handheld games: Playing cards, aliens, fish and slot machines are just some of the electrode shapes you'll see!
There are two main types of LCDs used in computers, passive matrix and active matrix.
Passive-matrix LCDs use a simple grid to supply the charge to a particular pixel on the display. Creating the grid is quite a
process! It starts with two glass layers called substrates. One substrate is given columns and the other is given rows made
from a transparent conductive material. This is usually indium-tin oxide. The rows or columns are connected to integrated
circuits that control when a charge is sent down a particular column or row. The liquid crystal material is sandwiched
between the two glass substrates, and a polarizing film is added to the outer side of each substrate. To turn on a pixel, the
integrated circuit sends a charge down the correct column of one substrate and a ground activated on the correct row of the
other. The row and column intersect at the designated pixel, and that delivers the voltage to untwist the liquid crystals at that
pixel.
The simplicity of the passive-matrix system is beautiful, but it has significant drawbacks, notably slow response time and
imprecise voltage control. Response time refers to the LCD's ability to refresh the image displayed. The easiest way to
observe slow response time in a passive-matrix LCD is to move the mouse pointer quickly from one side of the screen to the
other. You will notice a series of "ghosts" following the pointer. Imprecise voltage control hinders the passive matrix's ability
to influence only one pixel at a time. When voltage is applied to untwist one pixel, the pixels around it also partially untwist,
which makes images appear fuzzy and lacking in contrast.
Active-matrix LCDs depend on thin film transistors (TFT). Basically, TFTs are tiny switching transistors and capacitors.
They are arranged in a matrix on a glass substrate. To address a particular pixel, the proper row is switched on, and then a
charge is sent down the correct column. Since all of the other rows that the column intersects are turned off, only the
capacitor at the designated pixel receives a charge. The capacitor is able to hold the charge until the next refresh cycle. And
if we carefully control the amount of voltage supplied to a crystal, we can make it untwist only enough to allow some light
through. By doing this in very exact, very small increments, LCDs can create a gray scale. Most displays today offer 256
levels of brightness per pixel.
Color and the Future
An LCD that can show colors must have three subpixels with red, green and blue color filters to create each color pixel.
Through the careful control and variation of the voltage applied, the intensity of each subpixel can range over 256 shades.
Combining the subpixels produces a possible palette of 16.8 million colors (256 shades of red x 256 shades of green x 256
shades of blue), as shown below. These color displays take an enormous number of transistors. For example, a typical
laptop computer supports resolutions up to 1,024x768. If we multiply 1,024 columns by 768 rows by 3 subpixels, we get
2,359,296 transistors etched onto the glass! If there is a problem with any of these transistors, it creates a "bad pixel" on the
display. Most active matrix displays have a few bad pixels scattered across the screen.
LCD technology is constantly evolving. LCDs today employ several variations of liquid crystal technology, including super
twisted nematics (STN), dual scan twisted nematics (DSTN), ferroelectric liquid crystal (FLC) and surface stabilized
ferroelectric liquid crystal (SSFLC).
Display size is limited by the quality-control problems faced by manufacturers. Simply put, to increase display size,
manufacturers must add more pixels and transistors. As they increase the number of pixels and transistors, they also
increase the chance of including a bad transistor in a display. Manufacturers of existing large LCDs often reject about 40
percent of the panels that come off the assembly line. The level of rejection directly affects LCD price since the sales of the
good LCDs must cover the cost of manufacturing both the good and bad ones. Only advances in manufacturing can lead to
affordable displays in bigger sizes.
For more information on LCDs and related topics, check out the links on the next page!