Browsing by Subject "Gallium nitride"

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Open Access
A 6-18 GHz GaN power amplifier MMIC with high gain and high output power density
(IEEE, 2019) Sütbaş, Batuhan; Özipek, Ulaş; Gürdal, A.; Özbay, Ekmel
A three-stage reactively-matched 6-18 GHz power amplifier MMIC design is presented. The design effort is focused on obtaining a low-loss output matching network for a high output power density. Active unit cells consist of an 8×125 μm transistor stabilized with a symmetrical parallel RC circuit. The wideband amplifier is fabricated using our in-house 0.25 μm GaN on SiC HEMT process. The fabrication technology details and overall device performance are reported. Experimental results show that the MMIC has a minimum gain of 22 dB and a maximum gain of 26.5 dB across the operation band. An average output power density higher than 3.3W/mm with an associated average power-added efficiency of 22.5% is achieved. The MMIC demonstrates output power greater than 9.5 W at the center frequency. This design is distinguished from recent studies with its low-ripple high gain and high output power density.
Open Access
AlGaN/GaN HEMT-based fully monolithic X-band low noise amplifier
(Wiley, 2005-04) Schwindt, R.; Kumar, V.; Aktas, O.; Lee, J. W.; Adesida, I.
A fully monolithic AlGaN/GaN HEMT-based low noise amplifier is reported. The circuit demonstrated a noise figure of 3.5 dB, gain of -7.5 dB, input return loss of -7.5 dB, and output return loss of -15 dB at 8.5 GHz. The dc characteristics of individual 0.25-μm × 150-μm transistors were: maximum current density of 1.0 A/mm, maximum transconductance of 170 mS/mm and a threshold voltage of -6.8 V. The devices have a typical short circuit current gain cutoff frequency of 24.5 GHz and a maximum oscillating frequency of 48 GHz. The devices demonstrated a minimum noise figure of 1.6 dB with an associated gain of 10.6 dB at 10 GHz.
Open Access
Analysis of defect related optical transitions in biased AlGaN/GaN heterostructures
(2010) Bengi, A.; Lisesivdin, S.B.; Kasap, M.; Mammadov, T.; Ozcelik, S.; Özbay, Ekmel
The optical transitions in AlGaN/GaN heterostructures that are grown by metalorganic chemical vapor deposition (MOCVD) have been investigated in detail by using Hall and room temperature (RT) photoluminescence (PL) measurements. The Hall measurements show that there is two-dimensional electron gas (2DEG) conduction at the AlGaN/GaN heterointerface. PL measurements show that in addition to the characteristic near-band edge (BE) transition, there are blue (BL) and yellow luminescence (YL) bands, free-exciton transition (FE), and a neighboring emission band (NEB). To analyze these transitions in detail, the PL measurements were taken under bias where the applied electric field changed from 0 to 50 V/cm. Due to the applied electric field, band bending occurs and NEB separates into two different peaks as an ultraviolet luminescence (UVL) and Y4 band. Among these bands, only the yellow band is unaffected with the applied electric field. The luminescence intensity change of these bands with an electric field is investigated in detail. As a result, the most probable candidate of the intensity decrease with an increasing electric field is the reduction in the radiative lifetime. © 2010 Elsevier Ltd. All rights reserved.
Open Access
Buffer effects on the mosaic structure of the HR-GaN grown on 6H-SiC substrate by MOCVD
(Springer New York LLC, 2017) Arslan, E.; Öztürk, M. K.; Tıraş, E.; Tıraş, T.; Özçelik, S.; Özbay, Ekmel
High-resistive GaN (>108 Ω cm) layers have been grown with different buffer structures on 6H-SiC substrate using metalorganic chemical vapor deposition reactor. Different combination of the GaN/AlN super lattice, low temperature AlN, high temperature AlN and AlxGa1−xN (x ≈ 0.67) layers were used in the buffer structures. The growth parameters of the buffer layers were optimized for obtaining a high-resistive GaN epilayer. The mosaic structure parameters, such as lateral and vertical coherence lengths, tilt and twist angle (and heterogeneous strain), and dislocation densities (edge and screw dislocations) of the high-resistive GaN epilayers have been investigated using x-ray diffraction measurements. In addition, the residual stress behaviors in the high-resistive GaN epilayers were determined using both x-ray diffraction and Raman measurements. It was found that the buffer structures between the HR-GaN and SiC substrate have been found to have significant effect on the surface morphology and the mosaic structures parameters. On the other hand, both XRD and Raman results confirmed that there is low residual stress in the high-resistive GaN epilayers grown on different buffer structures.
Open Access
Characterization of AlInN/AlN/GaN heterostructures with different AlN buffer thickness
(Springer New York LLC, 2016) Çörekçi, S.; Dugan, S.; Öztürk, M. K.; Çetin, S. Ş.; Çakmak, M.; Özçelik, S.; Özbay, Ekmel
Two AlInN/AlN/GaN heterostructures with 280-nm- and 400-nm-thick AlN buffer grown on sapphire substrates by metal-organic chemical vapor deposition (MOCVD) have been investigated by x-ray diffraction (XRD), atomic force microscopy (AFM), photoluminescence (PL) and Hall-effect measurements. The symmetric (0002) plane with respect to the asymmetric (101 ¯ 2) plane in the 280-nm-thick AlN buffer has a higher crystal quality, as opposed to the 400-nm-thick buffer. The thinner buffer improves the crystallinity of both (0002) and (101 ¯ 2) planes in the GaN layers, it also provides a sizeable reduction in dislocation density of GaN. Furthermore, the lower buffer thickness leads to a good quality surface with an rms roughness of 0.30 nm and a dark spot density of 4.0 × 108 cm−2. The optical and transport properties of the AlInN/AlN/GaN structure with the relatively thin buffer are compatible with the enhancement in its structural quality, as verified by XRD and AFM results.
Open Access
A charge inverter for III-nitride light-emitting diodes
(American Institute of Physics Inc., 2016) Zhang Z.-H.; Zhang, Y.; Bi, W.; Geng, C.; Xu S.; Demir, Hilmi Volkan; Sun, X. W.
In this work, we propose a charge inverter that substantially increases the hole injection efficiency for InGaN/GaN light-emitting diodes (LEDs). The charge inverter consists of a metal/electrode, an insulator, and a semiconductor, making an Electrode-Insulator-Semiconductor (EIS) structure, which is formed by depositing an extremely thin SiO2 insulator layer on the p+-GaN surface of a LED structure before growing the p-electrode. When the LED is forward-biased, a weak inversion layer can be obtained at the interface between the p+-GaN and SiO2 insulator. The weak inversion region can shorten the carrier tunnel distance. Meanwhile, the smaller dielectric constant of the thin SiO2 layer increases the local electric field within the tunnel region, and this is effective in promoting the hole transport from the p-electrode into the p+-GaN layer. Due to the improved hole injection, the external quantum efficiency is increased by 20% at 20 mA for the 350 × 350 μm2 LED chip. Thus, the proposed EIS holds great promise for high efficiency LEDs.
Open Access
Comparison of trimethylgallium and triethylgallium as "ga" source materials for the growth of ultrathin GaN films on Si (100) substrates via hollow-cathode plasma-assisted atomic layer deposition
(AVS Science and Technology Society, 2016-02) Alevli, M.; Haider A.; Kizir S.; Leghari, S. A.; Bıyıklı, Necmi
GaN films grown by hollow cathode plasma-assisted atomic layer deposition using trimethylgallium (TMG) and triethylgallium (TEG) as gallium precursors are compared. Optimized and saturated TMG/TEG pulse widths were used in order to study the effect of group-III precursors. The films were characterized by grazing incidence x-ray diffraction, atomic force microscopy, x-ray photoelectron spectroscopy, and spectroscopic ellipsometry. Refractive index follows the same trend of crystalline quality, mean grain, and crystallite sizes. GaN layers grown using TMG precursor exhibited improved structural and optical properties when compared to GaN films grown with TEG precursor.
Open Access
Controlled growth and characterization of epitaxially-laterally-overgrown InGaN/GaN quantum heterostructures
(IEEE, 2010) Sarı, Emre; Akyuz, Özgün; Choi, E. -G.; Lee I.-H.; Baek J.H.; Demir, Hilmi Volkan
Crystal material quality is fundamentally important for optoelectronic devices including laser diodes and light emitting diodes. To this end epitaxial lateral overgrowth (ELO) has proven to be a powerful technique for reducing dislocation density in GaN and its alloys [1,2]. Implementation and design of ELO process is, however, critical for obtaining high-quality material with high-efficiency quantum structures for light emitters [3]. ©2010 IEEE.
Open Access
Dark current reduction in ultraviolet metal-semiconductor-metal photodetectors based on wide band-gap semiconductors
(IEEE, 2009-10) Bütün, Serkan; Gökkavas, Mutlu; Yu, HongBo; Strupinski, Vlodek; Özbay, Ekmel
Photodetectors on semi-insulating GaN templates were demonstrated. They exhibit lower dark current compared to photodetectors fabricated on regular GaN templates. Similar behavior observed in photodetectors fabricated on epitaxially thick SiC templates. © 2009 IEEE.
Open Access
Demonstration of flexible thin film transistors with GaN channels
(American Institute of Physics Inc., 2016) Bolat, S.; Sisman, Z.; Okyay, Ali Kemal
We report on the thin film transistors (TFTs) with Gallium Nitride (GaN) channels directly fabricated on flexible substrates. GaN thin films are grown by hollow cathode plasma assisted atomic layer deposition (HCPA-ALD) at 200 °C. TFTs exhibit 103 on-to-off current ratios and are shown to exhibit proper transistor saturation behavior in their output characteristics. Gate bias stress tests reveal that flexible GaN TFTs have extremely stable electrical characteristics. Overall fabrication thermal budget is below 200 °C, the lowest reported for the GaN based transistors so far. © 2016 Author(s)
Open Access
Design of a GaN-based high gain X-Band power amplifier
(2024-05) Ağtaş, Utku
RF power amplifiers remain a vital element of space, airborne, and radar applications. Modern systems require high power and gain while maintaining high efficiency. However, obtaining these features with a compact design brings challenges. Monolithic Microwave Integrated Circuits (MMIC) provide flexibility and enhanced performance while operating at higher frequencies. Among other transistor technologies, GaN on SiC based high electron mobility transistors (HEMTs) provide extraordinary performance with high power density, thermal conductivity, and high band gap. In this thesis, we present a three-stage X-Band MMIC power amplifier (PA) based on NANOTAM 250 nm GaN-on-SiC process technology. The characterization steps of the transistors are discussed to extract process parameters for the design. The amplifier is designed on the Keysight ADS environment. The design is realized on a 3-inch GaN-on-SiC wafer. In the 8.5–10.5 GHz frequency band, measurements show that the PA achieves a 40 dB small signal gain, PAE higher than 40%, and average 20 W output power under 28 V, 100 mA/mm pulsed biasing conditions at room temperature. The MMIC occupies 10.26 mm2 area and has 1.96 W/mm2 power density.
Open Access
Design of an X-band GaN based microstrip MMIC power amplifier
(2019-02) Özipek, Ulaş
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.
Open Access
Design of high power S-band GaN MMIC power amplifiers for WiMAX applications
(IEEE, 2011) Cengiz, Ömer; Kelekçi, Özgür; Arıkan, Galip Orkun; Özbay, Ekmel; Palamutçuoǧullari O.
This paper reports two different S band GaN MMIC PA designs for WiMAX applications. First PA has a 42.6 dBm output power with a 55%PAE @ 3.5 GHz and 16 dB small signal gain in the 3.2-3.8 GHz frequency range. When two of these MMICs were combined by using off-chip Lange Couplers, 45.3 dBm output power with a 45%PAE @3.5Ghz and 16 dB small signal gain were obtained with less than 0.2 dB gain ripple in the 3.3-3.8 GHz frequency range. © 2011 IEEE.
Open Access
Design of multi-octave band GaN-HEMT power amplifier
(IEEE, 2012) Eren, Gulesin; Şen, Özlem A.; Bölükbaş, Basar; Kurt, Gökhan; Arıcan, Orkun; Cengiz, Ömer; Ünal, Sıla T.K.; Durmuş, Yıldırım; Özbay, Ekmel
This paper describes design, fabrication and measurement of 6 GHz - 18 GHz monolithic microwave integrated circuit (MMIC) amplifier. The amplifier is realized as coplanar waveguide (CPW) circuit using 0.3 μm-gate Gallium-Nitride (GaN) HEMT technology. The amplifier has a small signal gain of 7 ± 0.75 dB. The output power at 3dB compression is better than 24 dBm with 16%-19% drain efficiency for the whole 6 GHz-18 GHz frequency band under continuous wave (CW) power. © 2012 IEEE.
Open Access
Determination of energy-band offsets between GaN and AlN using excitonic luminescence transition in AlGaN alloys
(American Institute of Physics, 2006) Westmeyer, A. N.; Mahajan, S.; Bajaj, K. K.; Lin J. Y.; Jiang, H. X.; Koleske, D. D.; Senger, R. T.
We report the determination of the energy-band offsets between GaN and AlN using the linewidth (full width at half maximum) of an extremely sharp excitonic luminescence transition in Alx Ga1-x N alloy with x=0.18 at 10 K. Our sample was grown on C -plane sapphire substrate by metal-organic chemical-vapor deposition at 1050 °C. The observed value of the excitonic linewidth of 17 meV is the smallest ever reported in literature. On subtracting a typical value of the excitonic linewidth in high-quality GaN, namely, 4.0 meV, we obtain a value of 13.0 meV, which we attribute to compositional disorder. This value is considerably smaller than that calculated using a delocalized exciton model [S. M. Lee and K. K. Bajaj, J. Appl. Phys. 73, 1788 (1993)]. The excitons are known to be strongly localized by defects and/or the potential fluctuations in this alloy system. We have simulated this localization assuming that the hole, being much more massive than the electron, is completely immobile, i.e., the hole mass is treated as infinite. Assuming that the excitonic line broadening is caused entirely by the potential fluctuations experienced by the conduction electron, the value of the conduction-band offset between GaN and AlN is determined to be about 57% of the total-band-gap discontinuity. Using our model we have calculated the variation of the excitonic linewidth as a function of Al composition in our samples with higher Al content larger than 18% and have compared it with the experimental data. We also compare our value of the conduction-band offset with those recently proposed by several other groups using different techniques.
Open Access
Determination of the in-plane effective mass and quantum lifetime of 2D electrons in AlGaN/GaN based HEMTs
(2011) Celik O.; Tiras, E.; Ardali, S.; Lisesivdin, S.B.; Özbay, Ekmel
Magnetoresistance and Hall resistance measurements have been used to investigate the electronic transport properties of AlGaN/GaN based HEMTs. The Shubnikov-de Haas (SdH) oscillations from magnetoresistance, is obtained by fitting the nonoscillatory component to a polynomial of second degree, and then subtracting it from the raw experimental data. It is shown that only first subband is occupied with electrons. The two-dimensional (2D) carrier density and the Fermi energy with respect to subband energy (EF-E1) have been determined from the periods of the SdH oscillations. The in-plane effective mass (m*) and the quantum lifetime (τq) of electrons have been obtained from the temperature and magnetic field dependencies of the SdH amplitude, respectively. The in-plane effective mass of 2D electrons is in the range between 0.19 m0 and 0.22 m0. Our results for in-plane effective mass are in good agreement with those reported in the literature © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Open Access
Effect of growth pressure on coalescence thickness and crystal quality of GaN deposited on 4H-SiC
(Elsevier, 2010-09-25) Caban, P.; Strupinski, W.; Szmidt, J.; Wojcik, M.; Gaca, J.; Kelekci, O.; Caliskan, D.; Özbay, Ekmel
The influence of growth pressure on the coalescence thickness and the crystal quality of GaN deposited on 4HSiC by low pressure metalorganic vapor phase epitaxy was studied. It was shown that growth pressure has an impact on the surface roughness of epilayers and their crystal quality. GaN coalescence thicknesses were determined for the investigated growth pressures. The GaN layers were characterized by AFM and HRXRD measurements. HEMT structures were also fabricated and characterized. Among the growth pressures studied, 50, 125 and 200 mbar, 200 mbar was found to be most suitable for GaN/SiC epitaxy.
Open Access
Effect of substrate temperature and Ga source precursor on growth and material properties of GaN grown by hollow cathode plasma assisted atomic layer deposition
(IEEE, 2016) Haider, Ali; Kizir, Seda; Deminskyi, P.; Tsymbalenko, Oleksandr; Leghari, Shahid Ali; Bıyıklı, Necmi; Alevli, M.; Gungor, N.
GaN thin films grown by hollow cathode plasma-assisted atomic layer deposition (HCPA-ALD) at two different substrate temperatures (250 and 450 °C) are compared. Effect of two different Ga source materials named as trimethylgallium (TMG) and triethylgallium (TEG) on GaN growth and film quality is also investigated and reviewed. Films were characterized by X-ray photoelectron spectroscopy, spectroscopic ellipsometery, and grazing incidence X-ray diffraction. GaN film deposited by TMG revealed better structural, chemical, and optical properties in comparison with GaN film grown with TEG precursor. When compared on basis of different substrate temperature, GaN films grown at higher substrate temperature revealed better structural and optical properties.
Open Access
Effects of field plate on the maximum temperature and temperature distribution for gan HEMT devices
(American Society of Mechanical Engineers, 2016) Kara D.; Donmezer N.; Canan, Talha Furkan; Şen, Özlem; Özbay, Ekmel
Field plated GaN high electron mobility transistors (HEMTs) are widely preferred amongst other GaN HEMT devices because of their ability to regulate electric field at high power densities. When operated at high power densities, GaN HEMTs suffer significantly from the concentrated heating effects in a small region called hotspot located closer to the drain edge of the gate. Although; the stabilizing effect of field plate on the electrical field distribution in HEMTs is known by researchers, its effect on temperature distribution and the hotspot temperature is still not studied to a greater extend. For this purpose, finite element thermal modelling of devices with different sizes of field plates are performed using the joule heating distribution data obtained from 2D electrical simulations. Results obtained from such combined model show that the existence of a field plate changes the electrical field, therefore the heat generation distribution within device. Moreover; increasing the size of the field plate has an effect on the maximum temperature at the hotspot region. The results are used to analyze these effects and improve usage of field plates for high electron mobility transistors to obtain better temperature profiles. Copyright © 2016 by ASME.
Open Access
Electric field dependence of radiative recombination lifetimes in polar InGaN/GaN quantum heterostructures
(IEEE, 2009) Sarı, Emre; Nizamoğlu, Sedat; Lee I.-H.; Baek J.-H.; Demir, Hilmi Volkan
We report on external electric field dependence of recombination lifetimes in polar InGaN/GaN quantum heterostructures. In our study, we apply external electric fields one order of magnitude less than and in opposite direction to the polarization-induced electrostatic fields inside the well layers. Under the increasing external electric field, we observe a decrease in carrier lifetimes (τ) and radiative recombination lifetimes (τr), latter showing a weaker dependence. Our results on τr show an agreement with our transfer matrix method based simulation results and demonstrate Fermi's golden rule in polar InGaN/GaN quantum heterostructures dependent on electric field. For our study, we grew 5 pairs of 2.5 nm thick In0.15Ga 0.85N quantum well and 7.5 nm thick GaN barrier layers in a p-i-n diode architecture using metal-organic chemical vapor deposition (MOCVD) on a c-plane sapphire substrate. Devices with 300 μm × 300 μm mesa size were fabricated using standard photolithography, reactive ion etching and metallization steps. We used indium-tin oxide (ITO) based semi-transparent contacts in top (p-GaN) layer for uniform application of electric field across the well layers. The fabricated devices were diced and mounted on a TO-can for compact testing. © 2009 IEEE.