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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