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Proceedings of the IEEE

Issue 7 • Date July 2010

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Displaying Results 1 - 25 of 33
  • [Front cover]

    Page(s): C1
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  • Proceedings of the IEEE publication information

    Page(s): C2
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  • Contents

    Page(s): 1109 - 1110
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  • Vehicular Communications: Ubiquitous Networks for Sustainable Mobility [Point of View]

    Page(s): 1111 - 1112
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  • Challenges and Opportunities in GaN and ZnO Devices and Materials [Scanning the Issue]

    Page(s): 1113 - 1117
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  • Ultrafast Removal of LO-Mode Heat From a GaN-Based Two-Dimensional Channel

    Page(s): 1118 - 1126
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (807 KB) |  | HTML iconHTML  

    Dissipation of the Joule heat, accumulated in non-equilibrium longitudinal optical (LO) phonon modes, is considered in terms of LO-phonon lifetime. The dependence of the lifetime on electron density, hot-electron temperature, and supplied electric power are presented for a voltage-biased GaN-based channel with a two-dimensional electron gas (2DEG). An improved understanding of conversion of LO phonons into acoustic and other phonons is reached. A nonmonotonous dependence of the lifetime on the electron density is observed. The optimal 2DEG density for ultrafast decay of the LO-mode heat is estimated and explained in terms of LO-phonon-plasmon resonance. A new limitation of the frequency performance is predicted for heterostructure field effect transistors under the off-resonance conditions of operation. The shortest hot-phonon lifetime of ~60± 20 fs is found, at a high level of supplied power, in nearly lattice matched InAlN/AlN/GaN. View full abstract»

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  • Status of Reliability of GaN-Based Heterojunction Field Effect Transistors

    Page(s): 1127 - 1139
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (482 KB) |  | HTML iconHTML  

    GaN-based heterojunction field effect transistors (HFETs) will play major roles in the high-power, high-frequency military and commercial arenas for microwave and millimeter wave transmitters and receivers used in communications and radar devices. In fact, devices operative in the X-band (7-12.5 GHz) and beyond are already at market and boast quite impressive performances. Having improved the crystal quality now to levels where the reliability [expressed as mean time to failure (MTTF)] is claimed to exceed ten million hours, the work now needs to focus on which of the physical mechanisms responsible for degradation are the most important, and how the existing degradation accelerates subsequent degradation, ultimately resulting in device failure. Available data show that not all devices from the same wafer show similar longevity and the wide spread of activation energies reported for three-temperature extrapolation-based predictions of the lifetime are troubling. If we hope to make consistent, reliable predictions of device lifetimes, particularly when the devices are being pushed in radio-frequency (RF) operation to near their limits, more work will need to be done in characterizing the long term stability of the devices, and new physical models for the failure mechanisms will have to be developed. View full abstract»

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  • Small Signal Equivalent Circuit Modeling for AlGaN/GaN HFET: Hybrid Extraction Method for Determining Circuit Elements of AlGaN/GaN HFET

    Page(s): 1140 - 1150
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    The developments in AlGaN/GaN heterojunction field effect transistors (HFETs) are beginning to allow harnessing of the great potential of this technology in high-power radio-frequency (RF) applications. However, the integration of HFET into a circuit environment requires accurate small and large signal modeling of the device operating under various biasing conditions. The conventional small signal equivalent circuit modeling methods consist of “cold” measurements for extracting parasitic elements, and on-bias measurements in determining the intrinsic device circuit elements. Also, the optimization routines are often explored directly using the “hot” measurement data to minimize errors. In this paper, an 18-element small signal equivalent circuit model for AlGaN/GaN HFET is proposed and implemented, and contrasted to various de-embedding methods. Among the methods treated, the hot-FET optimization extraction based on the parasitic capacitances obtained from cold measurements leads to the smallest error between the simulated S-parameters and the measured ones at all bias points employed, with an average error of about 5%. This hybrid extraction algorithm is strengthened by imposing constraints to avoid any nonphysical convergence. We note that the extrinsic parasitics determined by this method differ considerably from the values obtained by cold-FET measurements, which implies that the assumption on the bias independency for extrinsic parameters in the latter method might be questionable for AlGaN/GaN HFET. View full abstract»

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  • GaN Power Transistors on Si Substrates for Switching Applications

    Page(s): 1151 - 1161
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    In this paper, GaN power transistors on Si substrates for power switching application are reported. GaN heterojunction field-effect transistor (HFET) structure on Si is an important configuration in order to realize a low loss and high power devices as well as one of the cost-effective solutions. Current collapse phenomena are discussed for GaN-HFETs on Si substrate, resulting in suppression of the current collapse due to using the conducting Si substrate. Furthermore, attempts for normally off GaN-FETs were examined. A hybrid metal-oxide-semiconductor HFET structure is a promising candidate for obtaining devices with a lower on-resistance (Ron) and a high breakdown voltage (Vb). View full abstract»

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  • Solid-State Lighting: An Integrated Human Factors, Technology, and Economic Perspective

    Page(s): 1162 - 1179
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (2507 KB) |  | HTML iconHTML  

    Solid-state lighting is a rapidly evolving technology, now virtually certain to someday displace traditional lighting in applications ranging from the lowest-power spot illuminator to the highest-power area illuminator. Moreover, it has considerable headroom for continued evolution even after this initial displacement. In this paper, we present a high-level overview of solid-state lighting, with an emphasis on white lighting suitable for general illumination. We characterize in detail solid-state lighting's past and potential-future evolution using various performance and cost metrics, with special attention paid to inter-relationships between these metrics imposed by human factors, technology, and economic considerations. View full abstract»

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  • GaN-Based Light-Emitting Diodes: Efficiency at High Injection Levels

    Page(s): 1180 - 1196
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    Light-emitting diodes (LEDs) have become quite a high-performance device of late and are revolutionizing the display and illumination sectors of our economy. Due to demands for better performance and reduced energy consumption there is a constant race towards converting every single electron hole pair in the device to photons and extracting them as well while using only the minimum required voltage. This raises the bar on GaN-based LEDs in terms of elimination of nonradiative recombination processes not just at low but just as importantly if not more at very high injection levels needed for high brightness sources, and design of heterostructures for efficient electron and hole recombination without carrier loss and voltage/photon energy conversion loss. The haunting efficiency droop observed in GaN-based LEDs at high injection levels has been attributed to nonradiative Auger recombination, but can be simply explained by electron spillover. Investigations of quantum well (QW) InGaN LED structures with different barrier heights, widths, and doping suggest that limited hole transport in the active region and the resulting electron spillover is responsible for efficiency droop at high injection levels. In this paper, highly critical, demanding, and challenging nature of high-efficiency high-brightness LEDs, in particular the basics surrounding the internal quantum efficiency of LED structures and the ongoing research/development, will be discussed. View full abstract»

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  • High Brightness GaN Vertical Light-Emitting Diodes on Metal Alloy for General Lighting Application

    Page(s): 1197 - 1207
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1999 KB) |  | HTML iconHTML  

    In this paper, we show the many advantages of the GaN-based vertical light-emitting diodes (VLEDs) on metal alloy over conventional LEDs in terms of: better current spreading, vertical current path for low operation voltage, better light extraction, flexible chip size scaling, higher driving current density, faster heat dissipation, and good reliability. The GaN VLED on metal alloy exhibits very good current-voltage behavior with low operated voltage and low serial dynamic resistance. The low operation junction temperature of GaN VLED on metal alloy demonstrates excellent heat dissipation capabilities. Chip size scaling without efficiency loss shows a unique property of GaN VLED on metal alloy. The GaN VLED on metal alloy also enables top surface engineering for efficient light extraction to further light output. A high-power white LED having efficiency of 120 lumen/W was achieved through a combination of reflector, surface engineering, and optimization of the n-GaN layer thickness. Coupled with good reliability and mass production ability, the GaN VLED on metal alloy is very suitable for general lighting application. View full abstract»

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  • Structural Defects and Degradation Phenomena in High-Power Pure-Blue InGaN-Based Laser Diodes

    Page(s): 1208 - 1213
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    To improve the lifetime of high-power pure-blue InGaN-based laser diodes, the need to reduce the number of newly created structural defects in active regions, consisting of multiple quantum well structures, is inevitable. We first report on detailed structural analyses of these new types of defects and discuss their formation mechanisms and reduction methodologies. We then fabricated laser diodes with current-injection free regions near the laser facets and confirm that this is an effective method for the suppression of degradation by catastrophic optical damage. Based on the analyses of aged devices by using fluorescence microscopy, we also discuss the degradation mechanisms of GaN-based laser diodes. View full abstract»

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  • Degradation Mechanisms of InGaN Laser Diodes

    Page(s): 1214 - 1219
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    We discuss various mechanisms of laser diode degradation based on our own experiments and on the available literature data. In most of the cases, degradation of InGaN laser diodes occurs through the increase of the threshold current with almost constant slope efficiency. The threshold current change follows frequently the square root on time dependence. Though this type of behavior has usually been attributed to magnesium acceptor diffusion, no firm proof of such a hypothesis has so far been presented. In contrast, there is an increasing number of reported experiments showing that the most important factor contributing to fast (hours), and medium time (hundreds of hours) degradation is the process of carbon deposition. This process involves photochemical reactions leading to the decomposition of hydrocarbons existing in the laser diode environment. This process resembles very closely the mechanism responsible for 980-nm laser diode degradation and known as Package Induced Failure. View full abstract»

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  • Wide Bandgap Semiconductor-Based Surface-Emitting Lasers: Recent Progress in GaN-Based Vertical Cavity Surface-Emitting Lasers and GaN-/ZnO-Based Polariton Lasers

    Page(s): 1220 - 1233
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    With edge-emitting GaN-based lasers in commercial systems, attention is shifting to more demanding and rewarding emitters. These encompass microcavity (MC)-based vertical cavity surface-emitting lasers (VCSELs) and polariton lasers. The impetus centers on applications such as high-speed/high-resolution laser printing/scanning technology, lighting, and new types of coherent but nearly thresholdless optical sources. Room-temperature operations of GaN-based VCSELs by electrical injection have been recently reported, and the research on GaN-based VCSELs is segueing into new opportunities such as polariton-based lasers. While still in its infancy, polariton lasing in GaN-based MCs at room temperature has been observed. Observation of spontaneous emission buildup in polariton lasing emission is attributed to a Bose-Einstein condensate of cavity polaritons. However, the realization of a polariton laser by electrical injection is still being pursued. In this paper, we discuss the recent progress in wide-bandgap semiconductor-based VCSELs and GaN-/ZnO-based polariton lasers. View full abstract»

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  • Intersubband Transition-Based Processes and Devices in AlN/GaN-Based Heterostructures

    Page(s): 1234 - 1248
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    We report on the physics, epitaxial growth, fabrication, and characterization of optoelectronic devices based on intersubband transitions in the AlN/GaN material system. While in 1999, only results of optical absorption experiments could be shown, photodetectors and modulators with operation frequencies beyond 10 GHz as well as optically pumped light emitters have been demonstrated recently. This is the reason for a comprehensive report on the most important properties of such devices. Beside some basic theoretical considerations, we will concentrate on the fabrication and characterization of modulators, switches, photodetectors, and light emitters. At the end of this paper, an outlook to future trends and developments in this emerging field will be given. View full abstract»

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  • Resonant Tunneling in III-Nitrides

    Page(s): 1249 - 1254
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    Wide-bandgap semiconductors can sustain high temperatures and high power operation in various important applications such as transistors, light-emitting diodes, and lasers. Although in embryonic stage, one can expect such a resilience in GaN resonant tunneling diodes (RTDs) and superlattices as well with distinct applications. Because of the negative differential conduction, the double barrier resonant tunneling structures could be the basis for new high-power coherent microwave sources operating in W-band and terahertz. In this paper, recent progress in wide-bandgap semiconductor RTDs is discussed. View full abstract»

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  • ZnO Devices and Applications: A Review of Current Status and Future Prospects

    Page(s): 1255 - 1268
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    ZnO is an attractive material for applications in electronics, photonics, acoustics, and sensing. In optical emitters, its high exciton binding energy (60 meV) gives ZnO an edge over other semiconductors such as GaN if reproducible and reliable p-type doping in ZnO were to be achieved, which currently remains to be the main obstacle for realization of bipolar devices. On the electronic side, ZnO holds some potential in transparent thin film transistors (TFTs) owing to its high optical transmittivity and high conductivity. Among the other promising areas of application for ZnO are acoustic wave devices, due to large electromechanical coupling in ZnO, and devices utilizing nanowires/nanorods such as biosensors and gas sensors and solar cells, since it is relatively easy to produce such forms of ZnO nanostructures, which have good charge carrier transport properties and high crystalline quality. Despite the significant progress made, there is still a number of important issues that need to be resolved before ZnO can be transitioned to commercial use, not to mention the stiff competition it is facing with GaN, which is much more mature in terms of devices. In this paper, recent progress in device applications of ZnO is discussed and a review of critical issues for realization of ZnO-based devices is given. View full abstract»

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  • Doping Asymmetry Problem in ZnO: Current Status and Outlook

    Page(s): 1269 - 1280
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    ZnO has gained considerable interest recently as a promising material for a variety of applications. To a large extent, the renewed interest in ZnO is fuelled by its wide direct band gap (3.3 eV at room temperature) and large exciton binding energy (60 meV) making this material, when alloyed with, e.g., Cd and Mg, especially attractive for light emitters in the blue/ultraviolet (UV) spectral region. Unfortunately, as with other wide-gap semiconductors, ZnO suffers from the doping asymmetry problem, in that the n-type conductivity can be obtained rather easily, but p-type doping proved to be a formidable challenge. This doping asymmetry problem (also dubbed as the p-type problem in ZnO) is preventing applications of ZnO in light-emitting diodes and potential laser diodes. In this paper, we provide a critical review of the current experimental efforts focused on achieving p-type ZnO and discuss the proposed approaches which could possibly be used to overcome the p-type problem. View full abstract»

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  • ZnO-GaN Hybrid Heterostructures as Potential Cost-Efficient LED Technology

    Page(s): 1281 - 1287
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (478 KB) |  | HTML iconHTML  

    Reliable and reproducible p-type doping is the main challenge for fabricating highly efficient ZnO-based light-emitting diodes. During the last few years, the lack of reliable p-type conductivity in ZnO has initiated research concerning the combination of ZnO and other semiconductors, which can then be doped p-type. One of these concepts is the combination of ZnO with GaN heterostructures aiming at the fabrication of hybrid LEDs. We discuss the problems as well as potential benefits from a combination of ZnO and GaN hybrid heterostructures in a single device. We also present our recent results on ZnO-GaN hybrid LEDs using an inverted LED concept. The hybrid LEDs have an external quantum efficiency of more than 35%. View full abstract»

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  • Ferromagnetism in ZnO- and GaN-Based Diluted Magnetic Semiconductors: Achievements and Challenges

    Page(s): 1288 - 1301
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    Both GaN- and ZnO-based diluted semiconductors (DMSs) have recently attracted considerable interest fueled by theoretical predictions of ferromagnetic ordering in these materials above room temperature, making them especially attractive for spintronics. The intense experimental research that followed has revealed, however, a great deal of controversy. The local structure and magnetic behavior of GaN- and ZnO-based DMSs were found to depend strongly on a preparation technique and growth conditions for the materials of the same nominal composition and the reported results varied considerably from group to group. This problem highlighted clearly the lack of theoretical understanding of physical mechanisms underlying ferromagnetisms in these materials and the inadequacy of standard characterization techniques used to probe structural and magnetic properties of the DMSs. In this paper, we report on the recent progress in the theoretical and experimental studies of ZnO- and GaN-based DMSs and make special impact on critical discussion of experimental methods employed for investigation of their magnetic and optical properties. View full abstract»

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  • Growth of Bulk GaN and AlN: Progress and Challenges

    Page(s): 1302 - 1315
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    GaN-based optoelectronic and electronic devices such as light-emitting diodes (LEDs), laser, and heterojunction field-effect transistors (HFETs) typically use material grown on foreign substrates such as sapphire, Si, and SiC. However, thermal and lattice mismatch present prevent attainment of quality films deemed necessary by ever increasing demand on device performance. In fact in LEDs intended for solid state lighting, internal quantum efficiencies near 100% might be needed, and further these high efficiencies would have to be retained at very high injection current levels. On the electronic device side, high radio-frequency (RF) power, particularly high-power switching devices, push the material to its limits. Consequently, as has been the case for other successful semiconductor materials systems, native substrates must be developed for the GaN family. In this paper, various approaches such as high-pressure nitrogen solution (HPNS), ammonothermal, and Na flux methods, and an intermediary technique called the hydride vapor phase epitaxy (HVPE; to a lesser extent as there is a review devoted to this technique in this issue) along with their strengths and challenges are discussed. View full abstract»

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  • The Ammonothermal Crystal Growth of Gallium Nitride—A Technique on the Up Rise

    Page(s): 1316 - 1323
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    Gallium nitride (GaN) is one of the most important wide band gap semiconductor materials in modern technology with even higher expectations for future applications it is ought to play a crucial role. Among this, the growth of lattice and thermally matched GaN substrates for the GaN device technology takes an essential piece. This paper is reporting on the achievements in the ammonothermal growth technique of GaN bulk crystals. Important features specific to the ammonothermal technique are focused on. Although only a few groups (currently <; 10 worldwide) are directly involved in the development of the ammonothermal bulk crystal growth technology, partly due to the extreme technological challenges, tremendous progress over the last decade has recently resulted in the fabrication of 2 inch large, free-standing, single crystalline GaN with excellent structural perfection. View full abstract»

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  • GaN Substrates for III-Nitride Devices

    Page(s): 1324 - 1338
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    Despite the rapid commercialization of III-nitride semiconductor devices for applications in visible and ultraviolet optoelectronics and in high-power and high-frequency electronics, their full potential is limited by two primary obstacles: i) a high defect density and biaxial strain due to the heteroepitaxial growth on foreign substrates, which result in lower performance and shortened device lifetime, and ii) a strong built-in electric field due to spontaneous and piezoelectric polarization in the wurtzite structures along the well-established [0001] growth direction for nitrides. Recent advances in the research, development, and commercial production of native GaN substrates with low defect density and high structural and optical quality have opened opportunities to overcome both of these obstacles and have led to significant progress in the development of several optoelectronic and high-power devices. In this paper, the recent achievements in bulk GaN growth development using different approaches are reviewed; comparison of the bulk materials grown in different directions is made; and the current achievements in device performance utilizing native GaN substrate material are summarized. View full abstract»

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  • Bulk ZnO: Current Status, Challenges, and Prospects

    Page(s): 1339 - 1350
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    Rediscovered in the last decade, zinc oxide (ZnO) shows a great potential for many optoelectronics and to some extent microelectronics applications. However, a clear majority of effort expended in this fast developing field has been limited to heteroepitaxial structures grown on foreign substrates with lattice-parameter and thermal-expansion mismatch with ZnO which is detrimental. Recognizing the importance, the effort has shifted to include developing technologies capable of producing freestanding ZnO wafers in large-scale for ZnO based device applications, which is the subject matter of this manuscript. Three competing approaches - hydrothermal method, melt growth (modifications of the well known Bridgman technique), and seeded vapor transport growth - have now reached or are approaching commercial viability. In this article, we discuss the progress, outstanding problems, and prospects of these growth methods employed for commercial manufacturing of ZnO wafers. View full abstract»

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North Carolina State University