Article

Microwave characteristics of liquid-crystal tunable capacitors

Inst. of Electron. Eng., Nat. Tsing-Hua Univ., Hsinchu, Taiwan
IEEE Electron Device Letters (Impact Factor: 3.02). 08/2005; 26(7):451 - 453. DOI: 10.1109/LED.2005.851118
Source: IEEE Xplore

ABSTRACT This letter investigates the microwave characteristics of the liquid crystal tunable capacitors for the first time. With the dielectric anisotropy properties, the liquid crystal capacitors present very different characteristics compared to the semiconductor or MEMS tunable capacitors. A quality factor of 310 with a control voltage of 5 V was achieved at 4 GHz. A tuning range of 25.3% for the control voltages from 0 to 5 V was obtained at 5 GHz. The results demonstrate the potential applications of liquid crystals as dielectric materials for capacitors with high quality factors and wide tuning ranges at high frequencies, particularly suitable for the future flexible electronics with transparent substrates.

3 Followers
 · 
302 Views
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: A regression-based technique is presented which distinguishes the dielectric loss from the resistive loss of high density planar capacitors in a very wide bandwidth of 0.1-8 GHz. Moreover, the procedure yields useful results if the capacitor deviates from a lumped element model and indicates when the used approximations break down or whether size-dependent loss mechanisms exist.
    Microelectronic Test Structures, 2008. ICMTS 2008. IEEE International Conference on; 04/2008
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Three different configurations of liquid-crystal tunable capacitors were investigated, including fractal structures, comb structures, and square structures. The tuning ranges of the fractal structures, the comb structures, and the square structures at 4 GHz from 0 to 5 V were found to be 8.81%, 7.17%, and 11.85%, respectively. The square structures have the largest tuning ranges mostly because of the uniform electric fields dominated. The Q-values of the fractal structures, the comb structures, and the square structures at 4 GHz with an operation voltage of 5 V were 68.2, 98.3, and 31.0, respectively. Compared to the other two structures, the square structures perform the highest inductive effects, leading to the lowest Q-values. In addition, the liquid crystals with different chiral doping concentrations were injected into the capacitor structures to investigate the impact of different liquid crystals on the tunable capacitors. The threshold voltages (V<sub>th</sub>) increase as the chiral doping concentrations enlarge. The steepness of the C-V curves increases as the chiral doping concentrations reduce. Furthermore, a liquid-crystal capacitor model was developed to qualitatively illustrate the performance of the liquid-crystal tunable capacitors. The model prediction and the measurement results were highly correlated
    IEEE Transactions on Electron Devices 08/2006; 53(7-53):1675 - 1682. DOI:10.1109/TED.2006.875818 · 2.36 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: El presente trabajo aborda de forma pormenorizada diferentes aspectos relativos al comportamiento eléctrico y óptico de varias tecnologías de materiales cristal líquido (CL), así como sus potenciales aplicaciones en el ámbito de los sistemas de comunicaciones. Inicialmente, se presenta una breve introducción sobre los cristales líquidos (CL). Se describen sus principales características eléctricas y ópticas, haciendo hincapié en las estructuras que se utilizan en este trabajo. También se hace una revisión sobre el estado actual y futuro de las pantallas de cristal líquido (LCD) de pequeño y gran formato, que es la aplicación industrial más importante a nivel de mercado de los dispositivos de CL, y, adicionalmente, se presentan otras aplicaciones para sistemas de comunicaciones que no están relacionadas directamente con la presentación de imágenes. A continuación, se presenta un protocolo experimental de caracterización eléctrica de dispositivos basados en CL. Previamente a la descripción del protocolo desarrollado, se incluye el estado del arte de los métodos de caracterización eléctrica que se utilizan actualmente para cada uno de los dispositivos utilizados en este trabajo, y se analizan los modelos más adecuados. En este contexto, se explica el procedimiento utilizado para la obtención de los parámetros de cada uno de los circuitos eléctricos equivalentes (CEE) en las diferentes tecnologías de CL estudiadas. Se ha prestado especial atención al diseño de formas de onda para el proceso de caracterización, enfocado en realizar la medida de impedancia para diferentes estados de transmisión. Posteriormente, los parámetros obtenidos del CEE son validados comparando la medida experimental de la corriente que circula por el dispositivo de CL para diferentes señales de tensión aplicadas entre sus terminales y la corriente simulada (con una herramienta comercial de simulación de circuitos eléctricos) en el CEE obtenido previamente. Por otro lado, gracias a un mejor conocimiento eléctrico de estos dispositivos, se propone el diseño, la caracterización eléctrica y la utilización de estos dispositivos basados en CL en aplicaciones de tipo no fotónico, en concreto, en sistemas de comunicaciones en el rango de radiofrecuencia. En este sentido, se han diseñado filtros y osciladores de frecuencia variable controlados electrónicamente (VCO) basados en dispositivos de CL Nemático Torsionado (TN). Estos circuitos se han implementado en el laboratorio utilizando componentes electrónicos comerciales, se han caracterizado experimentalmente y se han validado estos resultados con los modelos teóricos propuestos para dicho tipo de dispositivos. Finalmente, se ha realizado el desarrollo de aplicaciones de alto valor añadido basadas en dispositivos de CL en ámbitos como la seguridad y la discapacidad. En concreto, se ha desarrollado un sistema de ayuda a la visión en pacientes con problemas de baja visión que es capaz de realzar las formas y los contornos de los objetos y que permite a dichos usuarios moverse de forma autónoma en cualquier tipo de entorno. Además, se describe también de forma esquemática el protocolo de comunicaciones y la optimización electroóptica realizada para un sistema de visión integrado en un casco de bomberos, cuyo visor es una micropantalla de CL antiferroeléctrico de alta resolución, y que permite presentar información de contexto al bombero, proporcionándole una mayor seguridad y eficiencia en el trabajo. ________________________________________ This work tackles in detail different aspects regarding the electrical and optical behaviour of several liquid crystal (LC) material technologies, as well as their potential applications within the scope of communication systems. Initially, LC materials are briefly introduced and their main electrical and optical characteristics are described. Moreover, a revision of the current and future trends of small and large-sized liquid crystal displays (LCD) is presented. Nowadays, LC devices market is mainly focused on displays to be used either as conventional TV or integrated in many multimedia devices (PDA, glasses for 3D cinema, digital cameras, etc.). Additionally, other applications for communication systems that are not directly linked to the image presentations are introduced. An experimental protocol for electrical characterization of LC-based devices is presented. Previously to the protocol description, the state-of-art of the electrical characterization methods currently used is included, and the most suitable electrical models are analyzed. In this context, the procedure to derive the parameters for each electrical equivalent circuit (EEC) is explained. A special attention has been rendered to the design of voltage waveforms for the characterization process. Subsequently, the parameters obtained from the EEC are validated by comparing the experimental measurement of the electrical current across the LC device with the simulated electrical current (using a commercial simulation tool). On the other hand, as a consequence of the electrical model obtained for these devices, the use of LC-based devices for non-photonic applications is proposed and, particularly, for communication systems in the radio frequency range. In this context, voltage-controlled variable-frequency filters and oscillators (VCO), based on twisted nematic (TN) LC structures, have been designed. These circuits have been implemented in the laboratory using commercial electronic components, and experimentally characterized from the electrical point of view. These results have been validated by comparing with the theoretical results derived of each model of circuit. Finally, the development of high-added value applications based on LC devices in fields related to safety and disability is shown. In fact, an assisted vision system has been developed for visually impaired people. This system is able to highlight the shapes of objects allowing to users to move autonomously in unknown environments. Furthermore, we also describe schematically the communications protocol developed and the electrooptical optimization performed in a vision system integrated in a fire-fighter helmet. In this system, a high-resolution antiferroelectric LC microdisplay was used, which enables to show relevant information to the fire-fighter, improving the safety and efficiency in this kind of dangerous tasks.

Preview

Download
12 Downloads
Available from