Theses supervised by Prof. Dr. Ekmel Özbay
46 theses · İhsan Doğramacı Bilkent University
EM-tabanlı ekstrinsic parazitikleri çıkarılmış HEMT modeli kullanarak C– ve X–bandı uygulamaları için GaN-on-SiC geniş bant sürücü yükselteci tasarımı
Gallium Nitride (GaN) high electron mobility transistor (HEMT) technology is known for its high-power applications due to its wide band-gap and high electron mobility characteristics, which allow the use of larger voltages and currents in the circuit. This enables the design to output a higher RF power for the same gate periphery when compared to other technologies. Broadband driver amplifiers (DAs) operating in the C– and X–bands are commonly used for satellite communications, phased-array radar systems, and, signal generators. Designing an amplifier that achieves both, good small-signal and large-signal performance for such fractional bandwidths remains a challenge today. In this thesis, we present a GaN-on-SiC based broadband DA monolithic microwave integrated circuit (MMIC) operating at 5-12 GHz. The 3-stage MMIC exhibits a typical small signal gain of 29.7 dB with a ± 1.4 dB gain ripple and input and output reflection losses better than 10.5 dB and 8.5 dB, respectively. The average saturated output power is 2.65 W, with an average power density of 3.31 W/mm. The DA occupies a chip area of 2.7 x 3.2 mm². EM-based cold-FET technique is used to extract the extrinsic parasitic parameters (EPPs) for a typical 6×100 µm HEMT. Various HEMT structures are EM simulated discretely to obtain the scalability rules of the EPPs used for the HEMT model. The DA design is verified using the Process Design Kit (PDK) HEMT models, and the two sets of simulations are compared with the measurement results.
Simplex kodlanmış OTDR-tabanlı dağıtık sıcaklık algılama
Temperature sensing for environmental and structural monitoring is of utmost importance for certain applications, e.g., monitoring of wildfires, gas pipelines, electric lines, temperature of facilities such as nuclear reactors, etc. Conventional devices to measure temperature along many kilometers are not cost-effective, and are impractical to install each measurement device. The solution heralded was to utilize the back-scattered Raman signal inside the optical fiber to measure the temperature, as the Raman scattering is a manifestation of the interaction of light with molecular vibrations (optical phonons). Each scattering point along the fiber serves as an individual sensing element, and for that reason, this technology is called 'Distributed Temperature Sensing' (DTS). However, the problem with Raman OTDR-based DTS originates due to the Raman signal being weak as much as 70dB below the launched optical power, resulting in a low signal-to-noise ratio (SNR). Increasing optical power is not viable because of optical nonlinearities. To overcome this problem, advanced noise filtering techniques and interrogation methods can be used. An interrogation method named Simplex coding is widely used in the literature, which involves the modulation of the amplitude of the laser, based on a predetermined code word. In this way, more than one pulse can be present during each interrogation, which then suppresses the uncorrelated additive white Gaussian noise. Optimizing the SNR enables longer sensing distances and higher temperature resolution performance in Raman OTDR-based DTS systems. In this thesis, first, a commonly used Wavelet-transformation-based denoising is experimentally demonstrated in a multi-mode fiber equipped RDTS system. The method has improved the temperature resolution from 0.45◦C to 0.10◦C at 5.5km. Finally, 7-bit Simplex coding is experimentally demonstrated in a single-mode fiber equipped RDTS system. The Simplex coding method has improved the temperature resolution from 1.3 ◦C to 0.8 ◦C at 5km.
Alıcı-verici uygulamaları için GaN yükselteclerinin tasarımı ve geliştirilmesi
A transceiver, commonly known as a T/R module, is widely used in phased array radar systems for military, space, and commercial applications. Hybrid microwave integrated circuits and low-temperature co-fired ceramic technologies offer bulky solutions. However, advancements in GaN HEMT-based monolithic microwave integrated circuit (MMIC) technology have led to low-cost, compact, and efficient alternatives over the years. A microwave transceiver consists of a low noise amplifier (LNA), a high power amplifier (HPA), and a single-pole double-throw switch. The current thesis aims on indigenous design and development of LNA and HPA for an MMIC transceiver at X-band using NANOTAM's in-house 0.15/0.25 µm AlGaN/GaN fabrication process. Robustness, including survivability and reverse recovery time (RRT), and limiting output power are the primary focus of LNAs besides gain and noise figure (NF). The single-stage design based on cascode HEMT and three-stage designs based on source degenerated HEMTs are discussed in the frequency range of 8 to 12 GHz. It is shown that cascode HEMTs-based LNA have high survivability but at the cost of increased NF. In a three-stage design, RRT is studied as a function of the pulse width of the incoming signal. This LNA has the best combination of NF and survivability, surviving 42 dBm input power with sub-1.5 dB NF and having 23.2 dB gain with ±1 dB gain ripple. The output saturated power of the LNA is limited to 20 dBm using a novel un-gated HEMTs approach for easy integration with subsequent circuitry. Moreover, the design of a three-stage high power amplifier operating in the 8.5 to 11 GHz is discussed in detail. Two design approaches are described, and it is shown that reducing the power dissipation by reducing HEMT peripheries in the first and intermediate stages improves power added efficiency (PAE). The achieved PAE is higher than 43 % with more than 43 dBm output power in a compact size of 2.4 × 3.75 mm².
Silisyum karbür üzerinde galyum nitrat yüksek elektron hareketliliğine sahip transistörlerin (gan-on-sic hemt) güvenilirlik ölçümleri ve analizi
Gallium nitride (GaN) high electron mobility transistors (HEMTs) on semiinsulating SiC (GaN-on-SiC HEMT) have become the leading technology for high-power and high-frequency applications, yet their reliability is still under development. In particular, off-state operation under deep reverse bias is dominated by field-driven failure mechanisms such as impact ionization, hot-electron trapping, and gate and buffer leakage, which manifest as increased leakage currents, current collapse, and time-dependent breakdown. This thesis focuses on understanding and improving the off-state robustness of GaN-on-SiC HEMTs by systematically engineering the device layout, fabrication processes, and epitaxial design, using long-term off-state stress and leakage behavior as the primary evaluation criteria. After introducing GaN material properties, device structure, and relevant failure mechanisms, the thesis first establishes a baseline by characterizing the RFHTOL (RF-biased High Temperature Operating Life) behavior of the initial technology. Catastrophic failures under realistic RF stress motivated a series of geometric optimizations. By rearranging the lateral layout, in particular increasing the gate–drain spacing and rearranging the field-plate and drain pad connections, the breakdown voltage was significantly increased while maintaining essentially the same RF output power and efficiency. Next, fabrication-related reliability bottlenecks were addressed. Post-metal annealing and improved gate-foot etch resolved severe Q-point instability by restoring a true Schottky gate and eliminating residual SiN under the gate. An in-situ O2/Ar ion pre-gate treatment and a high-temperature PECVD first nitride layer were then introduced, reducing gate and drain leakage by more than an order of magnitude and virtually eliminating catastrophic HTRB failures up to 200 ◦C. These improvements were validated by DC-HTOL, HTRB, RF-HTOL, and step-HTRB tests. Finally, the thesis investigates three epitaxial engineering routes targeting electric field redistribution and buffer-related trapping. A detailed root-cause analysis of gate degradation under HTRB test, combining TEM, elemental analysis, and electrical data, show that an ultra-thin oxide layer beneath the gate enhance the impact ionization at the drain-side gate edge. A dedicated buffer-engineering as thinning the c-GaN and increasing carbon concentration via growth condition control redistributes the vertical field, and reduces the active impact-ionization volume. That results in a reduction of final gate and drain leakage after longterm HTRB. Building on this, two epitaxial concepts are also developed. An AlGaN/GaN/AlGaN back-barrier design achieves roughly 30% higher breakdown voltage and more than a two-fold increase in time-to-failure under HTRB, with preserved RF performance and strongly reduced dynamic dispersion. A novel Fe-free ultra-thin buffer architecture with only 200 nm total GaN thickness further improves breakdown voltage by approximately 78%, increases RF output power and transconductance, and yields an order-of-magnitude reduction in off-state leakage, with zero failures observed in HTRB over the investigated stress window. TCAD simulations and TEM analysis consistently show that these epitaxial concepts relax the peak electric field at the gate edge. Overall, the thesis demonstrates that off-state reliability of GaN-on-SiC HEMTs can be substantially improved without sacrificing RF performance by coordinated optimization of layout, process, and epitaxy, guided by field distribution and leakage-based reliability metrics.
Fiber optik gecikme hattı tabanlı 10 ghz faz gürültüsü ölçüm sistemi geliştirilmesi
Microwave photonics is an emerging field of study exploiting broadband, low loss photonics technology for high spectral purity microwave generation, processing and distribution. Fiber optical delay lines are such systems employed successfully for generation and phase noise analysis of microwave signals with high spectral purity. Low loss and wide bandwidth of the fiber optical delay line permits much larger delays to be realized at a reasonable loss at microwave frequencies. In this study, fiber optical delay line based frequency discriminator phase noise measurement system is designed and implemented to resolve ultra low phase noise spectra of optoelectronic oscillators. System design is described in detail including fiber optical and microwave component characterizations, selection criteria, system stabilization against environmental fluctuations and system calibration. Phase noise measurements for various RF synthesizers available in the laboratory are conducted with the developed system and compared to spectrum analyzer phase noise measurements to validate system calibration. Finally, phase noise spectra of optoelectronic oscillators (OEO) with 1 km and 2 km delay elements are demonstrated with the developed system. With 2 km OEO, system can resolve phase noise spectra as low as −140 dBc=Hz at 10 kHz offset from 10 GHz carrier frequency.
X-bant GaN tabanlı mikroşerit MMIC güç yükselteci
RF power amplifiers are crucial components of modern radar and communication systems. However, their design poses some challenges due to device limitations in high power and high frequency regime, as well as inherent difficulties of designing for nonlinear large-signal device operation. Gallium Nitride (GaN) based High Electron Mobility Transistors (HEMT) are promising candidates due to their superior material qualities, high power densities and ability to operate up to mm-wave frequencies. In this thesis, 0.25 μm GaN on SiC microfabrication process of Bilkent University Nanotechnology Research Center (NANOTAM) is presented. Transistor characterization procedure is demonstrated. Ideal transistor layout for design goals is selected and the transistor gate structure is optimized for X-band performance. A model library for microstrip passive circuit elements based on electromagnetic simulations has been developed. Finally, design and measurements of an X-band microstrip Class AB two-stage Monolithic Microwave Integrated Circuit (MMIC) PA, based on the same process are presented in detail. With die sizes smaller than 4.3 mm by 2.3 mm, fabricated MMICs operate at 8.5 - 11.5 GHz band with 24 dB small-signal gain. More than 13.5 W (41.3 dBm) output power (P6dB) and 31% - 38% power-added efficiency are achieved throughout the 8.5 - 11 GHz band in pulsed mode on-wafer measurements.
S-bant gan tabanlı düşük gürültülü yükselteç tasarımı ve karakterizasyonu
Low Noise Amplifiers (LNA) are widely preferred components in receiver frontend modules. The received signal level is generally very low and amplifying it without adding too much noise is very crucial in communication systems. In this thesis study design, fabrication and test of three Gallium Nitride (GaN) High Electron Mobility Transistor (HEMT) based Monolithic Microwave Circuit (MMIC) LNAs are presented. Inductive source feedback topology is used to obtain both better input return loss and noise figure. All three designs achieve higher than 20 dB gain, better than 10 dB input return loss and their noise figure values are 2 dB, 1.5 dB and 1 dB in S-band. High resistive gate biasing is utilized at third design to increase input power handling. Size reduction is very important in MMIC technology. The first design is 3 x 5 mm and the second design is 2 x 3.5 mm, % 46 size reduction is achieved. In GaN technology controlling SiN layer thickness is very problematic and this fabrication step affects capacitor values. The second and third LNA designs presented in this research, matching circuitries and implicitly overall characteristics are not influenced too much by a change of capacitor values. Targeted bandwidth is 2.7-3.5 GHz, achieved frequency range is 1.5 GHz (from 2.5 GHz to 4 GHz). The three LNA designs have 28.1 dBm, 33.4 dBm, and 35.9 dBm output third-order intercept point respectively. Output powers at 1-dB compression points are 18.2 dBm, 23.4 dBm and 25.9 dBm. For all three LNA designs, group delay is less than 0.3 nanoseconds.
S-bant galyum nitrür yüksek güçlü yükselteç tasarımı ve uygulaması
High power RF Microwave amplifiers are becoming more important as the telecommunications, defense and aerospace industries' demands develop. GaN on SiC technology offers higher power and better form factors for these applications compared to GaAs. In addition, SiC provides better mechanical properties and thermal performance. Design, manufacturing and measurements of a S-Band Power Amplifier by using a GaN discrete bare die transistor are presented. GaN on SiC technology, fabrication process, amplifier fundamentals and design steps are explained in detail. PCB laminate properties, manufacturing, wire bonding and importance of heat management are explained. Design, tapeout, characterization of a fabricated HEMT and its packaging are also mentioned. Power amplifier's small-signal gain of 14.5 dB is measured at center frequency. 41.5 dBm RF power at P6dB is measured at 200 µs pulse width 10% duty cycle at 3 GHz, reaching a power density of 5.4W/mm. Small-signal gain, IP3 measurements under different biases, AMAM and AM-PM distortions are also investigated in detail. EM simulations are performed in Keysight ADS design environment. Amplifier design is based on small-signal and loadpull measurements. De-embedding of fixture effects during HEMT characterization and their models are also investigated. Another hybrid amplifier design by using a packaged commercial GaN on SiC bare die power HEMT is also presented. Small-signal and power measurements are also offered.
Genişbant emiciler, ayarlanabilir renk filtreleri, ve çok işlevlimetayüzeyler olarak metamalzemeler
Metamaterials have enabled us to come up with artificial structures and designs that can perform optical functionalities which are not achievable with natural materials. Here we design and implement three important applications of metamaterials as: 1. Ultra-broadband absorbers, 2. Real-time tunable color filters, and 3. Wideband and wide-angle efficient beam deflector and Multi-functional angular filter. By lithography being a major hinder on the way to mass production and cost-effectiveness, most of our works are lithography-free. We have introduced Manganese (Mn) for the first time as a very promising metal for broadband absorption and have used it in all our works. Four different Mn-based broadband absorbers are designed and fabricated in different chapters. Mn is used in the Metal-Insulator-Metal (MIM) cavity, annealed MIM configuration, top-layer-patterned MIM configuration, and random nanopyramids. It is shown in all works that Mn has a much better performance compared to other metals. For instance, in the work based on random nanopyramids, we obtain ultraviolet (UV) to far-infrared (FIR) perfect absorption by exploiting a lithography-free method and only by coating a single Mn layer on a high-roughness substrate. Moreover, using the combination of the MIM cavity and an electro-optic material, we have shown that a lithography-free color filter can be achieved that covers the whole visible spectrum by changing the voltage from -12 to 12 volts. Finally, in a structure composed of Silicon nano-rods, an ultra-wideband and wide-angle highly-efficient beam deflection is obtained. What makes the same structure very promising is that it also has multifunctional applications as band-pass, band-stop, and low-pass angular filter. Keywords: Broadband absorbers, Manganese, electrically tunable, beam deflection, and angular filtering.
Normalde kapalı YEMT aygıtların tasarım, fabrikasyon ve karakterizasyonu
GaN-based high-electron-mobility transistors (HEMTs) have been developing rapidly from the time when they were first demonstrated in the 1990s. They have consistently been presented as a displacement technology to silicon based power devices owing to the superior material properties of GaN such as high-electric breakdown field, high-electron saturation velocity, and high mobility. Normally-off GaN HEMT devices are particularly significant in power electronics applications. In this thesis, a comprehensive study of normally-off high-electron-mobility transistors is presented, including theoretical background review, theoretical analysis, physically-based device simulations, device fabrication and optimization and electrical characterization. p-GaN gate InAlN/GaN HEMT and recessed AlGaN/GaN MISHEMT devices have been successfully demonstrated.
Uzay aracı pasif radiatif soğutucu, kolorimetrik kırıcılık indisi sensörü, renk filtresi, ve komşu bantlarda geçirgen tek taraflı mükemmel soğurucu/ yansıtıcılar olarak dielektrik metayüzeyler
Metamaterials define the class of synthetic, man-made materials with exotic properties that cannot be observed with natural materials. Their sub-wavelength counterparts are called metasurfaces. In particular, dielectric metasurfaces are extensively studied due to the advantages they offer in comparison to metamaterials, which are mainly their reduced thickness and not suffering from ohmic losses that are present in metals. Here, we design and implement dielectric metasurfaces in four important application areas, namely 1. Passive radiative coolers for spacecraft, 2. Colorimetric refractive index sensors, and 3. Color filters based on monolayer graphene. 4. A metasurface with a resonant one-way absorption/reflection with transmissive sidebands functionality In the first work, we propose a facile, lithography-free fabrication route, exploiting oblique deposition to design an optical solar reflector, which constitutes the physical interface between the spacecraft and space. Our proposed metasurface is based on disordered and densely packed Indium Tin Oxide (ITO) nanorod forests. The excellent light trapping capability of the nanorod forests, randomness in the geometrical dimensions of these nanorods, combined with the lossy plasmonic nature of ITO in the thermal-infrared range led to strong coupling of thermal-radiation to broad plasmonic resonances, and consequently an experimental emissivity of 0.968, in a very wide range from 2.5 µm to 25 µm. In the solar spectrum, low-loss dielectric characteristic of ITO resulted in an experimental solar absorptivity as small as 0.168. This design with high-throughput, robustness, low-cost and high-performance, therefore, shows great promise not only for space missions but also for promoting environmentally friendly passive radiative cooling for our planet and thermal imaging in the field of security labeling. In the second work, we propose a highly-sensitive refractive index sensor, utilizing the excitation of guided-modes of a novel, 2-dimensional periodically modulated dielectric grating-waveguide structure. The optimized nanosensor can numerically excite guided-mode resonances with an ultra-narrow linewidth (full-width at half-maximum) of 0.58 nm. Sensitivity is numerically investigated by considering the deposition of dielectric layers on the structure. For a layer thickness of 30 nm, the maximum sensitivity reached as high as 110 nm/refractive index unit (RIU), resulting in a very high Figure of Merit of 190. The fabricated devices with 30 nm Aluminum Oxide and Zinc Oxide coatings achieved a maximum sensitivity of 235.2 nm/RIU with a linewidth of 19 nm. Colorimetric detection with polarization-insensitivity is confirmed practically by a simple optical microscope. Samples with different coatings have been observed to have clearly distinct colors, while the color of each sample is nearly identical upon azimuthal rotation. Excellent agreement is obtained between the numerical and experimental results regarding the spectral position of the resonances and sensitivity. The proposed device is, therefore, highly promising in efficient, highly-sensitive, almost lossless, and compact molecular diagnostics platform in the fields of biomedicine with personalized, label-free, early point-of-care diagnosis and field analysis, drug detection, and environmental monitoring. In the third work, we numerically propose a graphene perfect absorber that can be utilized as a color filter, utilizing the excitation of guided-modes of a dielectric slab waveguide by a novel sub-wavelength dielectric grating structure. When the guided-mode resonance is critically coupled to the graphene, we obtain perfect absorption with an ultra-narrow bandwidth (full-width at half-maximum) of 0.8 nm. The proposed design not only preserves the spectral position of the resonance, but also maintains $>98\% $ absorption at all polarization angles. The spectral position of the resonance can be tuned as much as 400 nm in visible and near-infrared regimes by tailoring geometrical parameters. The proposed device has great potential in efficient, tunable, ultra-sensitive, compact and easy-to-fabricate advanced photodetectors and color selective notch filters. In the fourth and final work, we numerically propose the one-way perfect absorption of near-infrared (NIR) radiation in a tunable spectral range with high transmission in the neighboring spectral ranges. This functionality is obtained by using a 2-dimensional, guided-mode resonance based grating-waveguide metasurface that acts as a frequency-selective reflector, a spacer dielectric, and an absorbing oxide layer. Within the bandwidth of the excited guided-mode resonance excited at 1.82 µm with a full-width at half-maximum of 19 nm), we confirmed perfect absorption when light is incident from one of the two opposite directions, whereas in the other direction, perfect reflection is observed. The forward-to-backward absorption ratio reached as high as 60, while the thickness of the entire structure is in the order of the operating wavelength. In addition to the spectral tunability of the excited resonances and their bandwidths, our proposed device supports transparency windows with 65$\%$ transmission in the adjacent frequency bands. Our 2D grating is also verified to enable near-absolute insensitivity to the polarization state of incident light. Geometrical parameter modification also gives our design great tunability, as we also designed a device with 300 nm absorption/reflection linewidth.
Ka-bant uygulamaları için GaN HEMT tabanlı MMIC tasarımı ve üretimi
Gallium Nitride (GaN) technology has recently dominated the high power applications in the mm-wave frequencies, and its commercial use is emerging with the upcoming 5G technology. High Electron Mobility Transistors (HEMTs) based on GaN show superior material properties and high power densities, which makes them promising candidates to utilize for Monolithic Microwave Integrated Circuits (MMICs) in high frequency applications. NANOTAM's 0.15 µm/0.2 µm GaN HEMT on Silicon Carbide (SiC) microfabrication process is used to fabricate the transistors and passive components. Process steps are explained, as well as in-house epitaxial growth. Fabricated transistors are characterized for their direct current (DC), small signal, and large-signal performances. T-gate structure of the transistors is optimized for the highest gain performance at 35 GHz. A three-stage MMIC amplifier is designed, fabricated in two process cycles, and measurements are performed on-wafer at room temperature. The best performing MMIC shows a small-signal gain higher than 23.1 dB with an output power of 31.9 dBm and a power-added efficiency (PAE) of 26.5% at 35 GHz.
S22'de gözlenen kink-etkisi dikkate alınarak genişbantlı GaN LNA MMIC ve mikro/nano proses geliştirilmesi
Broadband low noise amplifiers (LNA) are one of the key components of the numerous applications such as communication, electronic warfare, and radar. The requirements for higher bandwidth, higher speed, higher survivability, higher reliability, etc. pushes the technological boundaries. The demand for high performance circuit components without a compromise stimulates the utilization of the high-end gallium nitride (GaN) technology to develop better monolithic microwave integrated circuits (MMIC) in a smaller footprint. To support the progress, the development of a proper GaN high electron mobility transistor (HEMT) technology and proper circuit models have become critical. To support the efforts and contribute to the progress, a 0.25 μm microstrip (MS) GaN HEMT technology is developed in Bilkent University Nanotechnology Research Center (NANOTAM). The technology development yield that the MS GaN HEMT technology is capable of supporting ≥4.4 W/mm output power (POUT), ≥50% power added efficiency (PAE), ≥15 dB gain, and ~1 dB noise figure (NF) at 10 GHz. Moreover, the gate structure of the technology is studied by evaluating the kink-effect (KE) in the output reflection coefficient (S22) of the HEMT to support the broadband operation. Besides the technology development, the small-signal (SS) and noise equivalent circuit models are studied, and the developed models present high convergence with the measurements. The accuracy of the models contributes to development of the cascode HEMT based LNAs even without fabricating the cascode HEMT. Furthermore, the developed models and the proper gate structure are used to develop the broadband QFN packaged GaN LNA MMIC for the mobile radio communications and the military & commercial radar applications. The results of the circuit models and the GaN LNA MMIC also yield that the developed MS GaN HEMT technology is capable for developing different solutions up to 18 GHz.
Girişimölçücü fiber optik dönüölçer performansı üzerine çevresel etkiler
Today main performance limitations for fiber optic gyroscope technology are its sensitivity to temperature fluctuations and vibration. Shupe error is the main error source for both disturbances. We propose an approach to reduce the thermal sensitivity by controlling the strain inhomogeneity through the fiber coil. The approach is based on advanced fiber coil modeling, which is verified by a series of experiments. Vibration is often a neglected disturbance by the researchers as it highly depends on the integrated platform. We propose a model for bias error formation due to optical power fluctuations under vibration. Model is composed of power fluctuation characteristics, spurious rotation rate formation due to mechanical Shupe error, and the suppression of the rotation rate by the closed-loop operation. Lastly, we introduce the concept of angle random walk performance degradation under vibration due to interferogram nonlinearity.
Çoklu bant ve termal ayarlanabilir ışık soğurucuları, çoklu termal kızılötesi yayıcı ve yüksek karşıtlıklı asimetrik iletim optik diyot tasarımı için plazmonik metamalzeme tabanlı yapılar
Alt dalga boyu nanoyapılara sahip metamalzemeler, doğal malzemelerle gözlemlenemeyen egzotik elektromanyetik özelliklere sahip bir sentetik malzeme sınıfını ifade eder. Negatif kırılma indisi, asimetrik ışık iletimi, görünmez perdeleme ve lazer bu özelliklere örnektir. Bu olası uygulamalar arasında, ışığın sınırlandırılması ve dalga boyu altı yapılar tarafından hasat edilmesi kavramı, termal emisyon, optik modülatör, algılama ve fotodedektörlere kadar uzanan yaygın uygulamaları nedeniyle büyük ilgi görmüştür. Burada, seçici termal emisyonlar ve sensörler için görünür ve yakın kızılötesi aralıklarda dört farklı uygulama alanında, yani 1) hibrit indiyum kalay oksit-Au meta malzeme soğurucu alt dalga boyu yapılarına sahip metamalzemeleri öneriyor ve tasarlıyoruz. 2) Optik modülatör için bir alt dalga boyu VO2 kullanarak dar banttan geniş bant soğuruculara aktif ayarlama. 3) Çoklu termal kızılötesi uygulamalarla uyumlu, spektral olarak seçici bir nanoanten yayıcı. 4) Doğrusal polarize dalgaların yüksek kontrastlı asimetrik iletimi gibi diyot. İlk çalışmada, görünür ve yakın Kızılötesi (nır) dalga boylarına [sırasıyla 773 nm ve 900 nm] odaklanan iki dar bant emme tepkisine sahip çok bantlı bir MPA ve NIR bölgesindeki başka bir pencerede geniş bant emici bir özellik öneriyoruz [1,530 nm'den 2,700 nm'ye kadar 1,170 nm bant genişliği ile]. MPA, bir SiO2 tabakası ile optik olarak kalın bir alt reflektörden ayrılan periyodik bir dizi kendinden hizalı hibrit indiyum kalay oksit (ITO)-Au bölünmüş halka rezonatöründen oluşur. Yarı analitik bir inceleme ile birlikte verilen sayısal hesaplamalara dayanarak, çift dar bant ve geniş bant tepkilerinin, altının optik tepkilerinin Ito ile plazmonik bir malzeme olarak hibridizasyonuna atfedildiğini görüyoruz. ITO'NUN görünür aralıkta düşük kayıplı bir dielektrik ve NIR bölgesinde kayıplı bir plazmonik malzeme olarak hareket ettiğini unutmayın. Ayrıca, metamateryalin birim hücresindeki uygulanan simetri nedeniyle, önerilen MPA polarizasyona duyarsız ve çok yönlü emici özellikleri temsil eder. Önerilen altyapı, seçici termofotovoltaik cihazlarda, termal yayıcılarda ve sensörlerde potansiyel uygulamalar bulabilir. İkinci çalışmada, metal-dielektrik plazmonik bir yapıya gömülü vanadyum dioksit (VO2) tarafından sağlanan çeşitli işlevlere sahip bir MPA öneriyoruz. İlk tasarım amacı için, bir gümüş (Ag) ayna üzerinde bir silikon (Si) nanograsyonun, yakın Kızılötesi (NIR) bölgesinde çoklu rezonans tepkilerine sahip olması önerilmektedir. Sonra, doğru konumda eklenen ince bir VO2 katmanı yapıyı aç/kapat anahtar ve rezonans ayarlayıcı olarak kullanılmasını sağlar. VO2'NİN geçirgenlik verilerinin Si'ninkine benzer olduğu VO2'NİN izolatör fazında, NIR bölgesi içinde çift güçlü bir rezonans davranışı elde edilir. Sıcaklığı arttırarak, VO2 durumu izolatörden metale dönüşür, böylece emme bantları yakın spektral pozisyonlara sahip üç farklı rezonans zirvesine dönüşür. Bu dönüşüm üzerine, yeni bir rezonans ortaya çıkar ve mevcut rezonans özellikleri spektral alanda Mavi/Kırmızı kaymalar yaşar. Bu zirvelerin süperpozisyonu, genel emilim bant genişliğini geniş yapar. Si, küçük bir termo-optik katsayıya sahip olmasına rağmen, ultra küçük boşluklarda güçlü ışık hapsi nedeniyle, Si nanogratings içinde önemli bir ayar elde edilebilir. Bu nedenle, önerilen hibrit tasarım, geniş bir yelpazeyi kapsayacak şekilde çok rezonanslı ayarlanabilir özellikler sağlayabilir ve doğrusal olarak termal olarak ayarlanabilir ve geniş bant Mpa'ların tasarımı için umut verici bir strateji olabilir. Önerilen çift ayar özelliği sayesinde, rezonans dalga boyları geniş bir dalga boyu aralığını kapsayan sıcaklığa karşı büyük hassasiyet gösterir. Genel olarak, önerilen tasarım stratejisi, ince bir VO2 tabakasının plazmonik emicilerle entegrasyonu ile sağlanan çeşitli işlevleri göstermektedir. Üçüncü çalışmada, çoklu Kızılötesi uygulamaları karşılamak için bir metal-izolatör-metal konfigürasyonu (Gümüş (Ag) tabakaları arasına sıkıştırılmış silikon dioksit (SiO2)) kullanarak boşluk yüzey plazmon modlarının uyarılmasına dayanan bir dalga boyu seçici nanoantenna yayıcı tasarlıyoruz. 3-5 mikron ve 8-12 mikron atmosferik şeffaflık pencereleri nispeten düşük emisyon korurken tasarım I olarak adlandırılan önerilen tasarım, atmosferik emme bantlarıyla eşleşen, nm, nm, and nm rezonans dalga boylarında üçlü dar mükemmel emilimleri sağlar. Daha sonra, Tasarım II olarak adlandırılan tasarım I'in işlevselliği, nanoantenna yayıcıdan gelen güneş ışınımı yansımasını en aza indirmek için yakın Kızılötesi bölgede geniş bant emilimini içerecek şekilde genişletilir. Son olarak, önerilen nanoantenna yayıcının (tasarım II) Kızılötesi imzasının gerçek zamanlı olarak ayarlanmasını sağlamak için tek ve üç katmanlı grafen tanıtılmıştır. Ayrıca, üç katmanlı grafen yapısının, SiO2 tabakasının içsel titreşim modları (optik fononlar) ile ilgili istenmeyen bir emme rezonans dalga boyunu, tek katmanlı olan için %25.53'e kıyasla %53.19 oranında baskılayabildiği gösterilmiştir. Tasarım I'in spektral analizi, tasarım II'nin analizi için sayısal simülasyon alanının genişletildiği hem analitik hem de sayısal yaklaşımlar kullanılarak doğrulanır. Tasarım I ve tasarım II'nin termal karakteristik analizleri (grafen tabakaları olmadan/olmadan), kara cisim radyasyonunun Kızılötesi imzalarının, tüm dalga boyu spektrumu için sırasıyla oda sıcaklığından 500 K'ye kadar geniş bir sıcaklıkta en az %96 ve %91 oranında önemli ölçüde azaldığını ortaya koymaktadır. Dördüncü ve son çalışmada, yakın Kızılötesi (NIR) bölgesinde yüksek kontrastlı ileri-geri oranına sahip dar bantlı bir optik diyot sunuyoruz. Tasarım, yaklaşık %88'lik bir ileri iletime ve %3'ten daha az bir geriye doğru iletime sahiptir ve 1550 nm dalga boyunda 14.5 dB'den daha büyük bir kontrast oranı sağlar. Yapı, her ikisi de silisyum nitrürden (Si3N4) yapılmış bir dielektrik levha dalga kılavuzunun üstünde tek boyutlu bir kırınım ızgarasından oluşur ve hepsi birlikte bir dielektrik substrat üzerine gömülü bir gümüş (Ag) ince film üzerine yerleştirilir. İnce bir gümüş tabaka üzerinde dielektrik bazlı bir kırınım ızgarası dalga kılavuzunun kullanılması, metalik tabakanın her iki arayüzünde uzun ve kısa menzilli yüzey plazmon polaritonları (Spp'ler) olarak bilinen iki yüzey plazmon modunun eşzamanlı uyarılmasına yol açar. Metalik tabakanın üst arayüzünde uyarılan Spp'lerin radyasyon modlarına bağlanmasının sonucu olan plazmon tünelleme etkisi, yüksek asimetrik iletim (AT) özelliği sağlar. Önerilen yüksek kontrastlı AT Cihazının spektral tepkisi, analitik bir yaklaşım olarak hem sıkı birleştirilmiş dalga analizi hem de sayısal olarak sonlu fark zaman alanı kullanılarak doğrulanır.
Megapiksel SWIR görüntüleme sistemi geliştirilmesi ve karakterize edilmesi
This thesis aims to develop a megapixel SWIR camera electronics and share the knowledge that has been acquired during the design phase as a detailed document that explains the theory and design steps to stand as a reference for academic researchers and engineers who work in this field. Valuable information from many different resources including books, presentations, technical articles, theses, etc. has been compiled and presented in this thesis following the design order. This thesis reviews the nature of light and its detection mechanisms and then concentrates on the idea of developing a product to detect light in the SWIR band. The sensors to detect light in the SWIR band are designed and manufactured in NANOTAM. There is a need to develop both hardware and software to process the sensor outputs which carry information about the captured image to generate a meaningful and useful set of data to be displayed. The objective is to come up with an imaging system that uses the in-house manufactured sensors and digitally visualizes the SWIR band with the best performance possible. The components that will make up the imaging system are examined thoroughly to be able to inherit correct design procedures and go for the best performance while preventing costly over-design. Different image sensor types such as CCD, CMOS, InGaAs, etc., and their characteristics and requirements are studied in detail. Analog Front End electronics are designed to add the lowest possible noise and thus degrading the sensor performance minimally. Other electronic circuits such as Thermoelectric Cooler controller, power regulation, FPGA electronics, etc. are designed to meet required operations without causing disturbance to Analog Front End electronics. Designs are carried out by abiding by correct PCB design practices which are addressed in this thesis. This way, it is made sure that the sensor performance, which is the limiting factor, is not degraded by the external electronics.
Büyük ölçekli üretime uygun metamalzeme tasarım mimarisi kullanarak renk üretme ve iyileştirmesi
Metamaterials are a type of artificial matt that can impose exotic functionalities beyond natural materials. These specifically designed sub-wavelength structures acquire these functionalities from their collective geometric arrangement rather than their individual single-unit properties. As a result, metamaterials have shown promising applications, including negative refraction, artificial magnetism, asymmetric transmission, lasing, and cloak of invisibility. Among all these applications, the concept of color generation and enhancement using metamaterial designs have attracted much attention in recent years. We can achieve color generation from two primary sources: i) filtering white light, and ii) generating light from emitting materials such as quantum dots. In color generation using white light, a metamaterial design reflects or transmits a narrow portion of the incident spectrum. Thus, the design acts as a color filter. However, the source is already a narrowband color light in the second category. Thus metamaterials merely amplify the color intensity rather than manipulate its spectral response. In this thesis, metamaterial structures are designed, fabricated, and characterized in both categories mentioned above; The content of this thesis consists of two parts; i)In the first part, we generated additive red-green-blue (RGB) colors in reflectance mode with near-unity amplitude. For this purpose, we designed a multilayer structure made of metal-insulator-metal-semiconductor-insulator (MIMSI) stacks to achieve >0.9 reflection peaks with full-width-at-half-maximum (FWHM) values <0.3λpeak. The proposed design also shows near-zero reflection in off-resonance spectral ranges, which, in turn, leads to high color purity. Finally, we fabricated the optimized designs and verified the simulation and theoretical results with characterization findings. This work demonstrates the potential of multilayer tandem cavity designs in realizing lithography-free large-scale compatible functional optical coatings. ii)In the second part, we utilized a large-scale compatible plasmonic nanocavity design platform to achieve almost an order of magnitude photoluminescence enhancement from light-emitting quantum dots. The proposed design is multi-sized/multi-spacing gold (Au) nano units that are uniformly wrapped with thin aluminum oxide (Al2O3) layer as a foreign host to form a metal-insulator-semiconductor (MIS) cavity, as we coated them with semiconductor quantum dots (QDs). Our numerical and experimental data demonstrate that, in an optimal insulator layer thickness, the simultaneous formation of broadband Fabry-Perot (FP) resonances and plasmonic hot spots leads to enhanced light absorption within the QD unit. This improvement in absorption response leads to the PL enhancement of QDs. This work demonstrates the potential and effectiveness of a host comprised of random plasmonic nanocavities in the realization of lithography-free efficient emitters. Overall, this thesis presents an alternative perspective on applying large-scale compatible metamaterials in color generation. Furthermore, the proposed designs and routes can be extended toward other functional photoelectronic designs, where high performances can be acquired in scaleable architectures. Keywords: Optical materials, metamaterials, color filter, light emission, quantum dot, plasmonics, scalable photonics
Foto-dönüşüm, foto-algılayıcı, ışık emisyonu, algılama ve filtreleme uygulamaları için litografisiz mükemmel soğurucularda güçlü ışık-madde etkileşimi
The efficient harvesting of electromagnetic (EM) waves by sub-wavelength nanostructures can result in perfect light absorption in the narrow or broad frequency range. These metamaterial based perfect light absorbers are of particular interest in many applications, including thermal photovoltaics, photovoltaics, emission, sensing, filtering, and photodetection applications. Although advances in nanofabrication have provided the opportunity to observe strong light-matter interaction in various optical nanostructures, the repeatability and upscaling of these nano units have remained a challenge for their use in large-scale applications. Thus, in recent years, the concept of lithography-free metamaterial absorbers (LFMAs) has attracted much attention in different parts of the EM spectrum, owing to their ease of fabrication and high functionality. In this thesis, the unprecedented potential of these LFMAs will be explored. This thesis explores the material and architecture requirements for the realization of a LFMA from ultraviolet (UV) to far-infrared (FIR) wavelength regimes. For this aim, we theoretically investigate the required conditions to realize an ideal perfect absorber. Then, based on the operation wavelength and application, the proper material and design architecture is defined. Later, to experimentally realize these ideal LFMAs, lithography-free large-scale compatible routes are developed to generate nanostructures in centimeter scales. Finally, the application of these LFMAs has been demonstrated in various fields including filtering, sensing, emission, photodetection, and photoelectrochemical water splitting. This thesis study demonstrates that, by the use of proper material and design configuration, it is possible to realize these LFMAs in every portion of the EM spectrum with a vast variety of potential applications. This, in turn, opens up the opportunity of the practical application of these perfect absorbers in large-scale dimensions. In the last section of the thesis, we discuss the progress, challenges, and outlook of this field to outline its future direction
S-bant 5g T/R modülleri için yüksek güçlü ve düşük kayıplı SPDT anahtarının SiC üzerine GaN kullanarak tasarımı
Radio frequency (RF) switches are one the fundamental components of modern communication systems. They enable the routing of high-frequency signals into different transmission paths. Therefore, they play a crucial role in transceiver (T/R) modules. Especially, 5G technology creates a demand for compact switches with high power handling, high isolation, and low insertion loss. GaN on SiC high electron mobility transistor (HEMT) technology stands out with its exceptional electrical and thermal characteristics among other semiconductor technologies. However, switch performance is limited by selected topology and transistor capability. Notably, the T-gate dimensions of the HEMTs directly affect the small-signal and large-signal performance of the switch. This study focuses on designing a single-pole double-throw (SPDT) monolithic microwave integrated circuit (MMIC) switch using gate-optimized HEMT in AlGaN/GaN on SiC technology. The foot length of the gate is varied from 200 nm to 250 nm, and the head length is varied from 500 nm to 750 nm in the T-gate structure to optimize the RF performance. An asymmetric SPDT switch using transistors with 500 nm head length and 250 nm foot length is designed to demonstrate transistor performance. The switch achieved an insertion loss of better than 0.85 dB throughout the 3.2–3.8 GHz bandwidth. The low-noise path can handle 25 W power level, while the high-power path can withstand up to 50 W of RF power at 1 dB compression level. The isolation performance is about 25 dB, while the return loss of the switch is better than 12 dB. The switch occupies a chip area of 2 x 2.2 mm2.
Mükemmel soğurucu, sensör, ışınımsal ısı transferi, dalga ayırıcı ve 5G/ötesi uygulamarı için Van Der Waals tabanlı ayarlanabilir meta-yüzeyler
Metasurfaces are thin, subwavelength structures that have extraordinary properties that cannot be found naturally. Tunable metasurfaces drew attention not only for their lightweight designs but also with the tunning option, having multiple responses without complex fabrication steps for each desired response. Besides tuning the structures by their intrinsic properties, the addition of van der Waals materials which are a specific type of 2D materials, can expand their tuning flexibilities and offer a wide range of applications. Here, we propose and investigate tunable metasurfaces in the following areas: Perfect Absorption, Sensing, Radiative Heat Transfer, Beam Splitting and 5G/Beyond Applications as: 1. All-Dielectric Metamirror for thermally tunable spectrally selective absorber, 2. Metasurface Design for Phonon-Induced Transparency and Nearly Perfect Resonant Absorption, 3. Near-Field Radiative Heat Transfer in Parallel-Plate Structures, 4. Gradient Metasurfaces for Beam Splitting and Light Absorption, 5. 5G and Beyond applications of mentioned works and future outlook.
X-bant GaN tabanlı düşük gürültülü yükselteç tasarımı ve geliştirilmesi
Gallium nitride (GaN) high electron mobility transistor (HEMT) technology emerged as a preferable candidate for high-power applications. GaN HEMTs on silicon carbide (SiC) substrate provide the best combination of speed and power due to high power density, escalated saturated carrier velocity, high efficiency, enhanced electrical breakdown, and superior thermal conductivity. Over the years, GaN technology also started to take its place in low-noise applications due to built-in power handling capability at the receive end of transceivers for compact designs and high linearity. For GaN-based low-noise amplifiers (LNAs), improving the noise figure (NF) and getting it close to other competitive technologies is always challenging. Moreover, further improvement in the robustness of GaN-based LNAs in terms of survivability and reverse recovery time (RRT) is needed. For this purpose, NANOTAM's 0.15 µm GaN on SiC HEMT process is used to realize LNAs, one with survivability as high as 42 dBm and the other having NF as low as 1.2 dB. Survivability is investigated in terms of gain compression and forward gate current, while RRT is explored in detail with respect to the RC time constant of transistor and trap phenomenon. In the LNA design, the significance of inductive source degenerated HEMT, and the role of stability networks towards NF improvement are discussed in detail. Furthermore, thermal simulations and infrared (IR) thermographic measurements of the LNA monolithic microwave integrated circuit are correlated to unveil the maximum channel temperature buried inside the two-dimensional electron gas of HEMT.
5G uygulamaları ve termal radyasyon yonetımı ıcın metamalzeme tabanlı nanoyapı tasarımı
The properties of natural materials can be the only limiting factor in today's technologies. For this, researchers in the last decades found that engineering the features of naturally occurring materials in the subwavelength scales can drastically change their properties. These materials beyond the natural ones are called "metamaterials", where "meta" means "beyond" in Greek. Although the fabrication of these materials can be quite challenging, clever designs and exploitation of physical phenomena can lead to tunable responses, eliminating the need for multiple structures. Here, different strategies for designing tunable meta-surfaces for a wide range of applications will be presented by giving two examples. These applications are namely: 1. Graphene-based Metasurface Absorber for the Active and Broadband Manipulation of Terahertz Radiation, 2. Adaptive Thermally Tunable Radiative Cooling with Angle Insensitivity Using Phase-Change Material-Based Metasurface.
Zenginleştirilmiş ışık madde etkileşimleri için plazmonik nanoantenler ve grafen tabanlı ayarlanabilir nanofotonik aygıtlar
Focusing, manipulating and beaming of electromagnetic waves are important for many applications such as antennas, optical isolators, biological sensor, chemical sensors, and solar cells. There is an extensive research about the manipulation of light, and its interaction with different types of materials including subwavelength structures. However, manipulating light at the nanoscale has many difficulties due to the diffraction limit. In this thesis, we mainly focus on the characterization and experiments of subwavelength plasmonic structures. We investigated the spatial distribution of the electric field through subwavelength slits by using symmetric and non-symmetric periodic metallic grating structures in order to obtain one-way transmission, off-axis beaming, collimation and diode-like beaming. We also studied various plasmonic structures such as circular rings and fractal bowtie antennas. After combining them with Raman active molecules, we showed that these plasmonic structures can be used as efficient surface enhanced Raman spectroscopy substrates. Finally, we designed, fabricated and measured nanoantennas and split ring resonators on graphene in order to tune their optical response using the electrically controllable doping property of the graphene.
Sualtı uygulamaları için ultra düşük gürültülü fiber optik sensor
Fiber optic sensor have many advantages over the conventional sensor technologies such as electromangnetic compatibility, light weight, exible geometric design, sensing signal at a remote point and no electrical signal at the signal detection point, easy sensor multiplexing,compatibility to fiber optic communication. Experimental studies have been available for underwater acoustic sensor with using fiber optics. In this thesis mandrel type interferometric fiber optic acoustic sensor design is analyzed in terms of noise performance characteristics and design parameters' contribution to noise performance is evaluated. Acoustic signals create phase change at the fiber optic acoustic probe and phase change is detected by using an interferometer. Experimental setup is used to emulate the design of the sensor and different components are used to evaluate the effect on the noise performance. Phase generated carrier method is evaluated for phase detection method. Finally noise measurement is done with a experimental setup emulating the design. Noise level is measured around -100 db re rad/√Hz and responsivity of the sensor(-140 dB re rad/µPa) is used to convert noise unit to pressure units and 40 dB re µPa=√Hz noise level in terms of pressure is obtained. This level of noise floor is close to sea sate 0 (SS0) noise level of the ocean. Sea state 0 is the state where there is no wind, wave or shipping traffic. This state is the lowest possible ambient noise available in the ocean and it is quite rare to find such state of the ocean. Therefore self noise of the designed fiber optic acoustic sensor is not the limiting factor for acoustic signal detection. Any signal over the ambient noise of the ocean can be detected using the designed fiber optic acoustic sensor.