Negligible Contact Resistance Research Articles

Strain-Insensitive Outdoor Wearable Electronics by Thermally Robust Nanofibrous Radiative Cooler.

Stable outdoor wearable electronics are gaining attention due to challenges in sustaining consistent device performance outdoors, where sunlight exposure and user movement can disrupt operations. Currently, researchers have focused on integrating radiative coolers into wearable devices for outdoor thermal management. However, these approaches often rely on heat-vulnerable thermoplastic polymers for radiative coolers and strain-susceptible conductors that are unsuitable for wearable electronics. Here, we introduce mechanically, electrically, and thermally stable wearable electronics even when they are stretched under sunlight to address these challenges. This achievement is realized by integrating a polydimethylsiloxane nanofibrous cooler and liquid metal conductors for a fully stable wearable device. The thermally robust architecture of nanofibers, based on their inherent properties as thermoset polymers, exhibits excellent cooling performance through high solar reflection and thermal emission. Additionally, laser-patterned conductors possess ideal properties for wearable electronics, including strain-insensitivity, nonsmearing behavior, and negligible contact resistance. As proof, we developed wearable electronics integrated with thermally and electromechanically stable components that accurately detect physiological signals in harsh environments, including light exposure, while stretched up to 30%. This work highlights the potential for the development of everyday wearable electronics capable of reliable operation under challenging external conditions, including user-activity-induced stress and sunlight exposure.

A Synthetic Approach to Cold Pressure Welding on Cogged Surfaces

Pressure welding on cogged surfaces represents a new technological variant of the cold welding. The components that are made from a material with higher plasticity (aluminium, lead etc.), having flat surfaces, are pressed on or between the harder material components (copper, brass, carbon/stainless steel, titanium, etc.) that have cogged surfaces. The main particularity of this technique is to achieve an appropriate joint by deforming only the component with higher plasticity. Due to the low degree of deformation needed, reduced pressure forces are applied in comparison with the classical cold pressure welding. The welding in isolated catching nodes is achieved by gripping, while the aluminium is gliding on the flanks of the teeth. The tensile strength of the joint is relatively low reaching up to 10% of the aluminium part, but can be improved by applying a heat treatment. Welded joints were made in various combinations, resulting in bimetallic or multilayered workpieces. Due to the negligible contact resistance, these joints can be appropriately used for applications in the electrotechnical field.

Synergistic Effect of Solvent Vapor Annealing and Chemical Doping for Achieving High-Performance Organic Field-Effect Transistors with Ideal Electrical Characteristics.

Contact resistance and charge trapping are two key obstacles, often intertwined, that negatively impact on the performance of organic field-effect transistors (OFETs) by reducing the overall device mobility and provoking a nonideal behavior. Here, we expose organic semiconductor (OSC) thin films based on blends of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT-C8) with polystyrene (PS) to (i) a CH3CN vapor annealing process, (ii) a doping I2/water procedure, and (iii) vapors of I2/CH3CN to simultaneously dope and anneal the films. After careful analysis of the OFET electrical characteristics and by performing local Kelvin probe force microscopy studies, we found that the vapor annealing process predominantly reduces interfacial shallow traps, while the chemical doping of the OSC film is responsible for the diminishment of deeper traps and promoting a significant reduction of the contact resistance. Remarkably, the devices treated with I2/CH3CN reveal ideal electrical characteristics with a low level of shallow/deep traps and a very high and almost gate-independent mobility. Hence, this work demonstrates the promising synergistic effects of performing simultaneously a solvent vapor annealing and doping procedure, which can lead to trap-free OSC films with negligible contact resistance problems.

Extended Cox & Strack analysis for the contact resistance of planar samples with carrier-selective junctions on both sides

We review the famous Cox & Strack equation that is commonly applied in contact resistance measurements of samples with a negligible contact resistance at the sample backside. We apply geometric interpretations to extend the Cox & Strack model a) to samples that have a non-negligible contact resistance not only on the front- but also on the back-side and b) to junctions with a buried contact resistance underneath a conductive layer. Case a) is for example a symmetric sample with a single material junction on both sample surfaces. Case b) could be a poly-Si/SiOx/c-Si or a-Si/c-Si hetero-junction. We compare our analytic treatment with rigorous finite-element simulations and find a relative agreement between 2.5 % and 36 % depending on the sample geometry and resistance values. We apply the method to analyze the contact resistance of lifetime samples with both-sided n+/n-type poly-Si junctions.

Fabrication and Evaluation of Low-Cost CrSi2 Thermoelectric Legs

CrSi2 is a promising thermoelectric material constituted of non-toxic and earth abundant elements that offer good perspectives for the mass production of inexpensive and reliable thermoelectric modules for waste heat recovery. Realization of robust metallic contacts with low electrical and thermal resistances on thermoelectric materials is crucial to maximize the conversion efficiency of such a device. In this article, the metallization of an undoped CrSi2 with Ti and Nb using a conventional Spark Plasma Sintering process is explored and discussed. These contact metals were selected because they have compatible thermal expansion coefficients with those of CrSi2, which were determined in this study by X-ray Diffraction in the temperature range 299–899 K. Ti was found to be a promising contact metal offering both strong adhesion on CrSi2 and negligible electrical contact resistance (<1 μΩ cm2). However, metallization with Nb resulted in the formation of cracks caused by large internal stress inside the sample during the fabrication process and the diffusion of Si in the metallic layer. A maximum conversion efficiency of 0.3% was measured for a sandwiched Ti/CrSi2/Ti thermoelectric leg placed inside a thermal gradient of 427 K. The preliminary results obtained and discussed in this article on a relatively simple case study aim to initiate the development of more reliable and efficient CrSi2 thermoelectric legs with an optimized design.

Multi-channel impedance measurement system for polymer humidity sensors

The design of a high performance, multi-channel impedance measurement system for humidity sensing characteristics test of polymer humidity sensors is presented. The relay modules with negligible contact resistance are designed to accomplish 32-channel measurement and automatic switching range. The division of the wide impedance range and the selection of the corresponding feedback resistance for automatic switching range are so designed as to satisfy the actual scale range of the ADC module for getting high accuracy measurement. An algorithm of compensation considering the parasitic parameters both in the measurement and feedback circuits is designed to further improve measurement accuracy. The whole measurement is automatically implemented by a field-programmable gate array for signal generation and logic calculation and a digital signal processor for frequency-spectra analysis. The results of the uncertainty analysis and tests revealed that our system was quite adequate to efficient humidity sensing characteristics test as opposed to the existing devices.

Electron and hole mobilities in ambipolar MoS2 electric-double-layer transistor

We report transport measurements of ionic liquid-gated field-effect transistors with multilayer MoS2 channels. Ambipolar characteristics with high ON/OFF ratios (>105) are observed. The current–voltage characteristics agree well with those calculated using a simple model assuming a high recombination probability and negligible contact resistance, except when holes are doped in the vicinity of the source contact. Electron and hole mobilities are estimated using the current–voltage characteristics in both the unipolar and ambipolar modes, wherein a p–n junction is formed in the latter. The electron mobility depends significantly on the carrier distribution in a channel that is controlled by the gate voltage. This result is attributed to the significant field dependence of the electron mobility.

The diverse electronic properties of C/BN heteronanotubes with polar discontinuity

Linear interfaces from within the heteronanotubes between carbon nanotubes (CNTs) and h-BN nanotubes (BNNTs) introduce unexpected electronic and spintronic properties that are different from individual components. In particular, for heteronanotubes with zigzag boundaries (ZCBNNTs) polar discontinuities occur at the interfaces, generating an intrinsic electric field acting on the segment CNTs. In turn, the electric field will cause a charge transfer between the two interfaces to compensate for the potential difference, thus the interface states (or boundary states) show new phenomena. For example, the half-metallic can be found in ZCBNNTs. Furthermore, the length and diameter of the ZCBNNTs can effectively adjust the electronic properties, due to the compensatory effects of the free charge to the polarization charge. Our results shed light on the future design of miniature superconducting spintronic devices and the realization of high performance nanoelectronic devices with negligible interface contact resistance barriers.

The electronic properties of Au and Pt metal contacts on quasi-one-dimensional layered TiS3(001)

The interfaces of layered trichalcogenide TiS3(001), with metals Au and Pt, were examined using X-ray photoemission spectroscopy. In spite of the fact that both Au and Pt are large work function metals, no evidence of Schottky barrier formation was found with this n-type semiconductor. Two- and four-terminal field-effect transistor measurements performed on exfoliated few-nm-thick TiS3 crystals using pure Au contacts indicate that Au forms an Ohmic contact on TiS3(001), with negligible contact resistance. The absence of appreciable Schottky barrier formation is attributed to strong interactions with sulfur at the metal-semiconductor interface.

Fluid-based spectrally selective filters for direct immersed PVT solar systems in building applications

Liquid filters applied in hybrid photovoltaic-thermal (PVT) solar systems report potential benefits as they allow for selecting the wavelengths at which the solar cell operates with higher efficiency. Among the rest, low frequency photons are absorbed by the liquid, therefore not warming the cell up. In addition, direct immersion of photovoltaic cells in liquids enhances temperature control by cooling the cells under almost negligible contact resistance between the liquid and the cell. The characteristics of the liquids for direct immersed PVT application are described, defining an indicator (ideal filter window) to select the best liquid depending on the PV technology and the incident irradiance. Several liquids are assessed based on the required properties for the present application. As a conclusion, regarding optical, thermal, electrical and operational aspects, a mixture of deionized water and isopropyl alcohol results as proper candidate satisfying well all of them.

Resistivity and Barrier Height of Nano-Resistors Made from Zr-Doped HfO2 High-K Dielectric on Si Wafer

The resistivity and barrier height of the nano-resistor made from the Zr-doped HfO2 MOS capacitor has been investigated. Bumps corresponding to nano-resistors formed from the dielectric breakdown were observed from SEM micrographs. The resistivity and barrier height to Si of the nano-resistor were estimated based on assumptions of the uniform size and negligible contact resistance to the ITO gate electrode. I-V curves of the nano-resistor device were used to estimate the resistivity of the single nano-resistor, i.e., about 103 Ω∙cm in the high gate voltage rage, and the barrier height, i.e., 0.66 V in the low voltage range. Judged from the barrier height value, the nano-resistor was fully established after the device was stressed at -20V for 1 min. These basic electrical properties are critical for practical applications of nano-resistor based devices.

Decrease of contact resistance at the interface of carbon nanotube/electrode by nanowelding

Reliable interconnection between carbon nanotubes (CNTs) and external circuit is one of the prerequisite in CNT electronics. In this work, ultrasonic nanowelding was used to bond CNTs with metal electrodes. By exerting ultrasonic energy at the interface of CNT/electrode, a reliable joint with negligible contact resistance was obtained between CNTs and electrodes. The performance of welding is susceptible to the ultrasonic parameters such as ultrasonic power and clamping force, as well as the metal type. It is found that the metals with good ductility or low melting point are easier to achieve effective joints. Moreover, interfacial compounds are formed at the welded surface of metal Al and Fe, which is resulted from the interacting and chemical bonding of carbon and metal atoms. After nanowelding, the contact resistance between CNTs and electrode is decreased dramatically, and the two-terminal resistance of the sample approximates to the intrinsic resistance of the CNT itself.

Ultra-high gain diffusion-driven organic transistor.

Emerging large-area technologies based on organic transistors are enabling the fabrication of low-cost flexible circuits, smart sensors and biomedical devices. High-gain transistors are essential for the development of large-scale circuit integration, high-sensitivity sensors and signal amplification in sensing systems. Unfortunately, organic field-effect transistors show limited gain, usually of the order of tens, because of the large contact resistance and channel-length modulation. Here we show a new organic field-effect transistor architecture with a gain larger than 700. This is the highest gain ever reported for organic field-effect transistors. In the proposed organic field-effect transistor, the charge injection and extraction at the metal–semiconductor contacts are driven by the charge diffusion. The ideal conditions of ohmic contacts with negligible contact resistance and flat current saturation are demonstrated. The approach is general and can be extended to any thin-film technology opening unprecedented opportunities for the development of high-performance flexible electronics.

A Single‐Material Logical Junction Based on 2D Crystal PdS2

A single-material logical junction with negligible contact resistance is designed by exploiting quantum-confinement effects in 1T PdS2 . The metallic bilayer serves as electrodes for the semiconducting channel monolayer, avoiding contact resistance. Heat dissipation is then governed by tunnel loss, which becomes negligible at channel lengths larger than 2.45 nm. This value marks the integration limit for a conventional 2D transistor.

A Complementary Dual-Contact MEMS Switch Using a “Zipping” Technique

This paper presents a microelectromechanical systems contact switch having both hard and soft contact materials in a single cantilever-type switching device. It operates with a zipping mechanism within which both contact materials (Pt-to-Pt and Au-to-Au) make individual contact sequentially and then detach in a reverse sequence to take advantage of both contact materials: low contact resistance and high reliability in a hot switching condition. In addition, an extended gate electrode and double T-shape cantilever beam structures effectively facilitate the sequential actuation. The fabricated switch successfully demonstrated a “dual-contact concept”-it made two sequential contacts at 31 (Pt-to-Pt) and 56 V (Au-to-Au) and it was then detached at 49 (Au-to-Au) and 23 V (Pt-to-Pt) in a single switching operation. Also, it achieved a low contact resistance of 0.3-0.5 Ω (including beam and some portion of the signal line resistances) at gate voltage from 60 to 70 V owing to the Au-to-Au contact in the device. Simultaneously, negligible contact resistance variation was observed during 2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> cycles at a voltage/current level of 10 V/10 mA under hot switching and unpackaged environments, representing >100-fold longer lifetime than that of a conventional Au-to-Au cantilever switch fabricated on the same wafer.

The Application of a Nanostructured Material in the Fabrication of a P-I-N Diode Chip

This research focuses on the integration of conventional silicon electronics with Nanostructured materials for an improved efficiency in the detection of photons applied in Fibre Optic Communications. The PIN starting material, intrinsic region thickness and packaging technology determines the diode performance. Nanostructured materials are processed using different techniques such as ion beam sputtering, layer-by-layer deposition, Nano Milling/Etching System, spin casting, ion beam deposition etc. This research utilizes the unique effects of the wide range of phenomena offered by Nanostructured materials such as plasmonic enhancement of absorption, ultraviolet light absorption, generation of multiexciton, charge storage in surface and interface traps, and size-based spectral tuning, to improve the functions of the intrinsic region of the P-I-N diode. The traps extend the lifetime of non-circulating carriers thereby increasing gain. The presence of an introduced external electromagnetic field shifts free conduction electrons. Other achieved advantages include a very negligible package parasitic effect, negligible contact resistance, and a package inductance on the order of 0.01nH for the P-I-N diode. 

A Simple Thermal Resistance Model for Open Cell Metal Foams

This paper presents a methodology for obtaining the convective heat transfer coefficient from the wall of a heated aluminium plate, placed in a vertical channel filled with open cell metal foams. For accomplishing this, a thermal resistance model from literature for metal foams is suitably modified to account for contact resistance. The contact resistance is then evaluated using the experimental results. A correlation for the estimation of the contact resistance as a function of the pertinent parameters, based on the above approach is developed. The model is first validated with experimental results in literature for the asymptotic case of negligible contact resistance. A parametric study of the effect of different foam parameters on the heat transfer is reported with and without the presence of contact resistance. The significance of the effect of contact resistance in the mixed convection and forced convection regimes is discussed. The procedure to employ the present methodology in an actual case is demonstrated and verified with additional, independent experimental data.

GRAIN SIZE EFFECTS ON EFFECTIVE THERMAL CONDUCTIVITY OF POROUS MATERIALS WITH INTERNAL THERMAL CONTACT RESISTANCE

We study the effective thermal conductivity of granular porous materials with or without consideration of the thermal contact resistance. A negligible thermal contact resistance means the particles connect well with each other. Otherwise we assume thermal contact resistance layers between the particles forming a network frame in the system We calculate the effective thermal conductivity by a high-efficiency lattice Boltzmann method and examine its dependence on the grain size of porous materials. The results show that the effective thermal conductivity decreases slightly with the averaged grain size for negligible thermal contact resistance, consistent with existing observations in engineering materials studies. When the thermal contact resistance is significant, the effective thermal conductivity decreases sharply as the grain size gets smaller, which agrees with the experimental measurements on nanoporous materials where the particles are not able to contact well with each other and the resistance dominates the thermal transport.

Fabrication of high performance/highly functional field-effect transistor devices based on [6]phenacene thin films

Field-effect transistors (FETs) based on [6]phenacene thin films were fabricated with SiO2 and parylene gate dielectrics. These FET devices exhibit field-effect mobility in the saturation regime as high as 7.4 cm(2) V(-1) s(-1), which is one of the highest reported values for organic thin-film FETs. The two- and four-probe mobilities in the linear regime display nearly similar values, suggesting negligible contact resistance at 300 K. FET characteristics were investigated using two-probe and four-probe measurement modes at 50-300 K. The two-probe mobility of the saturation regime can be explained by the multiple shallow trap and release model, while the intrinsic mobility obtained by the four-probe measurement in the linear regime is better explained by the phenomenon of transport with charge carrier scattering at low temperatures. The FET device fabricated with a parylene gate dielectric on polyethylene terephthalate possesses both transparency and flexibility, implying feasibility of practical application of [6]phenacene FETs in flexible/transparent electronics. N-channel FET characteristics were also achieved in the [6]phenacene thin-film FETs using metals that possess a small work function for use as source/drain electrodes.

An Electrostatically Actuated Stacked-Electrode MEMS Relay With a Levering and Torsional Spring for Power Applications

This paper reports on a novel electrostatically actuated microelectromechanical systems (MEMS) relay for use in power-switching applications. It features a levering and torsional spring to enhance the stand-off voltage and contact endurance by means of an active-opening scheme. The proposed relay is based on a unique stacked-electrode structure and a soft insulating layer under the contact material that make it possible to obtain extremely low contact resistance, resulting in high current driving capability and reliable contact endurance. The fabricated relay demonstrated actuation voltages under 40 V, a switching time of 230 μs, and a maximum stand-off voltage of 360 V, which is the highest level among electrostatically actuated MEMS relays reported to date. The contact resistance was under 5 mΩ at 40 V of applied voltage, and more than 1 A could be carried. The contact reliability in a hot-switching condition was investigated for various dc current levels. At a current of 10 mA, the relay operated for more than 107 cycles before the test was stopped. In addition, the permanent contact stiction during switching operation at a 200-mA current level was overcome with a pull-off (active-opening) voltage of 90 V by the levering and torsional spring. Using this healing process, a device that failed at about 104 switching cycles in the 200-mA hot-switching mode was revived and reoperated with negligible contact resistance variation, lasting up to 4.9 ×105 cycles, constituting an order-of-magnitude enhancement in the lifetime even after failure.