The two broad areas of CEMDAS research at Texas A&M are devices and systems.

Device research is concentrated in three of the strongest areas of technology within the Department of Electrical Engineering:

• solid-state electronics
• microwaves
• electro-optics
• noise, speed and power dissipation in microelectronics, nanoelectronics and quantum computing
  

• optical and microwave communications
• monitoring and control of industrial equipment and processes
• biomedical imaging.
• fluctuation-enhanced chemical sensing
• nonlinear signal processing and noise mitigation

DEVICES

Solid State Electronics Research

    The rapid expansion of the semiconductor industry over the past several decades has been accompanied by a trend to ever-smaller feature sizes, which in turn generates an ongoing need for new and improved device designs and manufacturing methods. Universities play an important role in this process in ensuring that well-trained, innovative engineers are available for industrial employment. Both undergraduate and graduate students in the solid state electronics program at Texas A&M become familiar with the methods and equipment for thin-film deposition, photolithography, wet and dry etching, process diagnostics, and device evaluation. Our faculty members have recently been employed as visiting scholars in state-of-the-art integrated circuit processing facilities, thus ensuring that we remain current with this fast-moving industry.

    Research areas in solid state electronics include integrated circuit design, processing, and diagnostic techniques, vacuum microelectronics, and semiconductor materials for thermoelectric application.

Integrated Circuit Design, Processing, and Diagnostic Techniques

    The traditional complementary metal oxide semiconductor (CMOS) inverter employs a single n-channel and a single p-channel transistor in a series connection. The gates are tied to a common input point with the output taken from the common connection of the source and drain of the two transistors. The transfer characteristic of output voltage versus input voltage exhibits a high- output state for a low-input condition and a low-output state for a high-input condition. At Texas A&M, we have recently demonstrated an inverter that exhibits a stable state in the mid-range of input voltages, thus providing three levels of stable conditions as the input voltage is varied. This is believed to be the first multi-valued logic circuit design using straightforward variations of proven CMOS technology. Devices such as this can provide an increase in functional complexity without an increase in circuit size.
    Metallization remains a critical issue in the evolution of solid-state devices. Advances in the state-of-the-art in low-resistance ohmic contacts for gallium arsenide, gallium phosphide, indium phosphide, and a number of other III-V semiconductors using platinum-based regrowth techniques have been achieved by Texas A&M researchers. These techniques are currently being applied by companies such as Texas Instruments, Hughes, and Lockheed Martin in the fabrication of improved microwave circuits and devices. Another metallization issue is the use of copper as a replacement for aluminum in the next generation of integrated circuits. Techniques such as silicide regrowth and electroplating of thin copper films for interconnections are under investigation by CEMDAS researchers.
    Laser scanning is being used as a diagnostic technique for latchup sites in CMOS integrated circuits (ICs). The npnp structures which are the active elements in these ICs are latent bipolar circuits which can be bistable.  One of the bistable states, the on state, causes the part to draw large current and causes the CMOS circuit to stop operating. This phenomenon is called latchup because the circuit may resume normal operation once the power is removed. Researchers at CEMDAS are developing a laser scanning system that can determine the susceptibility of a production IC to latchup and create a map of the latchup sites on a wafer. The system is under computer control and the current drawn by the part is stored with position to produce a two-dimensional photocurrent image.

Vacuum Microelectronics

    The demand for flat panel displays has undergone a remarkable growth in recent years driven in large part by the need of laptop computers for light, rugged, thin displays. This demand is likely to grow with the anticipated arrival of the so-called information appliance as well as other consumer applications such as telephones, automobiles, and  TV receivers, in addition to myriad military and industrial applications. CEMDAS researchers are working to commercialize a unique field emission display (FED) in which a thin-film phosphor developed at Texas A&M will be applied to a FED architecture developed by Dr. Leonid Karpov of KYPWEE Display Corporation of Austin, Tex. This approach overcomes manufacturing difficulties and results in a monolithic, cost-effective, rugged display. Research is underway toward the realization of monochromatic and color displays suitable for handheld applications which will function at full-motion video rates.
    The limited electron mobility in semiconductors may eventually cause a return to vacuum devices for the highest speed active electronic components. Researchers at CEMDAS are developing production processes for the fabrication of porous silicon field-emission cathodes. These cathodes have been shown to produce a two order of magnitude increase in the emission current over untreated silicon cathodes. This improved performance is attributed to the submicroscopic roughness which substantially increases the geometrical enhancement factor. This multitude of emission sites per cathode should also improve uniformity and reliability as well as reduce noise.  Current work is directed toward the development of a vacuum triode amplifier and a light-emitting diode.

Semiconductor Materials for Thermoelectric Application

    As chlorofluorocarbon (CFC)-based refrigeration systems come under increasing criticism for their contribution to the depletion of the world s ozone layer, the Seebeck effect and its application in solid-state cooling devices (SSCDs) is being considered as a replacement technology for traditional refrigeration systems. SSCD technology appears to be on the verge of significant material breakthroughs that have the potential to make the thermoelectric coefficient of performance for SSCDs exceed that of CFC-based refrigeration. Researchers at Texas A&M are engaged in doping certain semiconductors with phonon scattering impurities and measuring the resulting effects on thermal and electrical conductivities with the long-term goal of improving the thermoelectric figure of merit for SSCD applications. The work is a collaborative effort between Texas A&M and Marlow Industries, Inc., of Dallas, Tex., the world s largest supplier of commercial SSCDs.

Microwave Circuit and Device Research

    As application of microwave technology in the commercial sector has escalated in recent years, so has the demand for inexpensive, power-efficient, small, and reliable components.  Such components are needed for wireless communications, satellite communications, sensors, radio-frequency identification systems, and security systems. Microwave component research at Texas A&M encompasses radio-frequency, microwave and millimeter-wave circuits and devices; active antennas and power combiners; wireless communications and sensors; wireless power transmission in space; antennas and phased arrays; microwave device and circuit modeling; and quasi-optical components. Optoelectronic circuits for microwave application are also of considerable interest.
    Many novel microwave circuits and devices have been invented and/or developed for the first time at Texas A&M. These include a frequency tunable microwave ring resonator with over 20% tuning range, various active antenna elements and power combiners created by integrating Gunn and FET devices with notch and patch antennas, an electronic tunable coplanar waveguide filter with over 26% tuning range, wideband voltage tunable oscillators, aperture-coupled microstrip interconnects, a 35 GHz rectenna detector with 60% conversion efficiency, new high-T superconductor characterization methods and devices, wideband coplanar waveguide components and hybrid couplers with octave bandwidth, a low-loss Fabry-Perot millimeter-wave filter, novel feedforward power amplifiers with low intermodulation, power amplifiers for wireless applications, wireless RF sensors, a 15 GHz PHEMT oscillator with 58% efficiency, a microstrip flat reflectarray antenna, 30 GHz low-cost phased arrays for beam steering, dual-frequency antennas, optoelectronic parametric amplifiers with > 20 dB gain, and traveling wave photodetectors.

Electro-Optic Device Research

    The electro-optics program encompasses a range of technologies that make use of optical and electronic phenomena. Research areas of primary interest include fiber optics, integrated optics, and semiconductor lasers. We seek to exploit the special attributes of optical fibers (high information capacity, low optical loss, small size, mechanical flexibility) and diode lasers (small size, low electrical power consumption, high-speed modulation capability) in solving problems of current technological interest.  Application areas addressed in our research include communication, sensing, and instrumentation.

Integrated Optics

    Integrated optics is concerned with the development of thin-film components for performing functions such as modulation, switching, and multiplexing of guided light beams. Much of the research is oriented towards applications in optical fiber communications, a field in which technology is evolving rapidly to keep pace with seemingly insatiable demands for bandwidth. We are exploring new materials and fabrication methods for integrated optical waveguides as well as the demonstration of new device concepts.
    A novel technique developed at Texas A&M for making low-loss optical waveguides using the static strain optic effect has made it possible for the first time to make guided wave devices in tungsten bronze ferroelectric materials SBN and BSTN. These  super-EO materials  feature exceptionally large electro-optic coefficients, which are exploited to realize optical modulators and switches with very low electrical power consumption. The discovery in our laboratories that these materials have very low susceptibility to optical damage makes them attractive candidates for fabricating nonlinear frequency converters and guided-wave devices for emerging applications at visible wavelengths.
    The recent emergence of wavelength-division-multiplexing in fiber-optic systems has created a need for tunable filters for combining and separating the wavelength channels. At Texas A&M, a novel, tunable add-drop filter has been produced using a phase matched strain-induced grating for polarization coupling in diffused waveguides in a lithium tantalate substrate. It is projected that extending this work to lithium niobate substrates will make it possible to meet the international standard 100 GHz spacing between adjacent wavelength channels in a polarization-independent filter with unprecedented tuning speed and tuning range.

Fiber-Optic Sensors

    A patented technique for making internal mirrors in optical fibers forms the basis for a new class of sensors with the fiber Fabry Perot interferometer (FFPI) as the active element. In extensive field tests, these sensors have been used to measure gas pressure in internal combustion engines, liquid pressure in pumps, and strain in civil structures. The fiber-optic sensors have shown an unprecedented combination of high sensitivity, ability to endure extreme temperatures, and immunity from electromagnetic interference.  Laboratory work is continuing on transducers for acceleration and temperature.
    Another class of fiber-optic sensors based upon the Sagnac interferometer is also being vigorously pursued. Exceptional performance for measurement of electrical current, rotation rate, and liquid flow rate has been achieved. Fiber-optic sensors developed at Texas A&M are being commercialized by Honeywell, Phoenix, Ariz.; and by FFPI Industries, Bryan, Tex.

Semiconductor and Fiber Lasers

    Semiconductor lasers are studied for application in fiber-optic networks. A theory developed to explain the phenomenon of nonlinear gain in semiconductor lasers, after experimental verification, has been used in the design of lasers that can operate at frequencies to 40 GHz. The theory has also been extended to the prediction of the noise and intermodulation distortion levels in subcarrier-multiplexed optical communication systems. A world record in frequency stability for semiconductor lasers has been achieved by locking the optical output to a fiber-optic resonator. Tunable, line-narrowed lasers are being studied for use in spectroscopy and for generation of microwave signals.  Application of erbium-doped fiber amplifiers for generation of high- power optical pulses for nonlinear wavelength conversion in optical fibers is under investigation.

System Research

    New microwave, electro-optic, and solid-state devices developed in our laboratories are frequently used in the demonstration of  new system capabilities or improved system performance. We are interested in applying these components in areas such as wireless communications, optical communications, fiber-optic network diagnostics, magnetic resonance imaging, high-resolution optical imaging, industrial equipment monitoring, control systems for industrial machinery and processes. Some examples of system research are given below.

Communication Systems

    Much of the technology developed by CEMDAS faculty is targeted for application in the fast-growing areas of wireless, satellite, and fiber- optic communication. Microwave front-ends to 30 GHz are assembled using field-effect transistors (FETs) or high-electron-mobility transistors (HEMTs) as the power sources. Demonstration of dense wavelength division multiplexing for fiber-optic systems using tunable lasers and add-drop filters under development at Texas A&M is planned. The same electro-optical components that provide almost unlimited bandwidth can also be applied to multiplexed intersatellite links as a step beyond microwave communication.

Biomedical Imaging

    The Magnetic Resonance Systems Laboratory (MRSL) is developing technology for low-field nuclear magnetic resonsance (NMR) microscopy applications. In conjunction with Dr. Russ Huson of the Department of Physics at Texas A&M, MRSL researchers have demonstrated the potential for MR imaging at 0.22 Tesla. Using a desktop-sized magnet, images of okra plants have been obtained with 150 micron in-plane resolution in just over 1 minute.  The magnet was constructed from less than $2,500 of Nd-Fe-B magnet material. Future improvements in radio-frequency coil design have the potential to improve the in-plane resolution to less than 50 microns.  Potential applications include inexpensive magnetic resonance imaging and spectroscopy systems for on-line industrial process control as well as laboratory applications.
    Low-coherence optical interferometry is being applied for imaging of the internal structure of biological tissues. A novel signal processing scheme makes it possible to obtain three-dimensional images with sub-micron resolution with reflected optical powers in the picowatt range. This technique reveals structure in biological samples not seen using conventional methods. The same system can be applied to imaging of solid-state materials and devices.

Industrial Equipment and Process Monitoring and Control

    Fiber-optic sensors developed at Texas A&M have undergone extensive field tests. Combustion pressure sensors have been tested in large stationary engines operated by natural gas transmission companies: El Paso Energy and Transco, and in diesel locomotive engines manufactured by General Electric and General Motors. Data from the engine sensors is used in conjunction with control systems to reduce emissions and improve fuel economy. Liquid pressure sensors have been tested in large industrial pumps operated by Citgo and by Borg Warner, where they have shown a unique ability to detect the onset of harmful conditions, such as cavitation and surge. Strain sensors have been installed on a Union Pacific Railroad bridge for structural health monitoring.
    The U.S. semiconductor community faces new challenges as it moves towards production of circuits with feature sizes of a quarter micron and smaller. CEMDAS researchers are working to identify the key issues and problems facing the semiconductor industry in a number of critical processes involved in integrated circuit manufacturing, including: (a) single crystals (substrate materials), (b) thin-film deposition techniques, (c) rapid thermal processing, and (d) surface preparation. In particular, processing issues related to 12- inch and larger diameter bulk crystal growth of silicon, epitaxial thin film growth, bulk crystal growth of other electronic materials, and chemical/mechanical polishing are considered worthy for further consideration in the proposed project. As an example of one such process, the Czochralski (Cz) technique is used extensively for bulk crystal growth in both industry and university research labs. Research is in progress to improve the control of the Cz process in order to increase the yield and the quality of the crystals. This research effort encompasses both modeling and application of modern multivariable control theory. The overall goal is to develop a control scheme for the process that is based on a model that is as material independent as possible. Thus with only a few minor adjustments, the same controller can be used to grow different materials.

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