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How Does TFT LCD Display Work

Views: 226     Author: Reshine Display     Publish Time: 2023-09-28      Origin: Site


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How Does TFT LCD Display Work

1. What is a TFT LCD?

One of the display technologies that is developing the fastest right now is the TFT LCD or thin film transistor liquid crystal display. A type of semiconductor device called a thin film transistor (TFT) is used in display technology to increase the product's efficiency, compactness, and cost. The TFT LCD is an active matrix display in addition to having semiconductor properties, which enhances the advantages of this semiconductor device. It controls pixels individually and actively rather than passively.

Since being used in conjunction with flat panel technology, particularly liquid crystal displays (LCD), TFT displays have become increasingly popular for use in LCD monitors and display screens, including computer monitors and smartphones. With this advancement, the lighter, less-bulky LCD began to replace the cathode-ray tube, also known as a CRT, as the dominant display technology. TFT technology found in LCDs today is primarily used to create high-resolution and high-quality displays.

2. TFT LCD structure

Three main layers make up the TFT LCD's construction. Glass substrates make up the two sandwiching layers; one of them has TFTs while the other has an RGB, or red-green-blue, color filter. A liquid crystal layer is located in the space between the glass layers.

Structure of TFT LCD

The deepest or rearmost layer on a device's circuit board is the TFT glass substrate layer. Amorphous silicon, a variety of silicon with a non-crystalline structure, is used to make it. The actual glass substrate is then covered with a layer of silicon. The TFTs in this layer are individually paired with each sub-pixel from the other substrate layer of the device (see Architecture of a TFT Pixel below) and regulate the voltage applied to each sub-pixel. In this layer, between the substrate and the liquid crystal layer, there are also pixel electrodes. A conductor is a component that allows electricity to flow into or out of another object, in this case, the pixels.

The other glass substrate is located at the surface level. The actual pixels and sub-pixels that make up the RGB color filter are located directly beneath this glass substrate. This surface layer has counter (or common) electrodes on the side closest to the liquid crystals that block the circuit that travels between the two layers to balance the electrodes of the aforementioned layer. Because they allow for transparency and have good conductive properties, indium tin oxide (ITO) electrodes are typically used in both of these substrate layers.

Polarizer filter layers are present on the exterior sides of the glass substrates, whether they are closest to the front or back. Only certain beams of light that are polarized in a particular way, i.e., whose geometric waves are compatible with the filter, can pass through these filters. Incorrect polarization prevents light from passing through the polarizer, resulting in an opaque LCD screen.

Liquid crystals are situated between the two substrate layers. Together, the molecules that make up a liquid crystal may move and behave like a liquid, but they maintain a crystal structure. For use in this layer, a variety of chemical formulas are available. To induce certain behaviors of passing light through the polarization of the light waves, liquid crystals are typically aligned to position the molecules in a particular way. Either a magnetic field or an electric field must be used to accomplish this; however, with displays, a magnetic field would be ineffective because it would be too powerful for the display itself. As a result, electric fields, which use very little power and require no current, are used.

The alignment of the crystals is in a 90-degree twisted pattern before applying an electric field to the crystals between the electrodes, allowing a properly crystal-polarized light to pass through the surface polarizer in a display's "normal white" mode. Electrodes that have been specifically coated in a substance that twists the structure in this particular direction are what causes this state.

The twist, or re-alignment, is broken when the electric field is applied, causing the crystals to straighten out. Although passing light can still pass through the back polarizer, light is not transmitted to the surface due to the crystal layer's failure to polarize the light for passage through the surface polarizer, creating an opaque display. When the voltage is reduced, only some crystals realign, letting some light through and producing various tones of grey (light levels). The twisted nematic effect is the name given to this effect.

One of the least expensive options for LCD technology, the twisted nematic effect also enables quick pixel response times. There are still some restrictions, however. The quality of color reproduction may not be excellent, and there are fewer viewing angles or angles from which the screen can be viewed.

Through in-plane switching (IPS) of the liquid crystals, these limits were overcome. IPS parallelizes the crystal alignment as opposed to aligning them perpendicularly to the electrodes. The matrix then streamlines light to a greater extent. Initial issues with slow response times were mostly fixed recently, so the advantages of better viewing angles and color reproduction now outweigh the drawbacks. However, compared to twisted nematic devices, it is a more expensive technology.

twisted nematic

The device's backlight, which can project light from the side or back of the display, is the source of the light that travels through it. The LCD must use the backlight in the LCD module because it cannot produce its light. Light-emitting diodes, also referred to as LEDs, are the type of light source that is used most frequently. Organic LEDs (OLEDs) have also become popular recently. If polarized properly, this light, which is typically white, will pass through the surface substrate layer's RGB color filter and display the color the TFT device specifies.

3. Driving a TFT LCD

There is a basic explanation of the Field Effect Transistor (FET) in the first paragraph under "Evolution of TFTs" in the previous article, "The History of Thin Film Transistor Displays." Since the TFT is a type of FET, it also adheres to the FETs' operating principle. Essentially, the signal current of a TFT can be controlled or changed by applying a voltage to the gate. On the TFT-based LCD panel, this current, known as the driving voltage, then flows from the source to the drain and sends a signal to its sub-pixel, allowing light to pass through.

4. A TFT Pixel's Architecture

Each pixel in an LCD can be identified by its three sub-pixels. The RGB colorization of that entire pixel is produced by these three subpixels. These sub-pixels, each with its own independent structural and functional layers as previously mentioned, serve as capacitors or electrical storage units within a device. According to the liquid crystal alignment, light passing through the filters and polarizer can be mixed into colors of almost any kind using the three sub-pixels per pixel.

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