LCD (theory) FAQ


  3.5) Driving Methods: Active Matrix Displays

In order to eliminate the problems of the STN/passive matrix display, the active matrix display was developed. Active matrix displays have a thin film Transistor or diode on the glass substrate that indirectly addresses each pixel element. Depending on the display type, application, or manufacturer, the TFT may be comprised of amorphous silicon (a-Si) or polycrystalline silicon (p-Si), The TFT completely isolates one pixel element from the others in the display and eliminates the problem of partially active pixels. Simply put, the TFT acts as a switch ! When a row of TFTs are addressed the gate lines are active-- the switch is turned on, this allows charge to flow from the columns into the pixels and set the image for the frame cycle. Once a row has been addressed, the gate line is reversed biased (the switch is turned off) to insure that no charge can pass from the columns into the pixel element. Thus, the pixel is now completely isolated as the rest of the display is addressed. The LC material acts as a capacitor and stores charge. After a charge is placed on a liquid crystal cell ( the defined pixel area), it begins draining similar to a discharging capacitor (an exponential function). As a result, unless the display can be written quickly (all 480 rows scanned and the return to the top of the display to rescan starting from row 1 for a VGA display) the image will not be uniform from the top to the bottom of the display as the LC material starts to untwist. In order to insure charge storage for one frame and carefully control charge on a pixel element, TFT displays incorporate a second capacitor in parallel with the LC material. The combined capacitance gives active matrix displays the essential capability to accurately maintain the amount of charge applied; thus reliable partial charges can be utilized and gray scale or full color displays are possible. With proper drivers and high quality TFTs, 256 gray scales have been obtained with quality that surpasses that of CRTs. TN material can also switch much faster than STN, thus 40 millisecond TFT displays are common yielding CRT like speed.

COMMONLY ASKED QUESTION NOTE: What is cross Talk? Cross talk is best described as the effect when a dark image (box or widow shape) is placed in the middle of a white background. Faint vertical and horizontal lines will be seen from the edge of the window proceeding to the edges of the screen. This can be caused by poorly designed drivers or poorly made TFTs. The off selected TFTs are not completely off and some charge, very strong at the edge of a window, has leaked into the pixels creating the effect. This is a common problem and is being addressed by the manufacturers of TFT based displays.

  3.6) Color Display Pixel Layout and Yields

In order to build a fully functional color VGA display, a TFT LCD must have 480 X 640 X 3 pixel elements. The 640 X 480 is the well understood VGA pixel layout (640 X 400 for the Apple Powerbook series and 640 X 480 for the 180c), except 640 red, 640 green, and 640 blue columns or stripes of color pixels are now required. This is a total of 921,600 TFTs that must work in order to build a perfect display. Using a semiconductor analogy, it is similar to building a 1 Megabit DRAM on a 10 inch glass plate; not an easy task considering a particle smaller than the diameter of the human hair can destroy a single TFT. Achieving a 100 % yield or a perfect display is virtually impossible, thus even though manufacturer yields are starting to reach 60% (for sellable devices) prices are very high. Furthermore, if 4 defective pixels are found on a color VGA screen, this already represents a 99.99 % pixel yield-- not bad for any process. For the most part, the most common pixel defect is caused by some form of contamination damaging a TFT and preventing it from turning the pixel off (seen as bright spots on a dark background). There are two layouts for pixels on TFT displays. The most common for computer applications is the STRIPE layout. A stripe layout has repeating stripes of red, green, and blue columns across the display. For multimedia and high density arrays (projection LCD modules), a triad pixel layout is used. A triad layout has the three color sub-pixels in a triangle shape. Figure 7 illustrates the difference between the two layouts.

Figure 7: Color Pixel Layouts

Stripe Layout				Triad Layout
RGB	RGB	RGB 			 R     R     R

Note: What is a color filter? A color filter works by absorbing specific wavelengths of light and only passing light of a certain wavelengths (In other words, a red filter will remove all wavelengths of light except for red -- thus it looks red !). White light is made up of a spectrum of wavelengths, so it can yield the red, green, and blue for displays. However, when filtering out the unwanted wavelengths, the overall brightness is reduced.

COMMONLY ASKED QUESTION NOTE: What is a pixel? Unfortunately, in most of the literature and magazines, there is not a clear definition as to what a pixel is. In its most basic form, a pixel is described as one element on a display screen. For a monochrome screen this is an adequate description. However a color pixel is actually made up of three subpixels: a red, green, and blue pixel. This is sometimes called a pixel triad. Therefore care must be taken in describing pixels. In terms of this document, a pixel is the entire element consisting of red, green, and blue sub subelements. A subpixel consists of the individual red, green, or blue elements. Gray scales for LC displays are always calculated as a function of subpixels.

  3.7) Color Displays: Gray Scales and Bits

Due to the overall poor performance of Passive Matrix color displays, only active matrix displays will be specifically discussed. However, major points are applicable to both display addressing technologies. Unlike an analog CRT, in a digital color TFT active matrix display, you literally get what you buy...forever. Even if you upgrade to a new video driver or display card, you will still have the same number of colors and gray scales. The number of colors is a direct result of the number of gray scales a display can reproduce. The standard VGA format is rated to display 256 colors, however it can select from a 18 BIT CLUT (color look up table) which means the choice of 262,144 colors(this calculation is based on a bit calculation for a pixel triad -- 2 ^ 18 --- see later section on calculations). Intrinsic gray scale reproducibility for TFT displays is a result of two factors: the quality of the driver ICs used on the display and the resistance of the gate metal(the rows of the display). The gate metal must carry a clear and undeformed pulse from one end of the display to the other( 640 X 3 = 19200 lines). If the pulse is not maintained the TN curve will not charge to the desired level and the correct color can not be displayed. Therefore, the more gray scales required, the greater the control that must be exerted over the gate lines. For example, most displays sold today can display 256 colors out of 4092 or 512. The 256 colors is based on the VGA video controller, the 4092 is a display limitation. 4092 possible colors indicates that a display can reproduce 16 gray scales. This is derived from 16 (red) X 16 (blue) X 16 (green) = 4092 possible colors. Once again, dithering can be used to extend this, but there are displays in limited production that can reproduce 256 gray scales or more than 16 million colors ! Most current TFT color displays feature 3 bit drivers (where 2 raised to the third power yields 8); these drivers can produce a total of 512 colors. This is more than adequate unless later on you decide you wish to pursue some multimedia functions which require more than 32 levels of gray scale. Although the controller and the computer may be fast enough to handle the functions, 8 or 16 gray scales will be inadequate-- your image will not be what you expect (It will look like a collection of color shadows). Sharp has recently demonstrated 10 inch 640 X 480 displays running on Apple Macintoshes displaying 64 gray scales. These 6 bit drivers are supposedly entering production and will enter the commercial market shortly. The color reproduction of these displays is excellent.

  3.8) Understanding Digital Color Pixels

4 Bits, 8 Bits, 16 Bits, 24 Bits Just how many colors can they actually generate ? Digital video divides the number of colors or gray scales into a distinct number of points. Based on these "POINTS" the system can generate a fixed number of colors or gray scales. Manufacturers tend to play games with numbers, so sometimes it is very difficult to understand "BIT" color talk. First of all, the bit system is based on the binary system so: 1 Bit color, which is 2 raised to the first power is 2. In other words a black & white display where the pixel has a state of being either on or off. This can currently be extended to 24 bits which (at 2 to the 24 power) yields more than 16 million gray scales. OK, now that we understand how gray scales are calculated, lets convert this to a color display: Once again, manufacturers play a game with numbers and here we introduce bpp or BITS PER PIXEL. Now depending on manufacturer, a pixel can be made up of 1 subpixel (the individual Red, Green, and Blue pixels) or can be a composite of all three colors. If we examine a 16bpp system the following calculations are applicable: 2 raised to the 16th power is: 65536. So if the system is 16 bpp for the combined primary colors, the system can produce a total of 65536 colors. If the system produces 16 bpp for all three colors then 65536 X 65536 X 65536 = 2.8 X 10^14 colors. The small table below summarizes the Bits confusion. Triad Pixel refers to a combination of the RED/GREEN/BLUE pixels. Subpixel refers to the individual red, green, or blue pixel. The number of gray scales for a monochrome display is always the same as a triad calculation bpp display. The numbers listed down the columns refer to how many gray scales or colors that a system configuration can produce. CRT based color is usually calculated as the triad pixel calculation.

Bits/Colors    Mono or Triad Pixel	Subpixel (R/G/B)
1			2		8
4			16		4096
8			256		1.68X10^7
16			65536		2.81X10^14
24			1.68X10^7	4.72X10^21

COMMONLY ASKED QUESTION NOTE: What is analog video? Unlike digital or bit based video analog video is based on a continuous flow of data. The wave form can be thought of as a continuous wave of points with the distance between points so small that it is impossible to differentiate between them. In other words, it can theoretically provide an infinite number of gray scales. VGA is an analog system and VGA CRTs are analog displays. The advantage of an analog display is that when you upgrade your video card and drivers to handle more colors, your existing monitor should be able to operate with the extended color ranges. NEC makes an analog XVGA TFT LCD, but due to power handling requirements, it is not suitable for battery based portable computers.

COMMONLY ASKED QUESTION NOTE: Why are TFT Color Displays Expensive ? There are numerous reasons for this. As discussed above, displays with large numbers of defective pixels can not be sold and as a result, yield is usually thought to be the major problem. In reality, one should be aware that the largest cost of TFT displays are the materials utilized for production. Since Japanese manufacturers have not standardized the size of displays yet, each manufacturer has specific material needs (glass, holders for machines, robots, and etc.). This fact alone keeps display prices extremely high since material and machine suppliers can not make standard parts for an entire industry at this time.

COMMONLY ASKED QUESTION NOTE: How many gray scales are required for multimedia operations ? Usually 64 gray scales or more are required for true multimedia operations. TFT LCDs with 64 gray scales will probably be available in volume within a year.

COMMENT: Why are some color TFT displays much brighter than others ? The brightness of a screen is determined by two related factors; the size of the screen and the aperture ratio of the pixels. On the surface of an active matrix array there are both pixels and electronics, as a result of the opaque electronics, some of the area that light could pass through is blocked. The ratio of light passing through the pixel to the entire area of the pixel and associated electronics is called the aperture ratio. The larger the ratio is, the more light that can pass through the pixel and the brighter the image on the display will be. Furthermore, if the display itself is bigger, there is more room for the pixels and the result is more light passing through the individual pixels. For this reason, DTIs 10 inch display found in the IBM Thinkpad 700c is much brighter than some of the smaller 8.4 and 9.5 inch TFT displays.

  3.9) Basic Principles of TFT Operation

For all intensive purposes, a TFT can simply be considered a switch; when selected (on) it allows charge to flow through it and when off it acts as an barrier preventing or at least restricting the flow of charge. As mentioned earlier, a TFT is a MOS FET device or a Metal Oxide Semiconductor Field Effect Transistor. The gate line can be considered the "switch" of the transistor, with this you turn it on, partially on, or off. The Source and drain are the entrance and exit, respectively, for the charge you want to pass through the switch. In the case of a display, this is the charge that you want to appear on the pixel. Looking at Figure 7, source and drain metal electrodes are separated by an amorphous silicon (a-Si) semiconductor layer; with the absence of charge the a-Si layer acts as an insulator or resistor and prevents the flow of charge from the source to drain; thus isolating the pixel from the rest of the display. SiNx or silicon Nitride is the gate insulator and forms the gate dielectric; electrons do not pass from the gate line into the transistor, but are used to influence the charge distribution in the semiconductor layer. A MOS FET that fits this description (you turn it on)is called an enhancement device. When a positive charge is placed on the gate line, electrons (or negatively charged particles) will begin to collect in the area above the gate, on the other side of the Silicon Nitride (SiNx) in the a-Si. When the charge on the gate is increased to a certain point, called the VT or threshold voltage, enough electrons will have collected in the a-Si to change it from an insulator to a conductor. In other words, you build up a channel of electrons, so if there are electrons at the source (high) and nothing at the drain (low), the electrons will begin to move through the electron filled channel until the charge is the same at both sides or you turn the transistor off. The result is a charging of the pixel and a change in the state of the liquid crystals. The unique aspect of this device is the nonlinear characteristics after the TFT passes through Vt. It exponentially moves to a conduction state (usually 6 to 8 orders of magnitude) and makes it very easy to turn a TFT on or off around the Vt value. For more information on MOS FET device operation, pick up a book on Semiconductor Physics or Solid State Physics. The above is only meant as a basic simplified description of MOS device operation.

  3.10) TFT Connections

The gate line of the TFT determines whether or not the TFT will pass a charge into the pixel. These are controlled by the row bus-lines. On a standard VGA display, the gate lines would be the 480 horizontal lines. The source lines of the TFT are connected to the column or data bus-lines. These lines provide the charge for the pixel or contain the data for the image. The drain lines of the TFT are directly attached to the ITO pixel, this transfers the charge from the semiconductor region into the pixel.

Figure 7: Thin Film Transistor Cross Section

^^^^^^^^^^^^^^^^^^^^^^^^	^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
	Source Metal	^	^	Drain Metal
			^	^		
		    $	SiNx	     $	
	*		a-Si	Semiconductor	 *		
*********					 *************
		~		SiNx	    ~
		~    __________________     ~
~~~~~~~~~~~~~~~~~   |		       |    ~~~~~~~~~~~~~~~~~~
		    |	gate	       |



  3.11) References

  3.12) Acknowledgments

I would like to thank Mike Schuster (SCHUSTER@PANIX.COM) for commenting on clearness and general understanding while compiling this FAQ.

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