What is the difference between TFT and LCD?
LCD (Liquid Crystal Display). Generally divided into monochrome and color LCD screens, monochrome LCD has almost quit from the notebook market, while the color LCD continues to develop. Color LCD is mainly divided into STN and TFT. Among them, TFT (Thin Film Transistor) LCD, also known as active electro-crystalline thin film transistor liquid crystal display, it is a true color LCD screen commonly known by many people; DSTN (Dual -Scn Twisted Nematic) LCD, which is a dual scan LCD screen. It is a display method of STN LCD and has now withdrawn from the market.
TFT is a variant of LCD, TFT, Thin Film Transistor, which is one of the active matrix type liquid crystal displays AM-LCD. TFTs are equipped with special light pipes on the back of the liquid crystal, which can be “active” on the screen. Independent pixels are controlled, which is the origin of the so-called active matrix TFT (Active Matrix TFT), which can greatly improve the reaction time. The reaction time of the TFT is relatively fast, about 80ms, and the STN is 200ms. Moreover, since the TFT is an active matrix LCD, the arrangement of the liquid crystals is memorable, and does not return to the original state immediately after the current disappears. The TFT also improves the STN flicker (water ripple)-blurring phenomenon, effectively improving the ability to play dynamic pictures. Compared with STN, TFT has excellent color saturation, reducing power and higher contrast, but the disadvantage is that it consumes more power and costs more.
Common liquid crystal displays are classified into four types according to their physical structure:
(1) TN-Twisted NemaTIc;
(2) Super twisted nematic (STN-Super TN);
(3) Double-layer super twisted nematic (DSTN-Dual Scan Tortuosity Nomograph);
(4) TFT-Thin Film Transistor.
The basic display principles of TN-LCD, STN-LCD and DSYN-LCD are the same, except that the twist angle of the liquid crystal molecules is different. The liquid crystal molecules of the STN-LCD have a twist angle of 180 degrees or even 270 degrees. The TFT-LCD uses a display mode that is distinct from the TN series LCD.
In a TFT-LCD, the function of the TFT is equivalent to a switching transistor. A commonly used TFT is a three-terminal device. A semiconductor layer is generally formed on a glass substrate, and has a source and a drain connected thereto at both ends thereof. And a gate insulating film is opposed to the semiconductor and a gate electrode is provided. The current applied between the source and drain electrodes is controlled by the voltage applied to the gate.
For the display screen, each pixel can be simplified in structure as a layer of liquid crystal sandwiched between the pixel electrode and the common electrode. More importantly, it can be considered as a capacitor from an electrical point of view. The equivalent circuit is shown in Figure 1. To charge the pixel P(i,j) of the j-row i column, the switch T(i,j) is turned on, and the target voltage is applied to the signal line D(i). When the pixel electrode is sufficiently charged, even if the switch is turned off, the charge in the capacitor is preserved, and the molecules of the liquid crystal layer between the electrodes continue to have a voltage application field. The role of the data (column) driver is to apply a target voltage to the signal line, and the gate (row) driver functions to turn the switch on and off. Since the display voltage applied to the liquid crystal layer can be stored in the storage capacitor of each pixel, the liquid crystal layer can be stably operated. This display voltage can be rewritten in a short time by the TFT, and therefore, even in a high definition LCD, it is possible to satisfy the requirement of not lowering the image quality.
The key to displaying an image is also the molecular orientation of the liquid crystal under the action of an electric field. Different display modes are generally achieved by the alignment treatment of the inside of the substrate to cause the alignment of the liquid crystal molecules to produce a desired structural deformation. Selecting a certain display mode, under the action of an electric field, the liquid crystal molecules change in orientation, and by matching with the polarizing plate, the intensity of the incident light after passing through the liquid crystal layer changes accordingly. Thereby achieving image display. In summary, TFT-LCD is different from the simple matrix of passive TN-LCD and STN-LCD. It has a thin film transistor (TFT) on each pixel of the liquid crystal display, which can effectively overcome the non-gating time. Crosstalk makes the static characteristics of the display LCD independent of the number of scan lines, thus greatly improving the image quality. The characteristics of the switching unit (ie, TFT) are required to meet the requirements of low on-state resistance and very large on-state resistance.