Base Doping Research Articles

Dependence of n-cSi/MoOx Heterojunction Performance on cSi Doping Concentration

In this work, we demonstrate a strong correlation between crystalline silicon (cSi) base doping concentration and the performance of cSi/MoOx heterojunction solar cell by investigating the structure numerically based on Silvaco TCAD simulation tool and experimentally. The doping concentration of n-type cSi was scanned in the 1 × 1015 – 2 × 1016 cm-3 range. Simulation results show that utilizing highly doped cSi wafer degrades the conversion efficiency of cSi/MoOx solar cell. Efficiency of 11.16% has been obtained from simulation results for 1 × 1015 cm-3 doping concentration while this value reduces to less than 4% for wafer with a doping concentration of 2 × 1016 cm-3. These simulation results were demonstrated experimentally and n-type cSi wafers with two different doping concentrations were considered, 1 × 1015 and 5.5 × 1015 cm-3. The key concept underlying this work is to differentiate explicitly the effect of cSi doping concentration on the performance of cSi/MoOx cell, thus a simple cell design is considered where n-type cSi wafers were heavily phosphorous-doped to form (n+) at the front of the Si and thermally evaporated MoOx films with various thicknesses (<15 nm) were inserted at the rear between cSi and Al contact. In accordance to simulation results, highly doped wafer exhibited low conversion efficiency of 3.32% while using lower doped wafer significantly improves the efficiency from 3.32 to 10.9%.

Current–voltage characteristics of Schottky diodes at high current densities under the injection of minority carriers

An analytical expression is derived for the current–voltage characteristic of a Schottky diode at a high injection level of minority carriers. It is shown that, even at very high current densities, the higher the base doping level, the larger the voltage drop across the diode. The physical mechanism responsible for this “paradoxical” result is analyzed. The validity of the analytical result is confirmed by a numerical calculation with software that takes into account the whole set of nonlinear effects caused by a high injection level in the base layer and by heavy doping of the emitter region.

1.0 eV GaAs based InAs quantum dot solar cells

There are great improvements in III-V semiconductor solar cells including GaAs single junction solar cell and tandem solar cells in last decades. The common commercially used triple-junction solar cell mostly utilize a Ge bottom cell with an (In)GaAs and InGaP middle and top cell respectively. The (In)GaAs middle cell absorbs limited solar spectrum and show minimum short-circuit current in multiple junction solar cell, which determine the overall current of the cell. Theoretically, higher energy conversion efficiency will be obtained when introducea subcell with 1.0 eV band gap in series to the InGaP/GaAs/Ge tandem structures. Lattice-matched GaInNAs and lattice-mismatched In0.3Ga0.7As are two common choices for achieving 1.0 eV band gap. However, the addition of N to GaAs has detrimental effects on the material quality, limiting its utility to approaches that required photocurrent. And the lattice mismatch makes thick In0.3Ga0.7As heterogeneous epitaxial difficult. The incorporation of low band gap nanoscale materials, such as quantum well and quantum dots, into current limiting junction of a multijunction solar cell can tuning the effective bandgap and enhance the low-energy photon absorption, thereby increasing the short-circuit current. The samples are grown by solid-source molecular beam epitaxy (V80H) on p-GaAs (001) substrates using an As4 source. Sample A is a standard GaAs solar cell with PIN structure. Sample B contains 10 period 8 nm In0.15Ga0.85As QW and sample C contains 10 period InAs dots-in-well structure. The intrinsic region of these samples was at the same length of 630 nm to obtain the same build-in electric field. Besides the instinct region, the three samples are designed the same. The ohmic contact layer’s doping concentration is N D/ N A=3×1018 cm−3 and its thickness is 300 nm. The emitter doping concentration is N D=5×1017 cm−3 and its thickness is 150 nm; The base doping concentration is N A=5×1017 cm−3 and its thickness is 300 nm. The As4 pressure and the InAs QDs growth rate are 1×10−5 Torr and 0.1 ML/s, respectively. The XRD pattern of sample B (QWSC) shows clearly defined and intense satellite peaks indicating steep interface and periodicity quantum well. However, the XRD pattern of sample C (the InAs dots in well solar cell) shows blurring diffraction peaks which means increased interface roughness. Uniform QD structure with density of 2×1010 cm−2 can be observed in SEM. From the STEM, we can see well-defined unrelated pyramidal quantum dots separated by a nominal 50 nm GaAs spacing. From the contrast, the pyramid InAs QDs are estimated to have the lateral dimension of 27 nm and the height of 12 nm. The PC response of sample A rise at 600 nm and fall off at 870 nm, which is corresponding to the GaAs bandgap according to the PL peak position; the PC response of QWSC (sample B) rise at 600 nm and fall off at 990 nm, which is corresponding to the In0.15Ga0.85As bandgap in PL spectrum. The incorporation of In0.15Ga0.85As quantum wells in GaAs standard solar extend the absorption of solar spectrum from 900 nm to 1000 nm. The wavelength region from 700 nm to 900 nm in PC of QWSC is weaker than the GaAs standard solar cell. The insertion of InAs QDs into QWSC can extend the infrared photon absorption to the wavelength of 1300 nm. The insertion of InAs quantum dots into In0.15Ga0.85As/GaAs quantum well solar cell extend the absorption of infrared light and the J sc reaches 13.68 mA/cm2, giving a 37.8% increase in short-circuit current over the baseline solar cell current. Because the voltage of multiple junction solar cell is the sum of the sub cell’s voltage. The voltage decreasement with the insertion of InAs quantum dots do not influence the overall voltage of multiple junction solar cell. In summary, the incorporation of quantum well and quantum dots in GaAs solar cell result in extended solar spectrum absorption and contribute to the sub-band gap current gains for GaAs solar cell. Especially, InAs dots-in-well solar cell can extend the absorption spectrum to the wavelength of 1300 nm, and increase by 37.8% in short-circuit current, which shows great potential in mitigating current mismatch of multiple junction solar cell. Furthermore, designing new multiple junction solar cell becomes possible.

Three-dimensional simulation of fabrication process-dependent effects on single event effects of SiGe heterojunction bipolar transistor**Project supported by the National Natural Science Foundation of China (Grant Nos. 61274106, 11175138, and 61601352).

The fabrication process dependent effects on single event effects (SEEs) are investigated in a commercial silicon–germanium heterojunction bipolar transistor (SiGe HBT) using three-dimensional (3D) TCAD simulations. The influences of device structure and doping concentration on SEEs are discussed via analysis of current transient and charge collection induced by ions strike. The results show that the SEEs representation of current transient is different from representation of the charge collection for the same process parameters. To be specific, the area of C/S junction is the key parameter that affects charge collection of SEE. Both current transient and charge collection are dependent on the doping of collector and substrate. The base doping slightly influences transient currents of base, emitter, and collector terminals. However, the SEEs of SiGe HBT are hardly affected by the doping of epitaxial base and the content of Ge.

Advanced surface passivation of epitaxial boron emitters for high-efficiency ultrathin crystalline silicon solar cells

In this work, we demonstrated an enhanced surface passivation of epitaxially grown boron-doped Si emitters by replacing thermal SiO2 as a passivation layer employed in a 15.9% efficient 21-µm Si solar cell (88 cm2) on stainless steel with a remote-plasma atomic layer deposition (ALD) of an Al2O3 film. A thin Al2O3 film deposited by remote-plasma ALD was very effective at reducing the emitter saturation current density (J0e) of epitaxial p+-emitter to 16.2 fA/cm2, compared to the J0e of 184.9 fA/cm2 by thermal SiO2 films. This reduction in J0e enables an increase in an implied open-circuit voltage (iVoc) from 630 to 688 mV. Quokka simulation shows that about a 1.1% absolute efficiency increase in the calculated baseline efficiency of a 15.1% of the ultrathin Si solar cell can be achievable by enhancing emitter surface passivation without changing the concentration in either the epi-emitter or epi-base. Finally, our results show that a high efficiency of 17.3% can be reached from the calculated baseline efficiency of 15.1% using the optimized conditions of an epitaxially grown emitter in combination with increasing the base doping concentration and improved base recombination lifetime.

Effect of Neutron Irradiation on Epitaxial 4H-SiC PiN UV&lt;sup&gt;&lt;/sup&gt;-Photodiodes

4H-SiC UV-photodetectors based on full-epitaxial p +p-n+ multilayer structures werefabricated. The diodes were irradiated with fast neutrons up to the fluence of 1·1014 cm-2 . Current-voltage characteristics, life time of non-equilibrium charge carriers as well as photosensitivityspectra of the diodes before and after irradiation were investigated. The studies showed that PiNUV-photodiodes with base doping below 1·1015 cm-3 retain their performance up to the fluence of5·1012 neutrons per cm2 . The further increase in fast neutron fluence stimulates the creation ofexcessive deep recombination centers. This leads to degradation of I-V-characteristics, reduction ofcarrier life time and, consequently, to degradation of the photosensitivity of devices.

Base Doping Effects on the Efficiency of Vertical Parallel Junction Solar Cells

In this work, we present a theoretical study of a vertical parallel junction silicon solar cell under monochromatic illumination. The internal quantum efficiency (IQE) and the photovoltaic conversion efficiency are calculated, taking into account the base doping density and illumination wave-length. The main purpose of this work is to show how conversion efficiency depends on internal quantum efficiency and the dependence of the later on the base doping density.

Displacement Damage in Bipolar Junction Transistors: Beyond Messenger-Spratt

Bipolar junction transistors (BJTs) with relatively large feature sizes, typical of older technologies and high-voltage devices, are susceptible to recombination in the neutral base region when they are damaged by atomic displacement. This results in gain degradation, which, as originally described by Messenger and Spratt, can be modeled as an increase in the ratio of BJT base to collector current ( $1/\beta $ ) when the device is exposed to high energy particles (e.g., neutrons). While this also occurs in smaller devices and in those intended for operation at lower voltages, recombination in the emitter-base depletion region typically dominates the radiation-induced gain degradation in these devices. This is a consequence of the recombination rate being highest in regions where the electron and hole concentrations are approximately equal. Recombination in the neutral base and emitter-base depletion region exhibit different dependences on applied voltage, base doping and neutral base width. These dependences result in inaccuracies in the standard Messenger-Spratt equation, even for devices with base widths as large as $\sim 10~\mu \text {m}$ . This paper presents a physics-based modification of the equation which provides a much better fit to experimental data, particularly at low voltages.

Performance of conversion efficiency of a crystalline silicon solar cell with base doping density

Abstract In this study, we investigate theoretically the electrical parameters of a crystalline silicon solar cell in steady state. Based on a one-dimensional modeling of the cell, the short circuit current density, the open circuit voltage, the shunt and series resistances and the conversion efficiency are calculated, taking into account the base doping density. Either the I-V characteristic, series resistance, shunt resistance and conversion efficiency are determined and studied versus base doping density. The effects applied of base doping density on these parameters have been studied. The aim of this work is to show how short circuit current density, open circuit voltage and parasitic resistances are related to the base doping density and to exhibit the role played by those parasitic resistances on the conversion efficiency of the crystalline silicon solar.

Influence of base carrier lifetime on the characteristics of 4H-SiC BJTs

The optimum range of the base electron lifetime (τnB0) in silicon carbide (SiC) bipolar junction transistors (BJTs) is qualitatively investigated in this paper, considering both the static and dynamic performance. The TCAD simulation results show that there is tradeoff between the current gain (β) and the turn-off time (tF). With τnB0 increasing, both β and tF increase. However, with a fixed base width, the base transport factor tends toward saturation when τnB0 exceeds certain value. Thus, the eclectic values of base electron lifetime with different base thickness are obtained. The influence of the base drive current is also discussed. According to the charge control theory, the excessive storage charge caused by overdrive has a great influence on turn-off delay time (tD). To solve this, a suitable base drive current is necessary. For the common 4H-SiC BJT structure, appropriate base drive under variable base and collector doping concentration is suggested.

Performance optimization of Pnp InGaAs/InP heterojunction phototransistors

Fabrication, physical simulation, and optimization of two-terminal Pnp heterojunction phototransistors (2T-HPTs) based on In0.53Ga0.47As/InP are reported. The parameters of fundamental models are determined by comparing the simulated current and response characteristics with the experimental results. To optimize the optical gain and device performance, the precise adjustment of the base doping level, base width, and compositional grading of base has been investigated. Properly reducing the base width or increasing the range of the compositional grading can greatly enhance the emitter injection efficiency. The effects of high-low doping in collector region and the insertion of a thin, undoped InGaAs layer in the base region of the HPT have also been investigated in detail. It is found the high-low doping in collector can form an electric field to aid carrier transport, and the intrinsic layer between emitter and base has functions of reducing knee voltage and the dark current of HPT.

Vertical GaN bipolar devices: Gaining competitive advantage from photon recycling

It is shown that peripheral current Ip flows in non‐self‐aligned mesa‐type p–n diodes but that Ip flowing in GaN diodes cannot be explained by elongated minority‐carrier lifetime associated with intrinsic photon recycling (IPR), i.e., reabsorption of radiative recombination. Accordingly, possible increase in the ratio of deep Mg acceptors (energy level: EA) due to extrinsic photon recycling (EPR) is proposed, and the effect of EPR is expressed as an effective EA. Due to the limit on the power dissipation of a semiconductor package, EPR works effectively in the case of zero‐offset bipolar‐junction transistors, whose base doping level is limited in terms of emitter‐injection efficiency.Measured and simulated forward‐current–voltage characteristics of non‐self‐aligned mesa‐type GaN p–n diodes.

New fast turn-on speed SCR device for electrostatic discharge protection

A new SCR with the variation lateral base doping (VLBD) structure (VSCR) is proposed to improve the turn-on speed for electrostatic discharge (ESD) protection. The turn-on speed of the SCR was determined mainly by the base transit time of the parasitic p-n-p and n-p-n transistors of the SCR, and the VLBD structure can reduce the base transit time of the bipolar transistors to improve the turn-on speed of the SCR. The experimental and simulation results show that the turn-on time of the VSCRs with the VLBD structure is 12% less than that of the MLSCR with the traditional uniform base doping without adding extra process masks and increasing the chip area.

Minority carrier injection and current–voltage characteristics of Schottky diodes at high injection level

Transport phenomena in Schottky diodes are analyzed at high injection levels of minority carriers. It is shown that the correct description of these phenomena requires that the mode of diffusion stimulated by the quasi-neutral drift (DSQD) should be considered. An analytical expression for current–voltage characteristics of a Schottky diode at high injection levels is derived. The expression predicts a seemingly paradoxical result: the higher the base doping level, the higher the voltage drop across a diode at the same current density. The analytical results are confirmed by computer simulations. The results may be important for analyses of SiC Junction Barrier Schottky (JBS) diodes at very high current densities (surge current mode).

Numerical Study of TCO/Silicon Solar Cells with Novel Back Surface Field

ZnS/Si/CuO heterostructure is investigated by a theoretical approach as a possible low-cost design for photovoltaic conversion in the track of the heterojunction with intrinsic thin layer solar cells. Our results indicate that, owing to perfect electron affinity and lattice matching properties, zinc sulfide with adequate Al doping can efficiently replace zinc oxide window layer as an emitter region for silicon-based solar cells. Lattice mismatch, energy band alignment at the interfaces and material resistivity are the framework parameters of the study. By focusing on the open circuit voltage parameter, the back metal/Si and silicon base doping were optimized so that the conversion efficiency was increased from 3.37% to 15.19%. The introduction of a cupric oxide (CuO) layer acting as a p + back surface field with a bandgap of 1.35 eV and appropriate doping as high as 7 × 1018 cm−3 can enhance the conversion efficiency to 17.30%, provided that the silicon material remains free from contamination by copper atoms and also by performing a suitable treatment of CuO to lower its resistivity.

Synthesis of π-Bridged Dually-Dopable Conjugated Polymers from Benzimidazole and Fluorene: Separating Sterics from Electronics

We describe the synthesis and characterization of three new alternating copolymers containing fluorene and a dually dopable benzimidazole moiety. Poly(2-n-heptyl-benzimidazole-alt-9,9-di-n-octylfluorene) (PBIF), poly(2-n-heptyl-benzimidazole-vinylene-9,9-di-n-octylfluorene) (PBIF-VL), and poly(2-n-heptyl-benzimidazole-ethynylene-9,9-di-n-octylfluorene) (PBIF-EL) were synthesized from Suzuki, Heck, and Sonogashira cross-coupling reactions in reasonable yield. The materials were characterized through ultraviolet–visible spectroscopy, photoluminescence, and cyclic voltammetry. The vinyl and ethynyl bridges in PBIF-VL and PBIF-EL were incorporated to separate benzimidazole and fluorene units while retaining conjugation. This allowed us to differentiate between steric and electronic contributions to the band gap (Eg) changes that occur upon acid/base doping of these materials. We demonstrate that the blue shift arising from acid doping PBIF is due to steric torsion, while the red shift found upon base doping P...

Mechanism of Fast Current Interruption inp−π−nDiodes for Nanosecond Opening Switches in High-Voltage-Pulse Applications

Pulsed-power applications supporting research in fusion, plasma, and particle physics require delivery of high voltages over very short times (nanoseconds, or less)---no mean feat of engineering. The authors study current interruption in a step-recovery diode (SRD), which could be a fast-opening switch for high-voltage pulsed electronics, and find the main governing parameters to be the diode's reverse current density and base doping concentration. This understanding allows the design of improved structures, taking advantage of modern semiconductor growth technology.

A simulation study on the electrical structure of interdigitated back-contact silicon solar cells

In this paper, a simulation for interdigitated back-contact (IBC) silicon solar cells was performed by using Silvaco TCAD ATLAS to investigate the cell’s electrical properties. The impacts of various parameters, including the depth of the front surface field(FSF), the FSF peak doping concentration, the depths of the emitter and the back surface field(BSF), the peak doping concentrations of the emitter and BSF, the base doping, and the bulk lifetime on the output characteristics like the light current-voltage curves and the internal quantum efficiency of the IBC solar cell, were investigated. The light absorption was determined by adjusting the antireflection coating and the Al thickness. The FSF must be thin and have a low doping concentration for high-efficiency IBC cells. If the conversion efficiency is to be improved, a thick emitter and a high doping concentration are needed. Because of the low resistivity of the Si substrate, the series resistance was reduced, but recombination was increased. With a high-resistivity Si substrate, the opposite trends were observed. By counter-balancing the series resistance and the recombination, we determined by simulation that the optimized resistivity for the IBC cells was 1 Ω·cm. Because all metal electrodes in the IBC cells are located on the back side, a higher minority carrier lifetime showed a higher efficiency. After the various parameters had been optimized, texturing and surface recombination were added into the simulation. The simulated IBC cells showed a short-circuit current density of 42.89 mA/cm2, an open-circuit voltage of 714.8 mV, a fill factor of 84.04%, and a conversion efficiency of 25.77%.

A novel approach for justification of box-triangular germanium profile in SiGe HBTs

This work presents a unique and robust approach for validation of using the box-triangular germanium profile in the base of SiGe heterojunction bipolar transistors, where the methodology considers the simultaneous optimization of the p-type base doping profile in conjunction with the germanium profile in the base. The study analyses the electron motion across the SiGe base in SiGe HBTs, owing to different accelerating electric fields. The analysis first presents a figure of merit, to achieve the minimum electron transit time across the base in conjunction with the increased current gain in n—p—n-SiGe HBTs, which shows a general trend vis-à-vis the advantage of a trapezoid germanium profile, but with additional accuracy as we considered simultaneously optimized p-type base doping. The effect of minority carrier velocity saturation is then included to make the study more detailed. The analysis then investigates the shifted germanium profile in the base to further minimize the base transit time. Finally, it is shown that a shifted germanium profile eventually evolves into a box-triangular Ge-profile in the SiGe base, which could simultaneously minimize the base transit time and reduce emitter delay by virtue of the high current gain. The analysis verifies that for an average Ge-dose of 7.5% Ge across the base, a box-triangular germanium profile in conjunction with an optimum base doping profile has an approximately identical base transit time and a 30% higher current gain, in comparison with an optimum base doping and triangular Ge-profile across the whole base.

Theoretical Investigation of Base Doping and Illumination Level Effects on a Bifacial Silicon Solar Cell

In this paper, we are investigating theoretically the behavior of a bifacial silicon solar cell in steady state with different illumination conditions. The bifaciality coefficient and the conversion efficiency are calculated for various rear side illumination conditions (traduced here by the illumination level) and back surface recombination velocity, taking into account the base doping density. The main purpose of the work is to show that bifacial illumination can improve significantly the conversion efficiency of the solar cell and to exhibit the role of the back surface recombination velocity, the base doping density and the rear side illumination conditions in the performance of the bifacial silicon solar cell.