What Are TFT Displays?

Many companies have adopted Thin Film Transistor (TFT) technology to improve color screens. In a TFT screen, also known as active matrix, an extra matrix of transistors is connected to the LCD panel—one transistor for each color (RGB) of each pixel.

These transistors drive the pixels, eliminating the problems of ghosting and slow response speed that afflict non-TFT-LCDs. The result is screen response times of the order of 25 ms, contrast ratios in the region of 200:1 to 400:1, and brightness values between 200 and 250 cd/m2 (candela per square meter).

The liquid crystal elements of each pixel are arranged so that in their normal state (with no voltage applied) the light coming through the passive filter is “incorrectly” polarized and thus blocked. But when a voltage is applied across the liquid crystal elements they twist up to ninety degrees in proportion to the voltage, changing their polarization and letting more light through.

The transistors control the degree of twist and hence the intensity of the red, green, and blue elements of each pixel forming the image on the display. Thin film transistor screens can be made much thinner than LCDs, making them lighter. They also have refresh rates now approaching those of CRTs because current runs about ten times faster in a TFT than in a DSTN screen.

Standard VGA screens need 921,000 transistors (640 x 480 x 3), while a resolution of 1024 x 768 needs 2,359,296, and each transistor must be perfect. The complete matrix of transistors has to be produced on a single, expensive silicon wafer, and the presence of more than a couple of impurities means that the whole wafer must be discarded.

This leads to a high wastage rate and is the main reason for the high price of TFT displays. It’s also the reason why there are liable to be a couple of defective pixels where the transistors have failed in any TFT display.

There are two phenomena that define a defective LCD pixel: a “lit” pixel, which appears as one or several randomly placed red, blue and/or green pixel elements on an all-black background, or a “missing” or “dead” pixel, which appears as a black dot on all-white backgrounds.

The former failure mode is the more common, and is the result of a transistor occasionally shorting in the “on” state and resulting in a permanently “turned-on” (red, green or blue) pixel. Unfortunately, fixing the transistor itself is not possible after assembly. It is possible to disable an offending transistor using a laser.

However, this just creates black dots that would appear on a white background. Permanently turned-on pixels are a fairly common occurrence in LCD manufacturing, and LCD manufacturers set limits, based on user feedback and manufacturing cost data, as to how many defective pixels are acceptable for a given LCD panel.

The goal in setting these limits is to maintain reasonable product pricing while minimizing the degree of user distraction from defective pixels. For example, a 1024 x 768 native resolution panel, containing a total of 2,359,296 (1024 x 768 x 3) pixels, that has 20 defective pixels would have a pixel defect rate of (20/2,359,296)*100 = 0.0008%. The TFT display has undergone significant evolution since the days of the early, twisted Nnematic (TN) technology based panels.

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