High Series Resistance Research Articles

Analysis of the mobility behavior of MOS2 2D FETs

In this work we analyze the behavior of 2D FETs, with channel length greater than the mean free path, and using MOCVD or CVD deposition method for the deposition of the 2D semiconductor layer, with different dielectric materials and EOTs. We show that transfer, output and conductance characteristics can be modeled with precision, considering the hopping transport mechanism as the predominant one, similarly to amorphous or polycrystalline TFTs. It was also observed that for the devices with channel length above one micrometer, mobility increased with the gate voltage as a power law. For channel lengths of 1 µm and 100 nm, mobility decreased with voltage, which in this case, can be attributed to other extrinsic effects, as the presence of high series resistance at the drain and source, which becomes more important as the channel length reduces, modifying its behavior with gate voltage.

Planar Micro-Supercapacitors with High Power Density Screen-Printed by Aqueous Graphene Conductive Ink.

Simple and scalable production of micro-supercapacitors (MSCs) is crucial to address the energy requirements of miniature electronics. Although significant advancements have been achieved in fabricating MSCs through solution-based printing techniques, the realization of high-performance MSCs remains a challenge. In this paper, graphene-based MSCs with a high power density were prepared through screen printing of aqueous conductive inks with appropriate rheological properties. High electrical conductivity (2.04 × 104 S∙m-1) and low equivalent series resistance (46.7 Ω) benefiting from the dense conductive network consisting of the mesoporous structure formed by graphene with carbon black dispersed as linkers, as well as the narrow finger width and interspace (200 µm) originating from the excellent printability, prompted the fully printed MSCs to deliver high capacitance (9.15 mF∙cm-2), energy density (1.30 µWh∙cm-2) and ultrahigh power density (89.9 mW∙cm-2). Notably, the resulting MSCs can effectively operate at scan rates up to 200 V∙s-1, which surpasses conventional supercapacitors by two orders of magnitude. In addition, the MSCs demonstrate excellent cycling stability (91.6% capacity retention and ~100% Coulombic efficiency after 10,000 cycles) and extraordinary mechanical properties (92.2% capacity retention after 5000 bending cycles), indicating their broad application prospects in flexible wearable/portable electronic systems.

Analysis of barrier inhomogeneities in Ti/p–type strained Si0.95Ge0.05 Schottky diodes using reverse current-voltage characteristics

Extracting electrical parameters such as barrier height inhomogeneities (BHi) from the forward bias can be very challenging in metal/semiconductor (M/Sc) contacts characterized by low barrier height and high series resistance. In this work we demonstrate that the reverse bias characteristics can be used as an efficient alternative method to investigate the inhomogeneity of low barrier height in M/Sc contacts. In particular, the BHi in Ti/p-strained Si0.95Ge0.05 Schottky barrier diodes (SBD) has been investigated using reverse current–voltage-temperature (IR-VR-T) characteristics over the temperature range of 100–300 K. The temperature dependence of the measured barrier heights (ΦRBp) using reverse-bias is successfully explained in terms of a thermionic emission (TE) current transport associated with Gaussian distribution of the barrier height due BHi. The mean homogeneous barrier height (Ф‾RBp) and its corresponding standard deviation (σRs) were determined for different reverse voltages. A decrease in Ф‾RBp and an increase in σRs with increasing reverse bias are observed, indicating a large barrier height distribution upon high applied reverse bias. The deduced average zero-field barrier height (ФFBp≈0.59eV) from IR-VR agrees well with the corresponding value determined from the forward characteristics. Moreover, the reverse bias extracted values for Richardson constant A* based on BHi model, are found in fair agreement with the reported theoretical value of 32 A cm−2 K−2 for our p-type strained Si0.95Ge0.05 alloy. Finally, the results obtained on Pd/n-type Si0.90Ge0.10 structure shown in this work fully support the utilization of such reverse bias method.

Characterization of Cost-Saving Monofacial PERC Module under Field Conditions in South Korea

The photovoltaic (PV) industry is constantly striving to increase module power output while decreasing costs. Importantly, the performance and reliability of cost-saving products should be evaluated before being launched into the PV market to avoid any unnecessary side effects. This study investigated the performance of a monofacial module employing a cost-saving bifacial cell installed at a carport in South Korea. The bifacial cell reduces costs, compared to monofacial cell, by using a lower quantity of aluminum paste on its rear; consequently, it has become a popular product in the PV industry. The monofacial module employing the bifacial cell showed an improved voltage temperature coefficient and low-light performance over monofacial cell-based module. Our field data highlight three conclusions from the bifacial cell-based module; it showed (1) different voltage temperature coefficients with better performance at lower temperatures, (2) better low-light performance owing to high series resistance, and (3) high current owing to its bifaciality. Notably, under high irradiance and temperature conditions, the bifacial cell performed worse than the monofacial cell. We concluded that this type of cell may perform well under northern European climatic conditions, though further investigation is required to optimize cell performance under various weather conditions.

Synthesis and electrochemical properties of porous carbon materials from sludge sources

Anaerobic digestion sludge technology is a green and efficient method of treating sludge. However, the presence of humic acid (HA) in sludge can inhibit methanogenic efficiency, and it is necessary to reduce its impact on biogas production by removal or pretreatment. HA, a graphene oxide material can be used to produce high-performance energy-storage materials. Thus, this study examined the sludge source of HA as a precursor, and different electrode materials were prepared by varying the reaction conditions. The structure and electrochemical properties of the electrode materials were analyzed. The results showed that the electrode material prepared using KOH as an activator at 700 °C exhibited optimal performance, with a high specific surface area (1480.53 m2·g−1), pore volume (0.943 cm3·g−1), specific capacitance (185.9 F·g−1), and equivalent series resistance (0.73 Ω). The material maintained 97.8% of its specific capacitance after 1000 cycles at a current density of 1 A·g−1 with benign cycle stability. This study confirmed that the production of HA as an electrode material at a low cost with good performance presents significant prospects.

On the role of inductive loops at low frequencies in PEM electrolysis

Inductive loops at low frequencies have been observed in the electrochemical impedance spectra (EIS) of various electrochemical cells. Although different physicochemical models for this phenomenon have been suggested in many other applications, this topic has not been widely discussed in the field of proton exchange membrane (PEM) electrolysis.In this article, low-frequency inductive loops in PEM electrolysis cells and their impact on cell performance are analyzed. We show that this phenomenon is reproducible and occurs with different cell materials and setups. Its impact increases with increasing current density and decreasing temperature. At extreme conditions (7 A∙cm−2, 40 °C) we show that the negative polarization resistance of the inductive process can exceed the capacitive polarization processes by a factor of three, resulting in a direct current resistance less than the high frequency series resistance of the cell.

High Breakdown Voltage GaN Schottky Diodes for THz Frequency Multipliers

Quasi-vertical gallium nitride (GaN) Schottky diodes on silicon carbide (SiC) substrates were fabricated for frequency multiplier applications. The epitaxial structure employed had an n− layer of 590 nm with doping 6.6 × 1016 cm−3, while the n+ layer was 950 nm thick, with doping 2 × 1019 cm−3. Potassium hydroxide (KOH) chemical surface treatment before Schottky contact metallization was employed to study its effect in improving the diode parameters. The KOH-treated diode demonstrated a breakdown voltage of − 27.5 V, which is the highest reported for this type of diode. Cut-off frequencies around 500 GHz were obtained at high reverse bias (− 25 V) in spite of high series resistance. The result obtained in breakdown voltage value warrants further research in surface treatment and post-annealing of the Schottky contact optimization in order to decrease the series resistance.

Terahertz Schottky Barrier Diodes Based on Aligned Carbon Nanotube Arrays

Carbon nanotubes (CNTs) with superior electrical characteristics are among the most promising candidate materials for constructing millimeter wave and terahertz wave electronic devices. However, the reported Schottky barrier diodes (SBDs) based on CNTs usually exhibited high series resistance and severe mismatch to lower impedance external circuits, which limits the devices for high frequency applications. In this work, CNT SBDs of lateral structure with a rectify current density of -0.78 mA/μm and a short circuit current responsivity of 6.8 A/W are demonstrated based on aligned carbon nanotube arrays (A-CNTs) with high-density and high-semiconducting purity on a quartz substrate. The A-CNT SBD with a width of 10 μm and an L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ch</sub> of 50 nm under zero bias exhibits an ultra-low intrinsic capacitance of 2.5 fF and an intrinsic cutoff frequency <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> of up to 840 GHz, which suggests that CNT-based SBDs are promising for millimeter wave and terahertz wave applications.

Batch Furnace Chemical Vapor Deposition of Pure Boron Layers on Si and GaN Substrates for Low-Leakage-Current Diode Fabrication

Boron deposition on both n-Si and n-GaN in the temperature range 250 - 500 °C, has been shown to form diodes with low saturation currents, i.e., electron injection from the n-substrate into the B-layer was efficiently suppressed. Moreover, down to 3-nm-thick B-layers on Si were shown to form a material barrier to Al, opening the possibility of fabricating Au-free gates for gallium-nitride high-electron-mobility transistors (GaN HEMTs). Several different chemical- and physical-vapor deposition (CVD/PVD) methods for depositing B have been studied for fabricating p+n-like Si diodes, called PureB diodes, all with comparable results. In this paper, the deposition of B-layers from diborane in a CVD batch furnace system is evaluated, particularly for use as a barrier material to enable Al-contacting of GaN diodes. These Al-B diodes could provide an option for fabricating low-leakage diodes that are compatible with complementary metal-oxide-semiconductor (CMOS) processing at industrially attractive high throughput. The bulk B has high resistivity, which, combined with the fact that non-uniformities in the nm range, are typical due to gas depletion along the furnace tube and gives uncontrollable, often high diode series resistance. A simulation study shows that Al-B could, nevertheless, be used as a gate stack in HEMTs for low-frequency power applications.

Electrical and Recombination Properties of Polar Orthorhombic κ-Ga2O3 Films Prepared by Halide Vapor Phase Epitaxy

In this study, the structural and electrical properties of orthorhombic κ-Ga2O3 films prepared using Halide Vapor Phase Epitaxy (HVPE) on AlN/Si and GaN/sapphire templates were studied. For κ-Ga2O3/AlN/Si structures, the formation of two-dimensional hole layers in the Ga2O3 was studied and, based on theoretical calculations, was explained by the impact of the difference in the spontaneous polarizations of κ-Ga2O3 and AlN. Structural studies indicated that in the thickest κ-Ga2O3/GaN/sapphire layer used, the formation of rotational nanodomains was suppressed. For thick (23 μm and 86 μm) κ-Ga2O3 films grown on GaN/sapphire, the good rectifying characteristics of Ni Schottky diodes were observed. In addition, deep trap spectra and electron beam-induced current measurements were performed for the first time in this polytype. These experiments show that the uppermost 2 µm layer of the grown films contains a high density of rather deep electron traps near Ec - 0.3 eV and Ec - 0.7 eV, whose presence results in the relatively high series resistance of the structures. The diffusion length of the excess charge carriers was measured for the first time in κ-Ga2O3. The film with the greatest thickness of 86 μm was irradiated with protons and the carrier removal rate was about 10 cm-1, which is considerably lower than that for β-Ga2O3.

Thermal sensing capability and current–voltage–temperature characteristics in Pt/n-GaP/Al/Ti Schottky diodes

We have discussed the thermal sensing capability under a constant current level and current versus voltage (I–V) traces by measuring the temperature of high series resistance Pt/n-GaP/Al/Ti Schottky structures in the 100−320 K range. The Rs values of 35 Ω and 4.50 × 103 kΩ for the device have been determined from I–V traces at 320 and 100 K, respectively. The thermal sensing (V–T) curves are expected to give a straight line at each current level. However, the V–T curves have deviated upward from linearity due to the high Rs value of the device after a certain temperature. The deviation point from linearity in V–T traces shifts to higher temperatures with an increase in bias voltage and current level. Thereby, the straight-line interval portion of the V–T curve has become too small with an increase in the current value. The thermal sensing coefficient α changed from 2.49 mV/K at 10 μA to 3.21 mV/K at 0.50 nA. Therefore, it has been concluded that the Pt/n-GaP/Al/Ti Schottky barrier (SB) is preferable for thermal sensor applications at the small current levels of 0.50, 1.0, 2.0, and 10.0 nA with high sensitivity up to a minimum temperature of 100 K. From I–V curves, qΦb0 and ideality factor values have ranged from 1.200 eV and 1.066 at 320 K to 0.854 eV and 1.705 at 100 K. It has been reported in the literature that the large SB height leads to a better temperature response.

Improvement in the Performance of P-Type Tunnel Oxide Passivated Contact Solar Cell With Selective Tunneling Junctions Underneath the Contacts on Front Side

Industrial silicon solar cells are now moving further from passivated emitter and rear contact (PERC) toward Tunnel Oxide Passivated contact (TOPCon) technology. The efficiency of the PERC solar cell is improved by TOPCon technology, in which the rear side is fully passivated by an oxide/poly-Si tunneling junction. The performance of a TOPCon solar cell may further be improved by the incorporation of selective tunneling junctions underneath the front contacts. Some fabrications have been reported on a similar structure for an n-type Si substrate but they have not achieved the expected results. Also, a detailed study has not been done yet. Using 3-D sentaurus technology computer aided design (TCAD) simulation software, a detailed analysis has been done on the modified structure through the optimization of the tunnel oxides, the interfaces, the n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">++</sup> poly-Si layer, the selective (SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">++</sup> -poly-Si) tunneling junctions underneath the front contacts, etc., which has not been done earlier. It is seen that a thick n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">++</sup> poly-Si layer causes parasitic absorption of light, whereas a very thin poly-Si layer cannot provide sufficient field for tunneling of carriers. A thicker tunnel oxide produces a high series resistance, whereas a very thin tunnel oxide cannot restrict the minority carriers to reach to the contacts. Therefore, an optimization is required and the optimization is done based on a fabricated bifacial p-TOPCon solar cell reported in the literature with an efficiency of 21.2% and bulk lifetime of 200 μs. After performing all the modifications in the fabricated reference cell and optimizing different parameters, the efficiency is improved by 3.5% in absolute. The efficiency is further improved to 25.4% with the bulk lifetime of 1 ms.

Hierarchical framework of CoZnS as a high-performance electrode material for supercapacitors

Efficient renewable energies are needed to meet the energy demands of modernized civilization. Therefore, this study evaluated composite Co 1- x Zn x S (0.04 ≤ x ≤ 0.34) (CZS-1 to CZS-6) thin film electrodes produced using a chemical growth approach as effective energy storage devices. Systematic integration of Zn 2+ in the CoS host formed the polycrystalline Co 0.24 SZn 0.76 (cubic) phase. The electrochemical measurements revealed a high diffusion rate and low series resistance, resulting in an excellent specific capacitance (C s ) of 1288 F g −1 at a current density of 2 mA cm −2 with better rate performance. In addition, the optimal electrode (CZT-5) showed a better power density of 8.86 kW kg −1 and an energy density of 44.4 Wh kg −1 with excellent capacitive retention (85% after 5000 cycles). The enhanced performance could stem from the spongy rose flower-like nanostructure with a covered network of polygonic petals. Consequently, the replacement of Co 2+ with Zn 2+ also contributed by exchanging the oxidation states of targeted atoms, including Co 2+ , Co 3+ , Zn 2+ , and S 2− . Moreover, surface topographical insight states that crystalline growth of the CZS samples with hillocks and valleys and CZS-5 sample exhibited maximum total roughness.

Characteristics of Dye-Sensitized Solar Cell under PWM Illumination: Toward Indoor Light-Energy Harvesting in the Solid-State Lighting Era

The dye-sensitized solar cell (DSSC) has been on the market as a permanent power source for indoor IoT edge devices. In recent years, indoor illumination technology has been experiencing a drastic transition from incandescent and fluorescent lamps toward solid-state lighting devices with light-emitting diodes (LEDs). In addition to the high power efficiency, a virtue of LEDs is their prompt response, which enables precise change of the illumination level using pulse-width modulation (PWM) of the current source, and thus PWM illumination is commonly installed in society. The light intensity change from off to on states of an LED under PWM driving is literally infinity, which causes the lighting to flicker. The lighting flicker induces not only an optical illusion but also biological effects, including serious health problems, which can be mitigated by raising the modulation frequency. Because the peak intensity of a PWM illumination can be 100 times that of the average intensity, the indoor solar cell, which has a relatively high series resistance, is expected to underperform. In this paper, the characteristics of a commercial indoor DSSC under PWM illumination are studied. It is found that while PWM illumination at low frequency seriously deteriorates the performance of the DSSC, it recovers at high frequency. The latter feature is not found in indoor amorphous-Si solar cells, and the electrochemical impedance spectroscopy revealed that it stems from the electrochemical nature of some components of the series impedance in the DSSC, offering a key piece of evidence of the superiority for use in the modern indoor application of the DSSC over traditional amorphous-Si solar cells.

Self-Operating Flyback Converter for Boosting Ultra-Low Voltage of Thermoelectric Power Generator for IoT Applications

This article presents a novel self-operating flyback converter to boost the ultra-low voltage produced by thermoelectric power generator (TEG). The proposed converter uses dual transformer, one for low-voltage startup and other to extract power from TEG. The use of separate transformer provides high-voltage transfer ratio, reduces high series resistance loss, and brings down the input impedance of circuit. The copper wire wounds ferrite toroid core transformer provides very low series resistance and negligible core loss, which improves the circuit efficiency. This boost converter operates with an input voltage range of 22–300 mV and provides maximum output power of 0.073 – 147 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\rm{mW}}$</tex-math></inline-formula> correspondingly. The peak efficiency of 68% with output power of 1.32 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\rm{mW}}$</tex-math></inline-formula> (voltage ∼1.83 V) is achieved at an input voltage of 50 mV. For regulated output voltage, a feedback loop is used to keep output voltage constant during sudden voltage rise of TEG. The proposed circuit uses discrete components, ferrite toroid core, and PCB, which has lower cost than available energy harvester module, which uses energy harvesting ICs, discrete components, and tiny transformer. The lower startup voltage, higher output power, and voltage with high conversion efficiency of this boost converter maximize the applications potential of the TEG as a portable power source for Internet of Things (IoT) devices.

Threshold voltage of p-type triple-gate junctionless transistors

The threshold voltage of rectangular p-type triple-gate junctionless transistors (JLTs) is studied experimentally using the transconductance derivative (dgm/dVg) method, after correcting the drain current from the impact of series resistance. The effect of series resistance on the dgm/dVg behavior is highlighted. In the investigated devices, the high series resistance affects the dgm/dVg behavior more than the short-channel effects. The results show that, in addition to the flat-band voltage, for the first time two threshold voltages Vth1 and Vth2 are observed within the partial depletion region in devices with channel length varying from 95 to 25 nm. Numerical simulations of the holes density distribution reveal the absence of corner effects due to the unique bulk neutral conduction, whereas Vth1 and Vth2 correspond to the threshold voltages of the side gates and top gate, respectively. The correct extraction of the flat-band voltage has been confirmed with numerical simulations of the holes density distribution. Experimental measurements of p-type JLTs with variable being the fin width indicate that the threshold voltages Vth1 and Vth2 are due to the different interface states density at the side and top gates.

A Novel Single-Stage Common-Ground Zeta-Based Inverter With Nonelectrolytic Capacitor

Aiming at the key problems of the existing nonisolated photovoltaic (PV) inverter, such as the common mode (CM) leakage current, the voltage up and down ability, and the service life restricted by the electrolytic capacitor, a novel single-stage common-ground zeta-based inverter with nonelectrolytic capacitor is proposed in this article. The proposed inverter is based on zeta dc converter and has the inherent voltage conversion characteristics of zeta converter, which makes it more suitable for PV system with wide input voltage fluctuation range. In addition, common-ground structure is adopted in this inverter, which can shorten the parasitic capacitor of the PV array to the ground. Therefore, it can effectively suppress the CM leakage current, reduce the shutdown frequency of the PV system, and improve the utilization rate of solar energy. Moreover, the proposed inverter does not contain electrolytic capacitors, which avoids the problem of high maintenance cost due to the short service life and high equivalent series resistance of electrolytic capacitors. First, the topology of the proposed inverter is given, and the modulation and operating principle are analyzed. Second, the passive components are analyzed. Finally, a 1 kW prototype is built for experimental verification. The experimental results verify the correctness of the theoretical analysis and simulation.

Revealing fundamentals of charge extraction in photovoltaic devices through potentiostatic photoluminescence imaging

Revealing fundamentals of charge extraction in photovoltaic devices through potentiostatic photoluminescence imaging

Structural and electrical properties of thick κ-Ga2O3 grown on GaN/sapphire templates

Thick (23 µm) films of κ-Ga2O3 were grown by Halide Vapor Phase Epitaxy (HVPE) on GaN/sapphire templates at 630 °C. X-ray analysis confirmed the formation of single-phase κ-Ga2O3 with half-widths of the high-resolution x-ray diffraction (004), (006), and (008) symmetric reflections of 4.5 arc min and asymmetric (027) reflection of 14 arc min. Orthorhombic κ-Ga2O3 polymorph formation was confirmed from analysis of the Kikuchi diffraction pattern in electron backscattering diffraction. Secondary electron imaging indicated a reasonably flat surface morphology with a few (area density ∼103 cm−2) approximately circular (diameter ∼50–100 µm) uncoalesced regions, containing κ-Ga2O3 columns with in-plane dimensions and a height of about 10 µm. Micro-cathodoluminescence (MCL) spectra showed a wide 2–3.5 eV band that could be deconvoluted into narrower bands peaked at 2.59, 2.66, 2.86, and 3.12 eV. Ni Schottky diodes prepared on the films showed good rectification but a high series resistance. The films had a thin near-surface region dominated by Ec − 0.7 eV deep centers and a deeper region (∼2 µm from the surface) dominated by shallow donors with concentrations of ≤1016 cm−3. Photocurrent and photocapacitance spectra showed the presence of deep compensating acceptors with optical ionization energies of ∼1.35 and 2.3 eV, the latter being close to the energy of one of the MCL bands. Deep level transient spectroscopy revealed deep traps with energies near 0.3, 0.6, 0.7, 0.8, and 1 eV from the conduction band edge. The results show the potential of HVPE to grow very thick κ-Ga2O3 on GaN/sapphire templates.

Design of microwave β-(Al x Ga x )2O3/Ga2O3 lateral Schottky barrier diodes

High power microwave Schottky barrier diodes (SBDs) are required to develop detectors and mixers in advance the millimeter-wave (mm-wave) wireless technologies such as fifth generation long-term evolution. To achieve high efficiency and sensitivity in SBDs, materials/structures with high mobility and low series resistance are needed. In this regard, we have proposed the two dimensional electron gas of β-(Al x Ga)2O3/Ga2O3 heterostructure channel lateral SBDs (LSBDs) for the first time. To design high performance LSBDs, detailed physical models of both β-(Al x Ga)2O3 and Ga2O3 were studied and characterized with experimental results. These physical models were used to perform realistic numerical simulations of DC, small signal and large signal conditions. It was observed that the structural and compositional aspects of heterostructure are largely influencing the small signal sensitivity and cut-off frequencies. Also the large signal analysis reveals that the LSBD can operate in linear detection of microwave signals, up to 38 dBm. By scaling down the device to a sub-micron channel length, a cut-off frequency of 1.5 THz was achieved in the proposed LSBD. Comparison with state-of-the-art SBDs, the observations from DC, small signal and large signal analysis recommend the proposed β-(Al x Ga)2O3/Ga2O3 LSBDs for high power and mm-wave/sub-THz detector/mixer application.