Search

The voltage and current oscillations occasionally occur during the reverse recovery transient of the superjunction MOSFET body diode. This paper identifies the unique reverse recovery oscillation characteristics of the superjunction MOSFET body diode. Base on the experimental investigation and theoretical analysis, the various reverse recovery oscillation mechanisms are clarified. At first, with a discrete 650 V/47 A superjunction MOSFET utilized, the high-frequency and low-frequency oscillation characteristics during the reverse recovery transient are obtained by the double-pulse test. After the theoretical analysis, it is found that the superjunction MOSFET body diode has various oscillation mechanisms depending on its drift region injection level. Under high-level injection condition, the high-frequency oscillation occurs due to the plasma extraction transient-time (PETT) effect. Under low-level injection condition, the body diode’s snappy reverse recovery results in the low-frequency LC oscillation. In the end of the paper, the oscillation behaviors under both high and low level injection conditions are reproduced by the mixed-mode numerical simulation, the simulation results validate the proposed oscillation mechanisms.

In this work, we have investigated the impact of interstitial Fe contamination on the effective minority carrier lifetime of n-type Cz silicon bulk material for high efficiency solar cells. The study covers a Fe concentration in the silicon bulk from 3.5 × 1012 cm-3 to 2.7 × 1014cm-3. We have added 5 different concentrations (30, 100, 300, 1000 and 3000 ppb) of Fe intentionally to a wet chemical process tank and measured the transfer to the silicon wafer surface mimicking a possible contamination during wet chemical processing. In order to fabricate carrier lifetime test vehicles, the silicon wafer is then passivated with thermal silicon oxide from both sides. The surface contamination is driven into the bulk by mimicking a high temperature process during solar cell manufacturing. Effective minority carrier lifetime is measured at injection levels from 1 × 1013 cm-3 to 3 × 1015cm-3. We have fitted the theoretical curve for interstitial Fe derived from the SRH theory to the measured values and extracted the Fe contamination concentration. This value is comparable to the calculated value extracted from the surface contamination measurement. For low level injection (LLI), we extracted the capture cross section for interstitial Fe to be 6.45 × 10-17 cm/s ± 2.23 × 10-17 cm/s. The measured Fe contamination levels are used for the conversion efficiency fitting of a n-type bifacial silicon solar cell using QUOKKA simulations. The simulations show that very low Fe contamination concentrations of [Fe]bulk = 3.5 × 1012 cm-3 ([Fe]surf = 6 × 1010cm-2) already degrade the solar cell efficiency by 10% relative.

A model consisting of Shockley Read Hall (SRH) recombination under steady state conditions of constant photon injection is proposed in this work to study the steady state minority carrier lifetime in CdS/CdTe thin film solar cells. The SRH recombination rate versus optical injection level is analytically approximated in the junction and neutral regions. In the neutral region, it is found that the recombination rate through certain defect levels has one constant value under lower optical injection conditions and another constant value under higher optical injection conditions with the transition occurring at a critical optical injection level. By simultaneously solving the equations of charge neutrality, charge conservation and SRH recombination in the neutral region, it is found that the compensation of doping and the reduction of minority carrier lifetime by donors in the p-type semiconductor can each be remedied by optical injection. It is also demonstrated that this optical-dependent SRH recombination is significant in large bandgap thin films. The measured minority carrier diffusion length in a CdS/CdTe solar cells, as determined from the steady-state photo-generated carrier collection efficiency, shows the predicted transition of minority carrier lifetime versus optical injection level. A numerical fitting of the indirectly-measured minority carrier lifetime by assuming the minority carrier mobility gives a non-intuitive picture of the p–n junction with a low free hole concentration but a narrow depletion region width.

We verify a fast and simple analytical model for locally contacted rear surface passivated solar cells by three dimensional numerical simulations. For low level injection conditions the analytical model shows good agreement with the numerical results. For intermediate injection levels and for the injection dependent bulk lifetime in oxygen contaminated silicon, the present analytical model deviates from the numerical results. The accordance clearly improves when including injection dependent recombination and conductance in the analytical model. For intermediate to high injection levels, it is crucial to implement additional corrections for injection effects, being emphasised by the low recombinative front side and bulk material of the device structure investigated in this work. When accounting for these deviations from low level injection, analytical modelling turns out being a fast way to predict the optimum contact spacing as well as the electrical parameters of locally contacted rear surface passivated solar cells even for intermediate injection conditions

Double side collecting silicon solar cells such as the emitter wrap through and the both sides contacted and collecting concept are one possibility to maintain high efficiencies even on low lifetime material. To investigate the intrinsic advantages and limitations of this concept we use the solution of the continuity equation under low level injection conditions for a double side collecting (n + pn + ) and a single side collecting (n + pp + ) solar cell. In general, compared to the single side collecting structure, the double side collecting structure yields the highest advantages concerning the roduct of the short circuit current density and the open circuit voltage Jsc·Voc on highly doped material with diffusion lengths in the range of half the cell thickness. A realistic calculation of the fill factor of such devices is done using two dimensional numerical simulations. It is shown that a major reduction of the fill factor is caused by laterally varying voltage induced non-generation losses and ohmic losses, both caused by the majority carrier flow in the base. Electro- and photoluminescence images verify and illustrate the lateral voltage distribution under different illumination levels and terminal voltages.

Silicon samples of various doping concentrations have been irradiated with MeV ions at low doses and different temperatures. Deep level transient spectroscopy (DLTS) has been used to characterise the distributions and productions of the divacancy center (V 2 ) and the vacancy-oxygen complex (VO). These two defects have been identified as being responsible for the charge carrier lifetime reduction. Due to their bandgap position and capture cross sections the singly negative charge state of the V 2 controls the low level injection lifetime, but the influence of VO becomes stronger for higher injection levels. The relative abundance of the two defects can be affected by using heavier ions than protons and electrons, which are commonly used today in the fabrication of silicon power devices. The reason for this is that the production of V 2 is enhanced by larger collision cascades, in comparison to VO, following heavier ion implantation. It will be shown that irradiation with heavy ions, instead of protons and electrons, will result in a better relationship between the high and the low level lifetime. This effect can be taken advantage of to improve the trade off between switching time and conduction losses in power devices. It will also be shown that the concentration ratio VO/V 2 can be further improved if the irradiations are performed at lower temperatures than room temperature.

The electrical behaviour of lateral bipolar transistors has a large effect on latch-up phenomena; in particular in the region between low level and high level injection, where the parasitic thyristor is switching on. By solving the continuity equation in this region under some approximations the base and collector current can be calculated rather accurately if the lifetime of the minority carriers in the base is known. The influence of this parameter is very important; therefore a few measurement techniques are discussed and the influence of the lifetime on the electrical characteristics is shown. Finally a comparison between the holding current, implementing and neglecting the odd shape of the beta in the transition region between high and low level injection, is made.

The effect of the semiconductor parameters on the injection of carriers in a metal-semiconductor-metal structure was analyzed. It was assumed that the semiconductor is completely depleted when no external voltage is applied. We found that in that case minority carrier injection dominates over majority carrier injection. Consequently, numerical analysis shows that the current density in a metal- n -type semiconductor-metal structure increases with increasing electron barrier height and/or with decreasing donor concentration. The hole injection current dominates over the electron injection current. These relationships are opposite to those occurring both in a single MS junction and in a partly-depleted MSM junction where the minority carrier injection ratio is known to be negligibly small.

For efficiency and convenience the voltage applied to a Si solar cell is often fairly rapidly driven from zero to the open circuit value typically at a common rate of 1 V per millisecond. During this time the values of current are determined as a function of the instantaneous voltage thus producing an I-V characteristic. The present paper shows that the customary expressions for the current as a function of cell parameters still remain valid provided that the diffusion length in the expression for the dark current is changed from its steady state value L to the effective diffusion length L1 given by L1 = L(1 + qV/kT.tau) to the -1/2, where V is the ramp rate considered constant and tau is the lifetime of minority carriers. This result is true to a very good approximation provided that low level injection prevails.

The i-region carrer lifetime in p-i-n diodes is frequently measured at a single bias current and the results extrapolated to higher or lower currents. This interpretation is erroneous, however, since high levels of d.c. bias injection can degrade the effective lifetime as seen at the device terminals. This paper shows the dependence of stored charge and carrier lifetime with d.c. bias current and device geometry. Experimental data are used to verify the analysis, which shows that this d.c. bias current dependence of the lifetime may be minimized by minimizing the low level injection carrier lifetime to i-region thickness ratio for a given device area.

In the literature several reports have indicated a strong increase of the minority carrier lifetime with injection level in defected single crystal and semi-crystalline silicon. These types of potentially low-cost materials are being developed for the photovoltaic industry. The lifetime increase, mainly observed at low injection levels, has usually been explained by the saturation of active traps with increasing injection level. However, a detailed experimental analysis of the recombination losses in materials showing this behavior, points to a strong local variation in the trap density, presumably associated with crystal microdefects. We demonstrate in this paper that the non-linearity of the recombination current at low illuminations observed in defected materials, can be explained by considering the recombination at grain boundaries and microdefects. The conditions are detailed under which the lifetime increases with injection level.

We have examined the role of constraints on carrier flow on electron-hole scattering of minority carriers. In a bipolar transistor, in which the flow of majority carrier current normal to the junction potential barriers is usually negligible, the effect of electron-hole scattering is less than would be inferred from measurements of transport in p-i-n diodes or in the Shockley-Haynes experiment. In addition, as a result of the momentum transferred fromthe minority carriers to the majority carriers, an electric field is developed which enhances the flow of minority carriers, particularly at high injection levels.

The low level injection d.c. and transient characteristics of a P+−N−N+ diode are discussed from a theoretical point of view, using the minority carrier lifetime ‘τepi’ in the lightly doped region and the parameter α defining the properties of the high-low junction, as basic parameters. A value of α less than unity is predicted for typical epitaxial silicon structures. Detailed experimental studies involving both lifetime measurements (using optical techniques) on the original epitaxial layer and substrate and d.c. plus transient measurements on completed devices confirm the existence of a ‘blocking’ high low junction (α less than unity). However the measured value of α is still considerably higher than that predicted from the theory, thus indicating conclusively the presence of additional recombination mechanisms at the high low junction interface. An interesting and useful result of the study is the fact that using a combination of d.c. and transient measurement techniques, both α and the effective epitaxial layer width Wepi can be uniquely determined using the given theoretical curves.

The theory underlying a method for measuring carrier diffusion length based on the surface photovoltage is extended to include bulk trapping effects at low injection levels. This is done through recognizing that even in the presence of trapping when their lifetimes may differ, excess electrons and holes both diffuse in the bulk with the same characteristic length. The theory has been examined in terms of quasi-equilibrium conditions across the surface space charge layer; however, the method is expected to work under less restrictive conditions. As with no trapping, the method should yield the minority carrier diffusion length in extrinsic material for majority carrier trapping in general, but in the case of minority carrier trapping, only for moderate trapping. When strong minority carrier trapping occurs in slightly extrinsic material, the effective diffusion length, owing to trap-modified ambipolar diffusion in a Dember field, may be greater than the usual minority carrier diffusion length which obtains in the absence of trapping. A treatment of recombination statistics is included to illustrate how trapping effects may arise from the presence of deep-lying impurities such as gold in solicon. It also provides a basis for a detailed examination of the low injection-level assumptions used in simplifying the problem of trap-modified carrier diffusion in the bulk. It is shown, for example, that in extrinsic semiconductor, the usual assumptions that the excess carrier densities be much smaller than their equilibrium values need apply to majority carriers only. For minority carriers, the limitations are far less severe in most cases of practical interest, and depend on the characteristics of the trapping centers involved.

The surface photovoltage (SPV) method of semiconductor minority carrier diffusion length measurement is simulated numerically by solving the semiconductor equations under monochromatic illumination. It is shown that the extracted diffusion length is dependent on the SPV level and the initial surface potential. This dependence arises because of carrier generation in the surface space charge region. For the p-type silicon investigated, the extracted diffusion length is found to be within 10% of the actual value even when the assumption of low level injection is violated.

The minority carrier lifetime is measured in p -type Ge up to a majority carrier concentration of 10 19 cm −3 using the photoconductive effect and the photo-magneto-electric effect. A stronger than linear dependence of the reciprocal lifetime on the carrier density is observed. Therefrom, from the weak temperature-dependence, from previous work and from the comparison with Si-data, we conclude that the dominant recombination mechanism at high carrier concentrations is the phonon-assisted band to band Auger-recombination. A transition coefficient of about 10 −31 cm 6 sec −1 is estimated.