TFT-LCD（Thin Film Transistor-liquid Crystal Display）TFT-LCD was invented in 1960. Over years of continuous improvements, it was formally adopted in commercial notebooks in 1991. As technologies mature gradually, TFT-LCD continues to replace CRT products in all kinds of applications and is now the mainstream of display technology.
The advantages of the LCD screen are low power consumption, low voltage run, high resolution, radiation-free, thinner and lighter screens, portability and long service time, which contribute to widespread applications in numerous fields, such as TVs, monitors, notebooks, mobile phones, GPS devices and PDAs.
A LCD display works in two basic modes: the normal black mode and the normal white mode. Here is an example of the normal white mode: As the picture below on the left (LCD gray-scale display theory) shows, when no voltage is applied on the liquid crystals, the cylindrical liquid crystals are aligned in an almost flat position, allowing the maximum amount of light to pass through both the front and back polarizers to achieve brightness. When a voltage is applied on the liquid crystals, they are aligned at angles: the higher the voltage, the steeper the angle. As a result, less light is transmitted through the polarizers. Light can barely pass when the liquid crystals stand vertically, resulting in a dark display. Accordingly, the voltage is adjusted to create the gray-scale. The picture below on the right (LCD V-T curve) shows that the transmittance of the panel decreases as the voltage applied on liquid crystals increases.
A TFT component is an electric-controlled switch. A scan line (gate electrode) controls the switch's opening and closing, while a data line (source electrode) supports different levels of gray-scale voltage needed for LCD illumination. When a high voltage is applied to the scan line, the TFT components are opened, introducing a gray-scale voltage from the data line. The gray-scale voltage is then applied on the liquid crystal layer through transparent pixel electrodes (drain electrodes), changing the angles of the liquid crystals to achieve the needed gray-scale.
The display area of the whole TFT-LCD panel consists of a matrix of pixels controlled by millions of independent TFT components. As the picture below shows, when the scan line opens the third line of the matrix, the data line will apply the necessary gray-scale voltage to the pixels in the third line and then close the third line, so the voltage can be kept. Meanwhile, the fourth scan line is opened and the necessary gray-scale voltage is applied to the pixels in the fourth line, which will then be turned off upon completion of the process. Then the fifth line is turned on to apply the necessary gray-scale voltage to the pixels in different lines of the panel in sequence.