Many devices now include displays for presenting visual information. In general, a display has several attributes that affect its suitability for a particular purpose. Among these attributes are size, brightness, contrast, resolution, clarity, viewing angle, and energy consumption. In general, it is beneficial to manufacture high-resolution, energy-efficient, high contrast, wide viewing angle, bright displays. Furthermore, it is desirable to manufacture such displays at a low cost and with high reliability.
a and 5b schematically show asymmetrical current flow through an exemplary diode of
Active matrix liquid crystal displays are widely used in a variety of applications, including notebook computers, flat panel monitors, handheld computers, cellular phones, and flat panel televisions. Active matrix liquid crystal displays may be fabricated by depositing and patterning various metals, insulators, semiconductors, and/or other materials on substrates. Such displays commonly employ semiconductor devices, such as amorphous silicon (a-Si) thin film transistors. Each pixel in the active matrix liquid crystal display may be coupled to an address transistor, which controls the voltage on each pixel and therefore its transmittance.
A growing application for active matrix liquid crystal displays is in large area televisions, which may have a diagonal size of up to 50 inches or more. However, thin film transistor controlled pixel arrays are difficult to manufacture for this application since a relatively large number of process steps are required to construct the thin film transistors. The total mask count may be 5 or 6 or more, which is burdensome. While the yields for small displays can be quite high, it is difficult to obtain an acceptable yield for large area displays. In addition, the design rules for patterning the various insulator, metal, and semiconductor layers are the same for small and large thin film transistor liquid crystal displays, requiring expensive photo-exposure equipment for large area substrates. This all increases the manufacturing expense of such thin film transistor liquid crystal displays.
Thin film diodes, including those referred to as metal-insulator-metal diodes, can be more economical to fabricate than a-Si thin film transistors. When a single thin film diode is used in series with a liquid crystal pixel, any variation in the thin film diode characteristic across the display area or over time or temperature can lead to a variation in the pixel voltage. This can result in poor gray scale control, poor uniformity, slow response time, and/or image sticking. In addition, it is difficult, if not impossible, to scale up single thin film diode liquid crystal displays to a diagonal size larger than about 10 inches without severe brightness gradients.
However, a differential circuit having two thin film diodes per pixel and two select lines for each row of pixels may mitigate, if not eliminate, the drawbacks of the single thin film diode approach.
The fabrication of a dual select diode active matrix liquid crystal display is typically less difficult than that of thin film transistor arrays. In particular, dual select diode active matrix liquid crystal displays can be fabricated in fewer mask steps (typically only two or three), with relaxed design rules that scale with the display size. When operated in a dual select mode, the pixel circuit acts as an analog switch. The dual select diode circuit is not a two-terminal switching device, but rather a three-terminal switching device, like those that incorporate a thin film transistor. A dual select diode display offers performance similar to that of thin film transistor liquid crystal displays, with accurate gray shade control, fast response time, and tolerance for variations in thin film diode characteristics over time and across the viewing area. Such a dual select diode liquid crystal display is also relatively insensitive to propagation delays on the select and data lines and can therefore be scaled up to a very large area, for example, exceeding 40 in. in diagonal size.
As shown in
The dominant conduction mechanism in many thin film diodes, such as SiNx diodes, is Frenkel-Poole conduction. Frenkel-Poole conduction is largely a bulk effect dependent on the increase in free electrons caused by the effective lowering of trap level energy in a strong electric field. The equation for Frenkel-Poole current I is:
Where S is the diode area, V is applied voltage, d is film thickness and
is the electric field. κ and α are constants that depend on the temperature and on the ratio of Si to N in the SiNx film (when SiNx is used to form the insulating layer in the diode). Ideally, Frenkel-Poole conduction is a bulk effect and the diode characteristics are symmetric, i.e. the current is the same for V+ and V−. In practice, some asymmetry in thin film diodes can be caused by the nature (e.g. the different work functions) of the contact metal or transparent conductor. The work function determines the barrier for electron injection from the contacts. Also, the film stoichiometry and interface at the bottom contact and the top contact can be different, further contributing to the asymmetry.
Some thin film diodes, such as metal-insulator-metal diodes can have appreciable residual asymmetry. In other words, a current to voltage relationship will not be the same in both directions across the diode.
A thin film diode or bi-directional thin film diode, as used herein, is a nonlimiting example of a nonlinear resistive element.
In one embodiment, diode asymmetry can be compensated for and an undesired DC component can be cancelled by utilizing a drive scheme in which the data voltage and/or select voltages are offset. Such an approach can be difficult to effectively implement unless the diode asymmetry is small and uniform across the display area.
Nonlinear resistive elements can be orientated to present substantially equivalent resistances upon application of opposite polarity select pulses to the select lines. A pixel in which the nonlinear resistive elements are orientated in the same direction may effectively compensate for asymmetry regardless of the magnitude of asymmetry in the nonlinear resistive elements, or the variation in asymmetry across the display area. By orientating the nonlinear resistive elements in the same direction, the voltage drop will be approximately the same across each nonlinear resistive element, and an unacceptable DC component across the liquid crystal capacitor can be avoided. In such an arrangement, both nonlinear resistive elements will have approximately the same current-to-voltage characteristics because asymmetry has been compensated for by arranging the nonlinear resistive elements in the same direction. The voltage at the pixel node should settle to (Vs++Vs−)/2 regardless of the degree of asymmetry or its long range variation across the display area. Therefore, if Vs+=−Vs−, the voltage at the pixel node should settle to zero during the select time. The asymmetry can vary from pixel to pixel without creating a DC component across the liquid crystal when two nonlinear resistive elements within one pixel have substantially similar current-voltage characteristics.
A transparent conductor layer may include indium-tin-oxide (ITO) or another suitable material. An insulating layer may include a silicon nitride (SiNx) or another suitable material. A conducting layer may include a metal, such as aluminum, copper, tin, etc., or another suitable conductor. It should be understood that the disclosed materials are provided for exemplary purposes, and that other materials may be used while remaining within the scope of this disclosure.
Although the present disclosure has been provided with reference to the foregoing operational principles and embodiments, it will be apparent to those skilled in the art that various changes in form and detail may be made without departing from the spirit and scope defined in the appended claims. The present disclosure is intended to embrace all such alternatives, modifications and variances. Where the disclosure or claims recite “a,” “a first,” or “another” element, or the equivalent thereof, they should be interpreted to include one or more such elements, neither requiring nor excluding two or more such elements.
This application claims the benefit of U.S. Provisional Application No. 60/560,431, filed Apr. 7, 2004, which is incorporated by reference.
Number | Date | Country | |
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60560431 | Apr 2004 | US |