Minority Carrier Distribution Research Articles

Understanding and Advancing Bifacial Thin Film Solar Cells under Dual Illumination

There is a great deal of interest in increasing the energy yield from thin film solar cells by implementing bifacial operation. However, deleterious band bending and high interface recombination at the back transparent electrode can cause problems. Herein, it is investigated how bifacial thin film devices perform when illuminated through the front, back, and simultaneously through both interfaces using numerical modeling. It is shown that the downward band bending near the back interface is reduced during illumination when the carrier concentration is low. This effect is not found when the doping is relatively high, but the minority carrier distribution is still modified. Under either condition, the power generated under bifacial illumination exceeds the sum of the power generated when the illumination is solely from the back or the front. It is also shown that the back‐illuminated device performance is independent of the angle at which the light enters the back of the device, which provides accommodation for scattered light. Finally, the power enhancement is calculated for bifacial devices relative to front‐illuminated devices, and is shown that any significant loss in frontside power generation is difficult to overcome with back illumination under real‐world albedo conditions.

Effect of shallow doping on performance parameters of single heterojunction solar cell

Abstract Fill factor and conversion efficiency of single heterojunction solar cell is analytically computed based on the shallow doping at GaAs quantum well region. Poisson’s equation is solved with suitable boundary condition applied at hetero-interface for both dark and illuminated conditions, corresponding to which open-circuit voltage and short-circuit current are computed. Realistic dependence of minority carrier distribution on material layers and diffusion widths are taken into account for simulation purpose. Result exhibits that variation of acceptor density leads to better outcome in terms of both efficiency and fill factor, which is also critically depends on length of quantum well region. Doping of AlGaAs barrier layer has negligible influence on fill factor and conversion efficiency. Optimized dimension of quantum well layer width is the critical parameter for design of efficient solar cell, as revealed from the analysis.

Comparative Analysis of the Matrix Method and the Finite-Difference Method for Modeling the Distribution of Minority Charge Carriers in a Multilayer Planar Semiconductor Structure

The stationary differential heat and mass transfer equation with discontinuous coefficients describes various time-independent physical processes, for example, the distribution of minority carriers from a stationary source in an inhomogeneous or multilayer structure. In this paper, we apply the matrix method and the finite-difference method for modeling the distribution of minority charge carriers generated by kilovolt electrons in multilayer semiconductor materials. The efficiency of the matrix method for solving stationary differential equations with discontinuous coefficients is demonstrated.

SiC gate-controlled bipolar field effect composite transistor with polysilicon region for improving on-state current

A novel silicon carbide gate-controlled bipolar field effect composite transistor with polysilicon region (SiC GCBTP) is proposed. Different from the traditional electrode connection mode of SiC vertical diffused MOS (VDMOS), the P+ region of P-well is connected with the gate in SiC GCBTP, and the polysilicon region is added between the P+ region and the gate. By this method, additional minority carriers can be injected into the drift region at on-state, and the distribution of minority carriers in the drift region will be optimized, so the on-state current is increased. In terms of static characteristics, it has the same high breakdown voltage (811 V) as SiC VDMOS whose length of drift is 5.5 μm. The on-state current of SiC GCBTP is 2.47 × 10−3 A/μm (V G = 10 V, V D = 10 V) which is 5.7 times of that of SiC IGBT and 36.4 times of that of SiC VDMOS. In terms of dynamic characteristics, the turn-on time of SiC GCBTP is only 0.425 ns. And the turn-off time of SiC GCBTP is similar to that of SIC insulated gate bipolar transistor (IGBT), which is 114.72 ns.

Computation-assisted performance optimization for photoelectrochemical photoelectrodes

The generation rate and collection efficiency of photocarriers fatally determine the photoelectrochemical performance of photoelectrodes. However, it is challenging to simultaneously reach a high generation rate and a high collection efficiency due to their conflictive dependence on the thickness of photocatalytic films, especially for metal oxide photocatalysts. Therefore, it is critical to select an appropriate thickness to reach the highest photocatalytic rate under certain light illustration. Herein, we proposed a physical model to predict the optimal thickness of photocatalytic films by combining computation and experiments. In this model, a photoelectrode was investigated by thoroughly considering the electric potential distribution in the whole photocatalytic film rather than only considering the depletion layer as previously. We solved the continuity equation and got the distribution of minority carriers in photocatalytic films. The used parameters for calculation were obtained through density functional theory calculation and experiments. The optimal thickness of photocatalytic films can be predicted with this model. We have used CuFeO2 films as the model material to verify the accuracy of the proposed model. Compared to the traditional trial-and-error process, our computation-assisted approach is highly efficient and can be broadly employed to other materials.

Effects of the Nature of Boundaries Conditions and Their Truncation Errors on the Distribution of Minority Carriers in Silicon Solar Cell

In this work, the effects of boundaries conditions and truncation errors in the distribution of minority carriers in the semiconductor are studied. It is a one-dimensional digital study of a polycrystalline silicon solar cell under polychromatic illumination in a dynamic state. Starting from the Boltzmann equation of semiconductors, the author establishes the general equation of particle transport. Assumptions made on the latter allow it to give the equation of distribution of minority carriers in a general way in its case to be studied. This dimensioned distribution equation reveals the parameters of influences on the distribution of carriers. It obtains a partial derivative equation for the carrier distribution function. The boundary conditions are then discretized to order one and then to order two. By considering boundary conditions and the nature of the carriers, the author numerically resolves the discretized general equation by assessing the influence of the nature of the boundary conditions and truncation errors and the influence of the discretization step on the density of the charge carriers by setting certain parameters and varying others. The work ends with a conclusion and logical follow-up to this work.

Perovskite Solar Cells with Front Surface Gradient

AbstractThe recombination flux in a solar cell is determined by not only recombination centers, but also the spatial distribution of minority carriers. For halide perovskite solar cells (PSCs), although there has been a tremendous amount of work focusing on defect passivation, the issue of carrier distribution is not as well studied as for other types of solar cells. Here in this work, with the incorporation of perovskite quantum dots, the concept of the front surface gradient in PSCs using a solution process is successfully realized. Evidenced by multiple characterization techniques, the minority carriers are pushed away from the defect‐rich surface by the gradient of valence band maximum, which effectively reduces surface recombination without compromising photocurrent. As a result, the normal structured hybrid PSCs and MAPbI3 cells exhibit open‐circuit voltages exceeding 93% and 90% of their respective Shockley–Queisser limits, and the power conversion efficiencies reach 22.36% and 20.53%, respectively.

Mathematical Modeling of Heat and Mass Transfer Phenomena Caused by Interaction between Electron Beams and Planar Semiconductor Multilayers

An analytical matrix method is proposed for the mathematical modeling of heat and mass transfer caused by the interaction between broad electron beams and planar semiconductor multilayers. Some possibilities of using this approach to estimating the excess minority carrier distributions in planar semiconductor multilayers are discussed. It is shown that the proposed matrix method allows calculation of the excess minority carrier distributions in a relatively short time with an accuracy sufficient for use in electron probe techniques.

Analysis of Quantum-well Heterojunction Emitter Bipolar Transistor Design

The paper presents the analysis of Quantum-well Heterojunction Emitted Bipolar Transistor Design based on physical parameters with numerical computations. The specific objective of this work is to enhance the physical performance of the Quantum-well Heterojunction Emitted Bipolar Transistor Design in real world applications. There have been considered on the III-V compound materials like GaAs for p-type layer, AlGaAs for n-type layer and InGaAs for quantum-well layer for different kinds of junctions which were developed in HEBT structure. In this analyses, the parameters for implemented HEBT structure were evaluated to find the multi-quantum-well band diagram, operating frequency (unity beta frequency), rise time, storage delay time, fall time, minority carrier distribution, current gain variation, voltage-current characteristics and phonon control on quantum-well device. In these analyses, the physical parameters were carried out based on the experimental studies from the recent research works and many literatures. The physical parameters which used in this HEBT structure have been provided to solve the real fabrication problems by using theoretical concepts. The quantum-well device based on III-V compound materials was performed by using numerical techniques with the help of MATLAB. The simulation results confirm that the developed HEBT structure was suitable for fabricating the real devices for high performance applications.

In English

Relaxation of the excess minority carrier distribution in macroporous silicon structure was calculated using the finite difference method. The initial distribution of the excess minority carriers has two maxima after carrier generation by electromagnetic wavelength 0.95 μm with small absorption depth. The first maximum of the initial distribution function is in macroporous layer, the second one is in monocrystalline substrate. Surface recombination leads to the diffusion of excess charge carriers to recombination centers and creates the non-homogeneity of the excess charge carrier distribution. A rapid maximum decrease of the excess carrier distribution function in a macroporous layer and near the boundary of a macroporous layer and monocrystalline substrate is found. The slow decrease of the distribution function in a monocrystalline substrate is evaluated. We observed one maximum of the excess minority carrier distribution after homogeneous carrier generation by electromagnetic wavelength 1.05 μm with big absorption depth. The rate of change in the concentration of excess minority carriers decreases in time in macroporous silicon layer due to high recombination and increases due to diffusion to the surface of silicon substrate after generation by the electromagnetic wave 0.95 μm with small absorption depth. The rate of change in the concentration of excess minority carriers decreases in time in the entire structure after generation by the electromagnetic wave 1.05 μm with big absorption depth and small non-homogeneity.

Collector modulation in high-voltage bipolar transistor in the saturation mode: Analytical approach

A simple analytical model is developed, capable of replacing the numerical solution of a system of nonlinear partial differential equations by solving a simple algebraic equation when analyzing the collector resistance modulation of a bipolar transistor in the saturation mode. In this approach, the leakage of the base current into the emitter and the recombination of non-equilibrium carriers in the base are taken into account. The data obtained are in good agreement with the results of numerical calculations and make it possible to describe both the motion of the front of the minority carriers and the steady state distribution of minority carriers across the collector in the saturation mode.

In English

We have obtained a simple expression that determines the effective minority carrier lifetime in macroporous silicon with periodic arrangement of infinitely long macropores as a function of bulk lifetime, surface recombination velocity of minority carriers, pore radius and the distance between the centers of macropores. This expression can be applied also to macroporous silicon with randomly distributed pores by replacing the pore radius and the distance between the centers of macropores with their average values. The distribution of steady-state excess minority carriers in macroporous silicon is calculated for the analytical model proposed by us. The calculation is made for the case when both the outer surface of macroporous silicon and the bottom of pores are illuminated with light. We observed two peaks of the distribution of steady-state excess minority carriers in macroporous silicon near the surfaces illuminated with light of wavelength 0.95 m. At the same time, if macroporous silicon was illuminated with light with the wavelength of 1.05 μm, we observed only one maximum in the distribution function of the excess minority carriers, in spite of the fact that the pore bottom was also illuminated with light. It is shown that the distribution of excess minority carriers in macroporous silicon with through pores is similar to the distribution in single crystal silicon. But in this case, the effective lifetime of minority charge carriers in the effective medium of macroporous silicon, which includes silicon and the surface of pores, corresponds to the bulk minority-carrier lifetime in monocrystalline silicon.

An analytical study of the minority carrier distribution and photocurrent of a p–i–n quantum dot solar cell based on the InAs/GaAs system

An analytical study has been carried out on the InAs/GaAs p+–i–n+ quantum dot solar cell, taking into consideration the contributions of each region of the cell to the total photocurrent. The expressions for the excess minority carrier concentration and photocurrent from the front and the rear regions of the device have been obtained and their variations with different device parameters have been studied. Also, based on the investigations reported by some researchers earlier, the photocurrent contribution from the intrinsic region of the solar has been studied, taking into account the quantum dot ensemble absorption coefficient, which depends significantly on the quantum dot size and size dispersion. It is observed that all the three regions of the cell contribute to the overall internal quantum efficiency (IQE) of the cell. The contribution of each region of the solar cell to the total IQE has been shown graphically. From these studies it is observed that the incorporation of the quantum dots in the intrinsic region enhance the photocurrent density and hence the IQE of such solar cell, as it absorbs low energy photons, which are beyond the absorption range of GaAs. Finally, the fill factor of the solar cell has been calculated.

Enhancement of Light-to-Dark Current Ratio Via Coupling Effect for MIS (p) Tunnel Diode Photo Sensors

The current of metal-insulator-semiconductor (MIS) tunnel diode is dependent on the Schottky barrier height and there are different mechanisms which dominate MIS tunnel currents while MIS tunnel diodes were biased at positive and negative voltages. In this work, coupled current-voltage behaviors were observed in a MIS(p) tunnel diode with an MOS structure coupled nearby (1). It was found that MIS(p) saturation tunneling current could be controlled by the voltage bias (VG) of the nearby MOS capacitor. While VG changes, minority carrier distribution at the fringe of the MIS tunnel diode also changes. As a result, by well-controlling VG, the light to dark current ratio Ilight/Idark can be effectively enhanced. In this study, two intensities of illumination on MIS(p) tunnel diodes of three different oxide thicknesses were displayed. It is shown under both intensities of illumination, the light to dark current ratio Ilight/Idark are all effectively enhanced by well-controlling VG. It is also found that the maxima of current ratio Ilight/Idark take place at different values of VG for different intensities of illumination. Coupling effect plays an important role on this phenomenon.

Photo Response Enhancement in MIS(p) Tunnel Diode via Coupling Effect by Controlling Neighboring Device Inversion Level

The current coupling effect in concentric circle MIS(p) structure was investigated. It is found that the saturation current of the central device could be affected by the distance between two electrodes (S) and the voltage bias (VG) of the nearby device. While the electrode separation and VG change, minority carrier distribution at the fringe of the MIS tunnel diode also changes causing the variation in the saturation current of the central device. In addition, it is also found that by well-controlling VG, the light to dark current ratio Ilight/Idark of the central device could be effectively enhanced. In this study, two intensities of illumination on MIS(p) tunnel diodes were displayed. It is found that the maxima of current ratio Ilight/Idark took place at different values of VG for different intensities of illumination. Coupling effect plays an important role in this phenomenon.

Minority carrier distribution in the front and base regions of a p/n GaAs-Si heterojunction solar cell and its spectral response

Minority carrier distribution in the front and base regions of a p/n GaAs-Si heterojunction solar cell and its spectral response

Dependence of the Photocurrent of a Schottky-Barrier Solar Cell on the Back Surface Recombination Velocity and Suggestion for a Structure with Improved Performance

Though Schottky-barrier solar cells have been studied extensively previously, not much work has been done recently on these cells, because of the fact that conventional p-n junction silicon solar cells have much higher efficiency and have attracted the attention of most of the researchers. However, the Schottky-barrier solar cells have the advantage of simple and economical fabrication process. In this paper, the effect of back surface recombination velocity on the minority carrier distribution and the spectral response of a Schottky-barrier silicon solar cell have been investigated and, based on this study, a new design of the cell with a back surface field has been suggested, which is expected to give much improved performance.

Compact Electrothermal Reliability Modeling and Experimental Characterization of Bipolar Latchup in SiC and CoolMOS Power MOSFETs

In this paper, a compact dynamic and fully coupled electrothermal model for parasitic BJT latchup is presented and validated by measurements. The model can be used to enhance the reliability of the latest generation of commercially available power devices. BJT latchup can be triggered by body-diode reverse-recovery hard commutation with high dV/dt or from avalanche conduction during unclamped inductive switching. In the case of body-diode reverse recovery, the base current that initiates BJT latchup is calculated from the solution of the ambipolar diffusion equation describing the minority carrier distribution in the antiparallel p-i-n body diode. For hard commutation with high dV/dt , the displacement current of the drain-body charging capacitance is critical for BJT latchup, whereas for avalanche conduction, the base current is calculated from impact ionization. The parasitic BJT is implemented in Simulink using the Ebers–Moll model and the temperature is calculated using a thermal network matched to the transient thermal impedance characteristic of the devices. This model has been applied to CoolMOS and SiC MOSFETs. Measurements show that the model correctly predicts BJT latchup during reverse recovery as a function of forward-current density and temperature. The model presented, when calibrated correctly by device manufacturers and applications engineers, is capable of benchmarking the robustness of power MOSFETs.

Subthreshold Swing Reduction by Double Exponential Control Mechanism in an MOS Gated-MIS Tunnel Transistor

The current of an MIS tunnel diode was tremendously changed (about five orders) while sweeping the gate bias of an MOS capacitor nearby. It was supposed that the effective Schottky barrier height of the MIS tunnel diode was affected by the bias of the nearby MOS gate capacitor via changing the minority carrier distribution at the fringe of the MIS tunnel diode. Therefore, the subthreshold behavior of the MIS tunnel diode current was controlled by the voltage bias of the nearby MOS capacitor, which is called MOS gated-MIS tunnel transistor. The minimum subthreshold swing of the MOS gated-MIS tunnel transistor could reach 15.3 mV/decade empirically.

The effect of front and rear surface recombination velocities on the photocurrent of a buried emitter silicon solar cell

Analytical work on the buried emitter solar cell (BESC) has been carried out based on the model presented previously by various researchers. The minority carrier distribution and the photocurrent contribution from the front region and also the rear region of the cell have been studied, taking into consideration the effects of front surface and rear surface recombination velocities. It is observed from these theoretical studies that reducing these recombination velocities, there is significant improvement in the photocurrent contributions form these regions. This emphasizes the role of a back surface field (BSF) in such devices. It may be mentioned here that in some recent experimental studies on new types of BESC solar cells, heavily doped BSF regions have been included by investigators.