Wide Bandwidth Research Articles

Exact solution to band structures of one-dimensional viscoelastic functionally graded phononic crystals

In this article, longitudinal wave (P-wave) band structures of functionally graded viscoelastic phononic crystals (PnCs) with exponential distribution properties are analytically investigated via an exact method. Differing from the laminated model, displacement and stress vectors of normal incident P-wave are obtained exactly by utilizing a stress-transform technique and the state space method. According to the continuous condition and Bloch theory, the eigenvalue equation of band structure is derived by applying transfer matrix method. The storage modulus is considered to be frequency-dependent while the loss modulus is neglected for the consideration of components viscoelasticity. Dispersion curves, obtained from the calculation of eigenvalue equation, are validated by the results of exist laminated model and transmission spectra which are also present to reveal the propagation characteristic of normal incident P-wave. Numerical examples indicate that band width and center frequencies of band gaps can be effectively altered by changing magnitude of exponential function, ratio between initial modulus and final modulus. Other new band gaps are opened as well as wider band width are achieved with the increase of gradient index к . The exact analytical solution presented in this paper can be the benchmark for those approximation solutions.

Preparation and electromagnetic wave absorption properties of CAU-Al MOF and graphene oxide aerogel composites

The application of microwave technology has penetrated all aspects of life, but also lead to the electromagnetic pollution, therefore the research of high efficiency microwave absorption materials has been widely concerned. In this work, a new type of Al based CAU-10-H MOF was prepared by hydrothermal method. The particles were polyhedral in size of 2 [Formula: see text]m. After high temperature annealing, the interplanar spacing increased with the annealing temperature, and the lattice structure was destroyed at 400°C. CAU-Al/rGO aerogels were prepared by freeze-drying of hydrogels. With the increase of CAU-Al content and the annealing temperature, the carbon crystallinity decreases, and the pores of the aerogels became larger and fluffier. The real parts of permittivity of CAU-Al/rGO aerogels were between 5 and 30, which can be tuned by the annealing temperature and CAU-Al content. For the microwave absorption performance, the CAU-Al/rGO aerogel with a ratio of 3:1 achieved the minimum reflection loss of −37.8 dB at 3 mm. Moreover, the CAU-Al/rGO aerogels showed a wide effective bandwidth, which reached 5.1 GHz under the thickness of 2.5 mm. Therefore, the CAU-Al/rGO aerogels could be used as a kind of broadband microwave absorber with excellent microwave absorption performance.

Pyroelectric Properties and Applications of Lithium Tantalate Crystals

Lithium tantalate crystals, as a type of pyroelectric material, stand out from many other pyroelectric materials due to the advantages of high Curie temperature, large pyroelectric coefficient, high figure of merits, and environmental friendliness. Due to the pyroelectric effect caused by their spontaneous polarization, lithium tantalate crystals have broad application prospects in wide spectral bandwidth and uncooled pyroelectric detectors. This article reviews the pyroelectric properties of lithium tantalate crystals and evaluates methods for pyroelectric properties, methods for modulating pyroelectric properties, and pyroelectric detectors and their applications. The prospects of lithium tantalate thin films, doped lithium tantalate crystals, and near stoichiometric lithium tantalate crystals as response components for pyroelectric detectors are also discussed.

Laminated acoustic metamaterials for low-frequency broadband ultra-strong sound insulation

In this paper, a kind of laminated acoustic metamaterials (LAMS), which can realize the bandwidth widening and amplitude improvement through the parallel coupling of in-plane gradient parameters and the series laminated design in the thickness direction of the local resonance units, is proposed. The designed LAMS consists of two different layers of single-layer acoustic metamaterials (SLAMs) laminated in series in the thickness direction. During the design process of the two layers of SLAMs, non-uniform mass distribution and non-consistent frame coupling layouts are respectively introduced to broaden the sound insulation frequency band of the acoustic metamaterials. Through careful design, the two layers of SLAMs have distinct working frequency bands, effectively preventing the occurrence of sound insulation valleys with low amplitude. Based on numerical analysis of two layers of SLAMs, this study discusses the results of transmission loss (TL) and the working mechanism of interlayer superposition coupling sound insulation. The analysis of TL for LAMS reveals its capability to achieve ultra-strong sound insulation effects across a low-frequency broadband range. The accuracy of the numerical analysis in this study is verified through semi-analytical methods and small-scale impedance tube experiments. The LAMS proposed in this paper holds significant potential for wide-band noise reduction control in various high-noise mechanical equipment or workplaces.

Sub-megahertz compact self-pulsed Raman laser

Broadband self-pulsed Raman laser operating at sub-megahertz (sub-MHz) has been widely used in optical sensing, high-spectral-resolution lidar, frequency-division multiplexing, and optical communication. However, it is a big challenge for achieving stable self-pulsed Raman laser oscillation in a wide pump power range. Here, we construct a compact Yb:YAG/YVO4 Raman laser for generating broadband self-pulsed laser operating at high repetition rate. The effects of cavity length (LC) and Raman crystal length (LR) on the performance of self-pulsed Raman laser have been investigated. By utilizing a 1.5 mm-thick YVO4 crystal as a Raman crystal, the repetition rate increases dramatically from 62 to 280 kHz when the LC decreases from 10 to 4 mm. Meanwhile, the pulse width of 0.66 μs and peak power of 1.1 W are achieved for the self-pulsed Raman laser with LC = 4 mm. For the 4 mm-long Raman laser cavity constructed with a 2 mm-thick YVO4 crystal, the pump power range enable for self-pulsed Raman laser oscillation extends from 1.8 to 3.9 W, the repetition rate has been further increased from 258 to 467 kHz. While the peak power and pulse width are 0.6 W and 0.98 μs, respectively. The Yb:YAG/YVO4 self-pulsed Raman laser oscillates in broadband multi-longitudinal-mode, the spectral bandwidth is 27.4 nm covering from 1051.1 to 1078.5 nm. Utilization of a tilted 2 mm-thick YVO4 crystal as output coupler to construct compact Yb:YAG/YVO4 self-pulsed Raman laser dramatically expands the pump power range for generating high repetition rate self-pulsed Raman laser. Sub-MHz self-pulsed Raman lasers with a wide spectral bandwidth have potential applications in materials processing, quantum information processing, and optical communications.

THz Radar Observations of Hydrometeors in a Spray Chamber

AbstractA THz radar, with its wide bandwidth, is capable of high‐resolution imaging down to the centimeter scale. In this study, a THz radar is applied to detect hydrometeors generated in a spray chamber. The observed backscattering signals show fluctuations at centimeter scales, indicating various hydrometeor distribution patterns along the radar beam. A co‐located High‐Speed Imaging (HSI) sensor is used to measure the Drop Size Distributions (DSD) in the spray chamber. The radar sampling beam is well aligned with the HSI probes, allowing an objective comparison between the remote sensing and in situ observations. In this study, the observed radar power is compared with the power estimated from the HSI measurements. Results show great consistency, with power difference smaller than 0.5 dB. This study demonstrates the feasibility and great potential of using a THz radar for ultra‐high‐resolution observations of clouds in a laboratory facility, and in the real atmosphere.

Electromagnetic Interference Shields Based on Highly Crystalline Single-Walled Carbon Nanotubes.

Although the convenience provided by electromagnetic waves used for information exchange is increasing, the energy of unwanted electromagnetic waves unintentionally emitted from devices is increasing as the devices work with higher frequency. In view of this vulnerability, thin films as lightweight electromagnetic wave shields against noise will be necessary for information protection. We propose the fabrication of lightweight electromagnetic wave shields using highly crystalline single-walled carbon nanotubes (HC-SWCNTs), which can be made large and flexible using a method based on a wet process, utilizing the optical and conductive properties of HC-SWCNTs. Electromagnetic wave shields are mainly classified into conductive, dielectric, and magnetic absorbers. We have developed a material synthesis technology for HC-SWCNTs and attempted to form an aqueous composite film using HC-SWCNTs and an organic binder. As a result, we found that the high crystallinity of CNTs suppresses the contact resistance between CNTs and we succeeded in constructing a flexible electromagnetic wave shielding film that can absorb electromagnetic waves in a wide bandwidth equivalent or superior to that of metal foil. This thin film can be applied to curved surfaces as desired because of its wet process, and it is expected to be a lightweight shield that can be used ubiquitously.

Lossless Phase‐Change Material Enabled Wideband High‐Efficiency Spatial Light Phase Modulation at Near‐Infrared

AbstractHigh‐efficiency spatial light phase modulation with wide operating bandwidth is highly significant yet challenging. Dynamic metasurfaces leveraging active materials with tunable optical response provide a promising solution. Current work is generally confronted with restricted operation bandwidth and diminished modulation efficiency, constrained by the limited tunable range and inherent absorption of active materials particular at optical frequency. Recently, the emergence of lossless phase‐change material Sb2Se3 has garnered widespread attention. Its unique characteristics, including near‐zero absorption at near‐infrared and a substantial refractive index contrast ≈0.93 during phase transition, enable the possibility of high‐performance spatial light modulation. Pioneering studies have validated the capability of lossless phase‐change metasurfaces for wavefront control, but are typically restricted to limited efficiency. Here, a hybrid phase‐change metasurface utilizing over‐coupled resonances supported by Sb2Se3 nanoholes is proposed. For the first time in optical frequency, high‐efficiency 4‐level phase modulation covering over π range is experimentally demonstrated with a sizable operating bandwidth of 42 nm and a minimum reflectance of exceeding 0.5. Leveraging optically driven localized phase‐transition technique, dynamic beam deflection is further demonstrated. The work validates the tremendous potential of phase‐change metasurfaces in achieving advanced spatial light control, signifying significant progress for the development and application of phase‐change photonic devices.

A fast and effective approach for microstrip filter design using GA and TL-model

Microwaves and RF technology and their components like filters, antennas, etc. are commonly used in wireless networking and communication systems, wireless security systems, radar systems, and environmental remote sensing. In this paper, a fast and effective procedure has been proposed for microstrip filter design using a genetic algorithm (GA) with a transmission line model (TL). GA is modified to be highly efficient and accurate by encoding the topology. For the fixed filter topology, the electrical parameters of the filter are encoded in a single chromosome. To make the proposed procedure fast and effective, a transmission line model has been employed to compute the fitness value in GA. To demonstrate the effectiveness of the proposed procedure, a wideband second-order bandpass filter with a center frequency of 2.3 GHz is examined with a pair of short-circuited stubs and a pair of open-circuited stubs. The optimized design is validated using full-wave methods (MoM and EM simulator CST). The results show a low insertion loss of 0.1 dB and return loss better than 30 dB and a wide bandwidth of 46.95% with − 3 dB cutoff frequencies at 1.76 GHz and 2.84 GHz.

Dual-Core Photonic Crystal Fiber Polarization Beam Splitter Based on a Nematic Liquid Crystal with an Ultra-Short Length and Ultra-Wide Bandwidth

This paper presents a novel pentagonal structure dual-core photonic crystal fiber polarizing beam splitter (PS-DC-PCF PBS) filled with a nematic liquid crystal (NLC) in the central hole. Unlike previous designs with symmetric arrangements, the upper and lower halves of the structure have different air hole arrangements. The upper half consists of air holes arranged in a regular quadrilateral pattern, while the lower half features a regular hexagonal arrangement of air holes. By filling the central hole with birefringent liquid crystal, the birefringence of the structure is enhanced, reducing the coupling lengths along the x polarization and y polarization directions. The polarization properties, coupling characteristics, and the influence of different structural parameters on the extinction ratio of the polarizing beam splitter are analyzed using the full-vector finite element method. Simulation results demonstrate that the designed PS-DC-PCF PBS achieves a maximum extinction ratio (ER) of 72.94 dB with a splitting length of only 61.9 μm and a wide operating bandwidth of 423 nm (1.324–1.747 μm), covering most of the O, E, S, C, L, and U communication bands. It exhibits not only ultra-short splitting lengths and ultra-wide splitting bandwidth but also good manufacturing tolerances and anti-interference capabilities. The designed PS-DC-PCF PBS could provide crucial device support for future all-optical communication systems and has potential applications in fiber optic communication or fiber laser systems.

Broadband Quad-Element MIMO Antenna with Enhanced Isolation for Sub-6 GHz and Sub-7 GHz 5G Applications

In this manuscript, a broadband quad-element MIMO antenna with enhanced isolation for n77, n78, and n79 bands of Sub-6 GHz and the n46 and n96 bands of Sub-7 GHz under 5G applications is designed. The designed antenna element is a rectangular monopole antenna comprising a semi-circular cut and a semi-circle embedded to obtain a wide bandwidth. These four-element antennas are aligned orthogonally to achieve the compact size and high isolation features of MIMO performance. The overall area of MIMO antenna is 40×40x1.6 mm3. MIMO antenna has a high isolation of <- 21 dB, an envelope correlation coefficient (ECC<0.002), a high diversity gain (DG> 9.99), an MEG gain of -3 dB, and a peak gain of 5.08 dBi.The results of the experiment are satisfactory with the S-parameter simulation results According to the results, a broadband quad-element MIMO antenna is a suitable candidate for Sub-6 GHz and Sub-7 GHz 5G applications.

A 195–244 GHz CMOS self-mixing frequency tripler with >23 dBc fundamental rejection

This paper presents a wideband and high-fundamental rejection self-mixing frequency tripler in a 40-nm CMOS. The proposed tripler consists of an input half-shielded transformer balun, an even-harmonics generator, a single-balanced mixer, a slow-wave coupling-line filter and an output half-shielded Marchand balun. The half-shielded coupling structure improves the coupling coefficient of the coupling-line components and also features high design freedom. Consequently, the half-shielded transformer balun achieves a broadband performance. The half-shielded Marchand balun achieves a 1-dB bandwidth of 109 GHz (49.7 %) and a minimum insertion loss of 1 dB. The slow-wave coupling-line filter enhances the fundamental rejection of the proposed tripler with low insertion loss of 1.5 dB, wide 1-dB bandwidth of 162 GHz (62.8 %) and compact footprint (0.01 mm2, 0.00535 λ2). The slow-wave coupling-line filter achieves a broadband low-frequency rejection over 18.6 dB from 65 to 88 GHz. Due to the above-mentioned technology, the tripler achieves a 3-dB bandwidth of 49 GHz from 195 to 244 GHz and a fundamental rejection larger than 23 dBc. Compared with the state-of-the-arts, the proposed self-mixing tripler features wide bandwidth, high fundamental rejection and compact core area among silicon-based triplers.

Synergistic enhancing effect for electromagnetic wave absorption via melamine resin-based magnetic core-shell microspheres/multi-walled carbon nanotubes

Rational design of corrosion-resistant electromagnetic wave (EMW) absorbing materials with wide bandwidth, strong absorption, and low filling ratio remains a significant challenge. In this work, the melamine resin-based magnetic core-shell microspheres (MF@Fe3O4@SiO2) were synthesized via dispersion polymerization and in-situ growth method. Experimental and simulation results indicated that the content and magnetism of the magnetic shell coordinated with the MF-core could be tactfully regulated with a small amount of incorporated Ni content. Moreover, the dispersion and corrosion resistance of MF@Fe3O4 core-shell microspheres were obviously enhanced by coating SiO2. It is worth noting that after the composite of MF@Fe3O4@SiO2 with MWCNTs-20, the presence of numerous heterogeneous interfaces, the tactfully improved conductive loss, and the optimized impedance matching synergistically result in superior EMW absorption properties. When the filling amount was 30 wt%, F9N1-S/MWCNTs-20 with a matching thickness of 1.76 mm, exhibited the maximum effective absorption bandwidth (EABmax) of 6.55 GHz, while maintaining a reflection loss (RL) of −62.94 dB at 1.53 mm. When the thickness of F9N1-S/MWCNTs-30 was increased to 3.24 mm, the RLmin at low frequencies was achieved at −67.14 dB. Besides, the simulation of radar cross section values ascertains the enormous potential of F9N1-S/MWCNTs to achieve stealth under radar detection. This study introduces a novel method for producing lightweight and efficient EMW absorbers with corrosion resistance.

Design, Fabrication, Characterization, and Simulation of AlN-Based Piezoelectric Micromachined Ultrasonic Transducer for Sonar Imaging Applications.

To address the requirements of sonar imaging, such as high receiving sensitivity, a wide bandwidth, and a wide receiving angle, an AlN PMUT with an optimized ratio of 0.6 for the piezoelectric layer diameter to backside cavity diameter is proposed in this paper. A sample AlN PMUT is designed and fabricated with the SOI substrate-based bulk MEMS process. The characterization test result of the sample demonstrates a -6 dB bandwidth of approximately 500 kHz and a measured receiving sensitivity per unit area of 1.37 V/μPa/mm2, which significantly surpasses the performance of previously reported PMUTs. The -6 dB horizontal angles of the AlN PMUT at 300 kHz and 500 kHz are measured as 68.30° and 54.24°, respectively. To achieve an accurate prediction of its characteristics when being packaged and assembled in a receive array, numerical simulations with the consideration of film stress are conducted. The numerical result shows a maximum deviation of ±7% in the underwater receiving sensitivity across the frequency range of 200 kHz to 1000 kHz and a deviation of about 0.33% in the peak of underwater receiving sensitivity compared to the experimental data. By such good agreement, the simulation method reveals its capability of providing theoretical foundation for enhancing the uniformity of AlN PMUTs in future studies.

Design of a Neural Network Model for Point-Defect Microcavities in Two-Dimensional Silicon-Based Dielectric Column Photonic Crystals

Currently, circuits are becoming more and more highly integrated, but current electronic chip technology has difficulty in meeting the requirements for increased data transmission speed and capacity because of its characteristics of increased energy loss due to interactions between electronic components. In contrast, photonic technology is a promising solution due to its characteristics of high speed, wide bandwidth, and low interaction. Photonic crystals are materials with an artificial periodic dielectric structure, with photonic bandgap and localization properties that are critical to their performance. Photonic crystals, which use photons as an information transfer medium, allow flexible control of photon propagation, just as electrons are controlled in semiconductors, and the study of multifunctional photonic crystals is important for the construction of integrated optical circuits. This paper proposes a neural network-based model to analyze and predict the optical properties of point defect microcavities in 2D silicon-based dielectric column photonic crystals. By modeling the complex relationship between the structural parameters of the photonic crystal and its optical response, a theoretical approach for the design of 2D silicon dielectric column photonic crystals can be provided.

A metasurface-based high gain circularly polarized patch antenna with wide global bandwidth

This paper introduces a novel metasurface (MS)-based circularly polarized (CP) microstrip patch antenna. Utilizing a square microstrip patch with plus- and diagonal-shaped slots on a 1.6 mm thick FR-4 substrate as the host antenna, the design incorporates a 4 × 4 square array MS on another 1.6 mm thick FR-4 substrate, separated by a 3.4 mm thick Teflon layer. The antenna offers a −10-dB impedance bandwidth from 5.45 to 7.55 GHz and an axial ratio (AR) bandwidth between 6.02 and 6.35 GHz, achieving a maximum realized gain of 6.3 dBic at 6.25 GHz, exhibiting right-handed circular polarization (RHCP). An equivalent circuit model is developed to validate impedance response, and the fabricated antenna closely matches simulated results. With a low profile of 0.42 λ o × 0.42 λ o × 0.033 λ o at 6.25 GHz, this CP antenna is suitable for applications like satellite communication for their uplinks and WLAN application.

2D Graphene Oxide Films Expand Functionality of Photonic Chips.

On-chip integration of 2D materials with unique structures and properties endow integrated devices with new functionalities and improved performance. With high flexibility in ways to modify its properties and compatibility with integrated platforms, graphene oxide (GO) is an exceptionally attractive 2D material for hybrid integrated photonic chips. Here, by harnessing unique property changes induced by photothermal effects in 2D GO films, novel functionalities beyond the capability of photonic integrated circuits are demonstrated. These include all-optical control and tuning, optical power limiting, and nonreciprocal light transmission. The 2D layered GO films are integrated onto photonic chips with precise control of their thickness and size. Benefitting from the broadband optical response of 2D GO films, all three functionalities feature a very wide operational optical bandwidth. By fitting the experimental results with theory, the changes in GO film properties induced by the photothermal effects are analyzed, revealing interesting insights about the physics of 2D GO films. These results highlight the versatility of 2D GO films in implementing new functions for integrated photonic devices for a wide range of applications.

Carbon nanotubes-encapsulated Co/Co7Fe3 nanocomposites: Achieving wideband electromagnetic wave absorption at ultrathin-thickness by regulating magnetic phase ratio

Nowadays, through various strategies such as composition regulation and structural design, the electromagnetic wave (EMW) attenuation ability of composites has been significantly improved. Nevertheless, at ultra-thin sample matching thicknesses, it is still a challenge for absorbing materials to possess high-intensity and wide bandwidth simultaneously. In this work, the cobalt/iron nitrilotriacetic acid chelates (Co/Fe-NAC) were fabricated by a one-step hydrothermal self-assembly strategy. Then, a series of carbon nanotubes-encapsulated Co/Co7Fe3 (Co/Co7Fe3@CNTs) nanocomposites were constructed by coating glucose hydrothermal carbon on Co/Fe-NAC precursors and subsequent calcination process. By controlling the Co/Fe ion content, the magnetic phase ratio in the product can be efficiently regulated to comprehensively optimize microwave absorption (MA) performance. The minimum reflection loss (RL) of the optimized sample is −42.6 dB at only 1.1 mm thickness, and the effective absorption bandwidth (EAB) reaches up to 10.1 GHz (7.9–18 GHz). Surprisingly, for all samples, the widest EAB can exceed 8.2 GHz at the ultra-thin thickness (thinner than 1.3 mm) and the strongest RL values are all below −30 dB (99.9 % EMW can be absorbed). Such excellent MA performance is attributed to the strong magnetic loss of highly dispersed magnetic nanoparticles, the strong dielectric properties of carbon nanotubes and shells, optimized impedance matching, as well as the synergistic effect of multi-dimensional heterogeneous components and structures. Moreover, the high-frequency structure simulator (HFSS) was used to further analyze the multi-polarization behaviors and heterogeneous magnetic coupling effect. This work may provide a new direction to the fabrication of efficient and ultra-thin absorbers, and Co/Co7Fe3@CNTs nanocomposites may become a promising candidate for practical application in the MA field.

High‐gain dual‐wideband ±45° dual polarization (DWDP) antenna for indoor distributed antenna system (IDAS) applications

SummaryThis paper presents two different configurations of ±45° dual‐wideband dual‐polarized (DWDP) antennas. The antenna comprises eight end‐loaded broadband dipoles grouped into two sets based on the bands of operation. The high‐frequency dipoles are strategically placed within the low‐frequency ones to form the elementary unit. Further optimization has been performed on the elementary unit in terms of the radiator placement to suppress sidelobe levels for array formation. Both configurations are meticulously fabricated using an all‐metal structure, demonstrating durability and the simplicity of fabrication. The antenna designs achieve ±45° polarization, high gain, satisfactory cross‐polarization discrimination (XPD), and wide impedance bandwidth covering 0.67–1.04 GHz and 1.54–3 GHz bands. Despite the nested configuration, mutual coupling is well within acceptable limits. Formation of the array based on the optimum elementary unit has been investigated by simulation, revealing reasonable gain enhancement and low sidelobe levels across all bands. This comprehensive approach yields a robust, high‐performing antenna configuration suitable for indoor distributed antenna system (IDAS) applications.

Performance parameters estimation of high speed Silicon/Germanium/InGaAsP avalanche photodiodes wide bandwidth capability in ultra high speed optical communication system

Abstract This paper has clarified the performance parameters estimation of high speed silicon/germanium/InGaAsP avalanche photodiodes wide bandwidth capability in ultrahigh speed optical communication system. The basic structure configuration of avalanche photodiode is clarified. The basic depletion region configuration schematic view is demonstrated. As well as the maximum electric field is indicated for impact ionization through avalanche photodiode in the presence of load. Various avalanche photo-detectors multiplication factor is clarified versus reverse bias voltage at room temperature. Different avalanche photo-detectors dark current is measured against avalanche photo-detectors volume at room temperature. Various avalanche photo-detectors effective doping concentration is demonstrated against ambient temperature variations. Si/Ge/InGaAsP avalanche photo-detectors excess noise factor is demonstrated versus the ionization ratio at room temperature (T = 300 K) and different ambient temperature (T = 350 K and T = 400 K). Various avalanche photo-detectors excess noise factor is measured clearly versus the multiplication factor at room temperature. Si/Ge/InGaAsP avalanche photo-detectors excess noise factor is demonstrated versus reverse bias voltage at room temperature. Si/Ge/InGaAsP avalanche photo-detectors noise equivalent power is clarified versus multiplication factor at room temperature. Various avalanche photo-detectors noise equivalent power is studied versus temperature variations. Si/Ge/InGaAsP avalanche photo-detectors sensitivity is measured in relation to the temperature variations. Different avalanche photo-detectors sensitivity is demonstrated in relation to reverse bias voltage variations at room temperature.