Search

Gallium nitride (GaN) high-electron-mobility transistor (HEMT) devices, which have wide application potential from power amplifiers to satellite, need to be thoroughly examined in terms of reliability in order to benefit the superior intrinsic properties of the device. The most critical parameter in the device reliability is the hotspot, or Tmax , which occurs somewhere on the subsurface and along the channel of the GaN HEMT, which is optically inaccessible due to optical path disability. Therefore, the Tmax value is underestimated in optical measurements, such as the thermographic IR and Raman methods. With 3-D electrothermal simulations, Tmax is obtained close to reality, but it requires a huge computation load and the complex modeling of semiconductor device physics. In 2-D or 3-D thermal simulations that do not use electrothermal simulations, since the self-heating is mostly modeled with a single heat source, neither the correct Tmax value is obtained nor the effect of bias conditions is considered. To address the aforementioned shortcomings, a hybrid method is demonstrated, which exploits the electrical measurements of GaN HEMT, which RF and reliability engineers often and easily do. It is demonstrated that Tmax can be determined quickly and close to the electrothermal simulations in a GaN HEMT device with a two-heat source method and finite element analysis (FEA) hybrid interaction with respect to various bias conditions. Moreover, the impact of the knee voltage is investigated with different knee-detection techniques. The proposed method provides GaN HEMT reliability engineers with an easy-to-implement alternative to reveal the hotspot location and the value.

This paper presents a method to reveal the channel temperature profile of high electron mobility transistors (HEMTs) in a multi-stage monolithic microwave integrated circuit (MMIC). The device used for this study is a two-stage X-band low-noise amplifier fabricated using 0.15 m GaN-on-SiC technology with 4x50 m and 4x75 m HEMTs at the first and the second stage, respectively. The surface temperature measured through infrared (IR) thermography has a diffraction-limited resolution. Moreover, it is impossible to measure sub-surface Tmax residing inside the two-dimensional electron gas of HEMT using IR thermographic measurements. Finite element analysis (FEA) thermal simulations are performed in this study to acquire the surface and sub-surface temperature profiles of the whole MMIC. IR measurements and FEA simulations are integrated through a correlation-based method verifying the accuracy of the FEA-based temperature profiles. This method leads to accurately finding the hotspots in the MMIC, thus revealing the Tmax of both stages. The correlation method using two filters approach to match the measurements and simulated temperature profiles of all the stages finds its application in MMICs’ high-temperature operating lifetime reliability tests.

High electron mobility transistors (HEMTs) based on gallium nitride (GaN) with a wide range of application potentials need to be rigorously examined for reliability to take advantage of their intrinsically extraordinary properties. The most vital parameter of the reliability, the hotspot, or Tmax, resides in the two-dimensional electron gas (2DEG) temperature profile inside the device where optical access is often restricted. The device surface temperature can be measured by widespread IR thermography with the limitation of diffraction-based IR transmission losses. However, Tmax on the sub-surface cannot be reached thermographically. Although finite element analysis (FEA)-based thermal simulations can easily reveal the 2DEG temperature profile, accuracy is tightly dependent on the realistic modeling of material/structure parameters. Because these parameters are rather sensitive to fabrication and processing, it is quite difficult to specify them accurately. To overcome these drawbacks, a method integrating both IR thermography and FEA thermal analysis is demonstrated on a fabricated high-power 40 × 360 μm packaged GaN HEMT as a proof-of-concept. Utilizing the simulation and measurement temperature profiles, a correlation algorithm is developed so that accuracy of the FEA thermal simulation is improved by calibrating the parameters specific to fabrication/process conditions by thermographic measurement. Then, it is quantitatively shown that the proposed method is able to find the 2DEG temperature profile and Tmax with an accuracy that best suits the intrinsic and extrinsic characteristics of the device under test. The method sheds light on GaN reliability engineering by providing a feasible and reliable alternative to realistically reveal hotspot information for device lifetime assessments.

Passive radiative cooling (PasRadCool), which emits thermal energy from objects to deep cold space through atmospheric transparency, offers complementary and alternative green energy solutions for passive cooling of buildings, clothing, and renewable energy harvesting. Depending on the spectral emissive/absorptive properties of the unit under test (UUT), radiative heat exchanges occur between the UUT, atmosphere, and sun, while at the same time non-radiative heat exchange occurs. The performance of the PasRadCool is determined by the combined thermal and thermodynamic effects of both exchange mechanisms. Although the non-radiative heat exchange, which consists of conductive and convective processes to the outer surfaces of the UUT and the surrounding air fluid, is very sensitive to environmental changes, the actual performance is not fully determined since this feature is considered statically in many studies. Herein, we propose a method that reveals the non-radiative thermal characteristics of the PasRadCool under real operating conditions. With a photonic radiative cooler structure, which we manufacture as a proof of concept, we perform nighttime field test measurements in varying non-radiative thermal conditions. The proposed method extracts the time-dependent non-radiative heat transfer coefficient of the UUT as accurately as possible. We also confirm that our experimental result shows good agreement with both numerical and analytical methods. The proposed approach, which highlights the realistic thermal management of PasRadCool, is not specific to the circumstances of our study and can be applied to all PasRadCool situations with different geometry, material, and environmental conditions.

In wireless communication systems, angular positioning is an important aspect affecting the behavior of an antenna reception performance. In this study, a broadband, high gain rectangular microstrip patch antenna using planar-patterned metamaterial concept and its measured angular power reception performance are presented. The metamaterial antenna’s −10 dB bandwidth is achieved as 2.42 GHz in the frequency range of 2.89–5.31 GHz with the peak gain of 5.56 dBi. Angular power reception performance of the proposed antenna is measured for different selected intervals in indoor and outdoor environments with the help of Software Defined Radio. The antenna is orientated vertically/horizontally and rotated in the azimuth O plane from 0° to 360° with angular resolution of ΔO = 45°. The measured received power distributions are shown at angle versus distance and compared.

Due to several advantages over conventional devices for the control of electromagnetic (EM) radiation, the demand for metasurface utilization based on artificially engineered micro and nanostructures is boosted, especially in new generation devices. Among the metasurfaces family, there has been a growing interest in Huygens’ metasurfaces that are easy to fabricate due to their lower aspect ratio compared to their counterparts and also provide alternative electromagnetic radiation control by tuning the dipolar electric and magnetic resonances. In this study, an all-dielectric Huygens’ metasurface consisting of the high-refractive-index nano shells embedded in the low-refractive-index environment is designed and extensively investigated numerically and analytically in the near-infrared spectrum. By simply tuning the nano shell inner radius, the effects on the dipolar resonances are unveiled specific to the proposed design. To assess the EM wave interactions in the designed Huygens’ metasurface, an analytical model based on the coupled discrete dipole approach is applied for selected distinct cases of the designed metasurface. It is shown that the spectral position of the dipolar resonances can be detuned or tuned simultaneously depending on the structural parameter of the meta-atoms arranged in a periodic array. This study sheds light on the physics and abilities of the nano shell structure as a Huygens’ metasurface for the potential applications of metasurface-based light–matter interaction including imaging and sensing.

Beam splitters are an indispensable part of optical measurements and applications. We propose a dynamic beam splitter incorporating all-dielectric metasurface in an elastic substrate under external mechanical stimulus of stretching. The optical behavior at 720 nm wavelength shows that it can be changed from a pure optical-diode-like behavior to a dynamic beam splitter. Although the structure is designed running at 720 nm, the design approach with appropriate materials can be used at any wavelength. Various cases, including wavelength and polarization dependencies, are thoroughly investigated to demonstrate the principles of operating conditions of two different regimes of the designed metasurface. (C) 2020 Optical Society of America

Günümüzde kullanılan ve en iyi bilinen gözetim ve çevre koruma sistemleri, çitler, duvarlar ve kameralar gibi geleneksel araçlarla birlikte kişi odaklı hizmetlerle sağlanmaktadır. Ancak, geleneksel teknikler her zaman etkili bir şekilde tehditleri tespit edemez. Üstelik, çeşitli sensörlerden faydalanan çitler reaksiyon süresi açısından meydana gelen sızmalara karşı zayıf kalabilmektedir. Bu çalışmada, yeterli reaksiyon süresi sağlayıp tehditleri önleyebilmek maksadıyla, laboratuvar şartlarında yazılım tabanlı telsiz destekli elektromanyetik çit uygulaması önerilmiştir. Önerilen çit, elektromanyetik girişimlere ve kasıtlı karıştırmalara karşı frekans değiştirmeye imkân sağlayan geniş bant metamalzeme antenler kullanılarak oluşturulmuştur. Deneysel sonuçlara göre, önerilen çit sistemi sınır hattı boyunca veya ötesindeki hareketliliği tespit etme amacıyla kullanılabileceği öngörülmektedir.

Recently, utilization of mobile robots has increased substantially. Accordingly, wireless communication is preferred in mobile robots. QBot 2 is an autonomous ground robot which is a new product of Quarc. The QBot 2 utilizes an onboard data acquisition card and a wireless embedded computer to measure the onboard sensors and drive motors. In this study, QBot 2 mobile robot is evaluated in terms of its ability to maneuver. In this way, controlling QBot 2 mobile robot in real time is planned while a command recognition system is developed. Connection with the QBot 2 is carried out in a wireless environment. A Simulink model is developed in MATLAB® environment. The created model is built with Quarc control software. Compiled model is downloaded with TCP/IP connection to QBot 2 and the application is carried out on an embedded computer. The QBot 2 mobile platform consists of two central drive wheels mounted on a common axis. This drive configuration is known as differential drive. The two drive wheels are independently driven forward and backward in order to actuate the robot. Motion of the wheels is realized using high performance DC motors. When the results are analyzed, 13 different motion types are observed in total. The observed motion types could be used as references in future works since many practical applications, such as the remote control of QBot 2 mobile robot via the human voice, require the availability of different motion types.

In this work, we present a high gain, broad bandwidth patch antenna which operates at 2.89–5.31 GHz using planar patterned metamaterial concept and its power reception performance. Ground plane is scraped with crossed strip-line gaps while the patch comprises separated micro triangular periodic gaps. The patterned metal patch and ground plane form a coupled capacitive-inductive circuit of negative index metamaterial. Moreover, metamaterial antenna's experimental and theoretical power reception performances are evaluated for different selected intervals and compared by using Software Defined Radio.

In this paper, a broad bandwidth, high gain rectangular patch antenna using planar-patterned metamaterial concept and its power reception performance are discussed. Top patch consists of separated micro triangular periodic gaps while the ground plane is etched with crossed strip-line gaps. The patterned metal patch and ground plane form a coupled capacitive-inductive circuit of negative index metamaterial. In addition, experimental and theoretical power reception performance of the metamaterial antenna for different selected intervals are compared by using Software Defined Radio.

Eye tracking in human-computer interaction is a situation often encountered in recent years. Pupil and eye gaze detection is used in health problems, security systems, ergonomic design, psychological analysis, market research, military areas and control applications. In this study, A realtime pupil-corneal reflection tracking with free head position is made by using Labview and Labview Vision Development Module. Literature in pursuit eye racking, we see that many different methods and algorithms used. Many of them use circle and ellipse detection algorithms. This is the long processing time of algorithms is one of the difficulties. With the proposed method in this study, pupil-corneal reflection is detected in 11 ms time at free head position.

Infrared (IR) sensors are intensively used in the thermal image processing applications. IR sensors have dominant effects on the performance enhancement of a thermal imaging system. If performance characteristics of IR sensors are known prior to the costly and time-consuming production phase, high performance IR sensors could be obtained rapidly and cost effectively. In this study, performance characteristics of noise and specific detectivity of HgCdTe IR sensor are evaluated at 77 K sensor operating temperature using a numerical simulation method. These characteristics can not be determined with analytical methods. Results of the study showed that the best detectivity was obtained when cut-off wavelength is 9 µm and the bias voltage is −10 mV. It is also clarified that the degradation of the detectivity is caused by the 1/f noise originating from the trap-assisted tunneling (TAT)