α-Si TFT (amorphous silicon thin film transistor) active matrix liquid crystal display is an active matrix liquid crystal display device based on amorphous silicon thin film transistor technology. Its core structure and working principle are as follows:
I. Core structure
Glass substrate
It consists of two glass substrates with a liquid crystal layer sealed in the middle.
The lower substrate is etched with scanning lines (row lines) and addressing lines (column lines), and TFT active devices and pixel electrodes are made at the intersection.
The upper substrate is covered with transparent electrodes (such as indium tin oxide, ITO) and polarizers.
Amorphous silicon thin film transistor (α-Si TFT)
Each pixel is equipped with a TFT as a switching element to control the on and off of the pixel.
The structure includes: gate, source, drain, amorphous silicon semiconductor layer (a-Si) and storage capacitor (Cs).
In the bottom gate structure (mainstream), the gate is located at the bottom of the substrate, which can block the backlight and avoid light leakage of the a-Si layer.
Liquid crystal layer and polarizer
The liquid crystal layer is sandwiched between two glass substrates, and the molecular arrangement is controlled by the electric field to adjust the transmittance.
The upper and lower polarizers are perpendicular to each other, and only light with a specific polarization direction is allowed to pass through.
Backlight and color filter
The LED backlight provides uniform white light, which is diffused by the light guide plate and illuminates the liquid crystal layer.
The color filter (RGB) decomposes white light into three primary colors, which are mixed to form a full-color image.
II. Working principle
Backlight excitation and polarization
The LED backlight emits white light, which is evenly diffused by the light guide plate and becomes single-direction polarized light through the lower polarizer.
TFT controls pixel transmittance
Row driver (Gate Driver): Open the gate of the TFT row by row to make the pixels of the corresponding row in a writable state.
Column driver (Source Driver): Write the grayscale voltage into the pixel capacitor and transmit it to the liquid crystal layer through the source.
Storage capacitor (Cs): Maintain voltage stability to ensure continuous deflection of liquid crystal molecules and avoid screen flickering.
Liquid crystal molecule deflection and light transmission adjustment
Liquid crystal molecules rotate under the action of electric field, changing the polarization direction of light.
After the transmitted light passes through the upper polarizer, only the light in the same vibration direction passes through, forming bright or dark pixels.
Color synthesis
The transmitted white light is decomposed into three primary colors by the RGB color filter, and mixed to form a full-color image.
III. Technical characteristics
Advantages
High resolution and large size: amorphous silicon technology is mature and suitable for manufacturing large-size screens (such as TVs and monitors).
Low cost: glass substrates are cheap and the manufacturing process is simple.
High contrast and wide viewing angle: active matrix drive eliminates crosstalk and supports wide viewing angle technologies such as IPS.
Environmental protection characteristics: no flicker, no radiation, low power consumption, in line with the "green computer" standard.
Limitations
Low electron mobility: the electron mobility of a-Si (about 0.5 cm²/V·s) is lower than that of polycrystalline silicon (LTPS), limiting high-frequency response.
Photosensitivity: a-Si is sensitive to light and requires a light shielding layer or bottom gate structure to prevent leakage.
Resolution bottleneck: Compared with LTPS, the pixel size is larger, making it difficult to achieve ultra-high resolution (such as mobile phone screens).
IV. Application scenarios
Mainstream market: medium and large-sized display devices such as TVs, computer monitors, laptops, and industrial control screens.
Technology evolution: By combining Mini-LED backlight, high refresh rate (such as 144Hz) and other technologies, the image quality performance is continuously improved.
Summary
α-Si TFT active matrix liquid crystal display is based on amorphous silicon thin film transistors. It achieves high-precision pixel control through active matrix drive. It has both cost advantages and mature processes. It is the mainstream technology in the field of medium and large-sized liquid crystal displays.