytical expression, and the influences of the artificial defect on carrier transport parameters are discussed in detail. The experimental results indicate that the surface recombination velocity and carrier diffusivity at artificial damaged location are dramatically increased compared with those in the healthy region. The carrier bulk lifetime of whole Si wafer is obtained to be about 38.33 μs by PCR scanning image measurements. Simultaneously, quasi steady-state photoconductance (QSSPC) method is used to measure the carrier effective lifetime of Si wafer, and it is about 33.85 μs. Therefore, the carrier bulk lifetime of Si wafer by PCR scanning image measurement is in good agreement with the QSSPC measurement. However, QSSPC measurement could obtain only the carrier effective lifetime of Si wafer. Furthermore, PCR scanning image measurement can be employed to measure the carrier transport parameters with high resolution in comparison with QSSPC measurement, and to evaluate the localized imperfection.

Low frequency conductance voltage analysis of Si/Ge/sub x/Si/sub 1-x//Si heterojunction bipolar transistors

Low-frequency-conductance-voltage (LFGV) method for analysis of heterojunction bipolar transistors (HBTs) is presented. The method gives accurate quantitative values for the important minority-carrier transport parameters that underlie the transistor performance, such as the base diffusion length, lifetime, diffusion coefficient and transit time. The method also allows a detailed analysis of the current gain and emitter injection efficiency. The analytical model and experimental methodology are demonstrated for a Si/Ge/sub x/Si/sub 1-x//Si HBT with a trapeziodal and linearly graded Ge profiles in the base. The LFGV method is general and can be applied to other bipolar transistors, including those based on III-V materials.

Minority-carrier transport parameters in CVD diamond

A study is presented on minority carrier transport parameters, in particular mobility–lifetime products, in natural and CVD diamond materials with different morphology. μ τ products are obtained by photocurrent versus field measurements and their values range between 10 −8–10 −5 cm 2 V −1 depending on grain size and orientation. Observed variations are mainly related to mobility changes, while minority carrier lifetimes are almost constant around 10 −10–10 −9 s. The origin of such a behaviour is analysed in terms of gap state distribution arising from structural defects, impurities and non-diamond microphases.

Energy band structure parameters and their data, derived from the measurements of minority carrier current density in heavily doped emitters of silicon devices

In the n(p)-type heavily doped emitter region (HDER) of silicon devices, at room temperature, we have investigated the minority-carrier lifetime and the energy band structure parameters such as band-gap narrowing, apparent band-gap narrowing, unperturbed Fermi energy shift, optical gap, and reduced interacting density-of-majority conduction (valence) band states effective mass. As used in our previous paper, the present treatment is also based on the two assumptions for minority-carrier transport parameters. Gaussian impurity density profile, and accurate expression for minority-carrier mobility. Our empirical models for minority-carrier lifetime and band-gap narrowing are proposed and determined such that their curves versus the impurity concentration lie in between 3heir existing experimental data. Then, from a conjunction between electrical and optical phenomena, it is found that our theoretical values of such the energy band structure parameters are in good accordance with our own corresponding data, derived from the measurements of minority-carrier saturation current density in the n(p)-type HDER of silicon devices.

Performance comparison analysis of GeSi and Si bipolar transistors

Performance improvements of Si/Ge/sub x/Si/sub 1-x//Si heterojunction bipolar transistors with linearly graded Ge profile in the epitaxial base layer are compared with Si transistors without Ge by using a combined DC and low-frequency (1 kHz) small-signal AC measurements. Quantitative analysis of the minority-carrier transport parameters in the GeSi base layer, and of the current gain and recombination current components, for both GeSi and Si transistors are reported for the first time. The two fold improvement of the base transit time in GeSi transistors is due to the built-in electric field from Ge grading, rather than to improved diffusivity.

Estimation of electrical bandgap narrowing in heavily doped p-gallium arsenide

For device modeling, the effect of heavy doping can be described by relating the effective intrinsic carrier concentration n/sub ie/ to the intrinsic carrier concentration in lightly doped material n/sub io/ with a nonphysical apparent bandgap shrinkage. The electrical measurements on bipolar devices provide the value of the equilibrium minority-carrier concentration and cannot directly give the amount of bandgap narrowing. The aim of this work is to extract this nonphysical, apparent bandgap shrinkage from the measurement of the minority-carrier transport parameters in p/sup +/-n GaAs diodes with the p/sup +/-layer hole density in the range from 6.3*10/sup 17/ to 1.3*10/sup 19/ cm/sup -3/. The recombination velocity at the surface of the p/sup +/ layer is calculated and compared with the available experimental data. >

3D-resolved determination of minority-carrier lifetime in planar silicon solar cells by photocurrent decay

In modern production schemes for Si solar cells, defect passivation and impurity gettering are often used to improve material quality and thereby cell efficiency. These processes generally alter the spatial uniformity of minority-carrier transport parameters over the wafer and may result in minority-carrier lifetime and diffusion length variations both lateral and in depth. In polycrystalline Si these spatial dependences are already present due to the nature of the material. We present an extension of the photocurrent decay method to determine the diffusion length in a three-dimensionally resolved fashion. From a single photocurrent decay curve the back-surface recombination velocity, the average minority-carrier diffusion length, and an asymmetry factor which qualitatively describes the depth dependence of the diffusion length are determined. This is done using the three observables: quantum efficiency, fundamental decay time, and intercept of the extrapolated decay curve with the time-zero axis. Lateral resolution is obtained by focusing the light beam to a small spot on the cell and measuring the current decay curve as a function of position on the cell. It is shown that light-pulse durations longer than the minority-carrier lifetime and wavelengths longer than 950 nm are required. These conditions are met by using modulated wavelength-tunable light from a Ti:sapphire laser. Measurements on monocrystalline cells show that the decay time is independent of wavelength and light-pulse duration, as predicted by theory. Furthermore, the intercept with the time-zero axis was shown to increase with increasing pulse duration and wavelength. Measurements on a set of polycrystalline Si cells were performed showing that gettering treatments during cell production result in depth-dependent lifetimes.

An analytical solution for the collection efficiency of solar-cell emitters with arbitrary doping profile

An analytical expression consisting of a rapidly converging integral series is derived to calculate the collection efficiency of solar-cell emitters in steady state. This general expression applies to emitters which feature an arbitrary doping profile and heavily doped regions. It does not rely on a particular description of the relation between minority-carrier transport parameters and doping density. It is shown that most of the earlier approximations are included in the solution. To demonstrate the advantages and feasibility of this approach, calculation results are compared with approximate and numerical solutions, and with exact solutions when available. >

Simultaneous extraction of minority-carrier transport parameters in crystalline semiconductors by lateral photocurrent

The mathematical analysis and parameter-extraction process for a new characterization method are presented. This method allows simultaneous measurement of the minority-carrier lifetime, diffusion coefficient, and diffusion lengths as well as surface recombination velocity. The technique employs semi-infinite two-dimensional photodiodes and uniform, instead of focused, illumination. The paper deals with the derivation of exact closed-form solutions associated with two-dimensional devices and discusses the simultaneous extraction of minority-carrier transport parameters.

Minority-carrier transport in nonuniformly doped silicon-an analytical approach

A general analytical solution for minority-carrier transport in a nonuniformly doped quasi-neutral silicon region is derived. It is shown that, in the cases of an exponential doping-density profile and a general power-law doping-density profile, a closed-form solution for the minority-carrier concentration can be obtained for doping densities up to 10/sup 20/ cm/sup -3/. In the analysis, the experimentally observed dependencies of minority-carrier lifetime, minority-carrier mobility, and bandgap narrowing on doping density are taken into account. Contrary to earlier analytical solutions, the solution is free of integrals of minority-carrier transport parameters over the semiconductor region under study. Three important bipolar device configurations in which a nonuniform doping density plays a role are analyzed with the analytical solution. The first is the drift-field solar cell, for which a factor-of-20 reduction in the dark saturation current compared with a uniformly doped solar cell is calculated. Second, the effective back-surface recombination velocity of a high/low junction back-surface field (BSF) cell is shown to decrease with increasing BSF region thickness. Third, the influence of surface recombination velocity on the minority-carrier concentration profile in a heavily doped emitter is reduced when a strong power law doping profile in an n-p junction device is used.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Current gain enhancement effect by gate doping in bipolar-mode field-effect transistor

The effect of the gate doping level on the current gain H/sub FS/ of a bipolar-mode field-effect transistor (BMFET) in the range of high drain current densities is investigated. It is demonstrated that the gate doping level can be chosen to optimize the current gain without significantly compromising the other features of the device. The behavior of H/sub FS/ is fully justified in terms of the doping dependency of the various minority-carrier transport parameters of the gate. >

Nondestructive SEM measurement of minority-carrier transport parameters of CuxS/CdS solar cells as a function of heat treatment

Electron-beam-induced-current techniques of a scanning-electron microscope have been extended to allow nondestructive measurements to be performed on p-n heterojunction devices consisting of thin layers sensitively influenced by surface effects and under conditions where junction collection efficiency is less than perfect. When applied to Cu <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> S/CdS solar cells formed on polycrystalline CdS with an epitaxial Cu <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> S layer that was heat treated at 180°C in a hydrogen-argon ambient, the dominant change was found to be the greater than two increases in junction collection efficiencies to a maximum and then a decrease for treatment times up to 120 min. No significant variations were found in the minority-carrier diffusion lengths which remained in the 0.20- to 0.26-µm range for the Cu <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> S and in the 0.41- to 0.46-µm range in the CdS. The Cu <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> surface-recombination velocity retained a constant magnitude equal to its diffusion velocity. Optimization of the collection efficiency changes should lead to improved device performance.