What is TFT LCD - Understand the Variants

TFT LCD (Thin-Film Transistor Liquid Crystal Display) technology is a type of active-matrix LCD that utilizes thin-film transistors to improve image quality and control each pixel individually. TFT LCDs are widely used in various applications, from televisions and computer monitors to smartphones and tablets. This essay explores the different variants of TFT LCD technology, their unique characteristics, advantages, disadvantages, and applications.

What is TFT Display

As active-matrix display, TFT LCDs operate by using an array of thin-film transistors and capacitors to control the light passing through the liquid crystal layer. Each pixel in a TFT LCD has its own transistor, which allows for precise control of the pixel's state, resulting in improved image quality, faster response times, and better overall performance compared to passive-matrix LCDs.

TFT LCD display is made up of multiple layers.

1.Backlight and Polarizers: The backlight (usually white LEDs) provides the necessary illumination. Two polarizing filters are placed at the front and back of the liquid crystal layer.
2.Liquid Crystal Layer: Sandwiched between the polarizers, this layer contains liquid crystal molecules that twist and align in response to electric fields.
3.Thin-Film Transistors: Each pixel has its own transistor, which controls the voltage applied to the liquid crystal layer. This determines the alignment of the liquid crystals and modulates the amount of light passing through.
4.Color Filters: Red, green, and blue color filters are placed over the sub-pixels to produce the full spectrum of colors.

One of the major advantages of TFT displays is that they control each pixel individually and continuously. That can lead to faster response times and accurate color controlling.

Advantages of TFT LCD Display

High Resolution: TFT displays can achieve high resolutions, allowing for sharp and detailed images.
Wide Viewing Angles: These displays offer wide viewing angles, ensuring that the content on the screen remains visible from different perspectives.
Fast Response Times: TFT displays have fast response times, making them suitable for applications that require quick transitions and motion rendering.
Color Accuracy: Thanks to their advanced technology, TFT displays provide excellent color accuracy, making them ideal for applications that require vibrant and lifelike visuals.
Improved Image Quality: The use of thin film transistors in TFT displays results in better image quality, with higher contrast ratios and more accurate colors.

TFT Display Panel Types: TN vs IPS vs VA

Early TFT LCD displays were based on Twisted Nematic (TN) technology but suffered from poor viewing angles. To overcome this problem, engineers and scientists developed in-plane switching (IPS) and vertical alignment (VA) technologies. IPS TFT displays can reach wide viewing angles of up to 178 degrees. The viewing angle of VA TFT displays is not as wide as IPS, but they have the best contrast. Therefore, a VA TFT monitor is the best choice for gamers interested in HDR image content. And the IPS TFT display is the most widely used.

1. Twisted Nematic (TN) TFT LCD

TN panels use simple driving schemes where the liquid crystals twist and untwist at varying angles to modulate light. When there is no voltage difference between the two transparent electrodes, liquid crystal molecules are twisted 90 degrees, in combination of upper and bottom polarizers, allows light to pass through LCD. As voltage applied, crystal molecules are untwisted and aligned to the same direction, blocking light.
Advantages: Fast response time and low production cost.
Disadvantages: Poor color reproduction and limited viewing angles.
Applications: Gaming monitors, budget laptops, and entry-level smartphones.

2. In-Plane Switching (IPS) TFT LCD
IPS panels crystal molecules are parallel to the glass substrates at initial stage, LCD is off. When the in-plane electrodes is charged, drives liquid crystals to align horizontally, allowing for consistent light modulation regardless of viewing angle.
Advantages: Superior color accuracy and wide viewing angles.
Disadvantages: Higher production costs and slower response times compared to TN panels.
Applications: High-end monitors, professional displays, smartphones, tablets, and televisions.

There are several IPS variants in LCD market.
Super IPS (S-IPS): Improved response times and color accuracy.
Advanced Super IPS (AS-IPS): Enhanced color reproduction and viewing angles.
Enhanced IPS (E-IPS): Balances cost and performance.
PLS (Plane to Line Switching): Samsung's variant offering similar benefits at lower costs.
AH-IPS (Advanced High-Performance IPS): Higher resolution and better color accuracy.
H-IPS (Horizontal IPS): Improved pixel structure for brightness and contrast.

3. Vertical Alignment (VA) TFT LCD
VA panels drive liquid crystals to align vertically when no electric field is applied. When a voltage is applied, the molecules tilt and modifying light direction, enhancing contrast ratios and black levels.
Advantages: Excellent contrast ratios, deeper blacks, and wider viewing angles than TN panels.
Disadvantages: Slower response times compared to TN panels and potential color shifting at extreme angles.
Applications: Televisions, computer monitors, and high-end displays requiring deep blacks and high contrast.

Four VA variants exist to improve VA panel's performance.
Multi-Domain Vertical Alignment (MVA): Enhances viewing angles and response times while maintaining good contrast ratios.
Patterned Vertical Alignment (PVA): Samsung's variant with improved contrast ratios and color reproduction.
Advanced MVA (AMVA): Further improvements in response times and color accuracy.
Super MVA (S-MVA): Better viewing angles and faster response times, approaching IPS performance.

TN LCD provides the best refresh rate and economic solution. While IPS has the best overall visual performance, and it is more expensive than the other two. Here is the details comparative analysis on the three LCD panel types.

Transistor Technology in TFT LCD - classification by material

The active material of TFT is silicon film. In light of the structure and technology of silicon film, the TFT substrate can be divided into amorphous silicon (a-Si), indium gallium zinc oxide (IGZO), low-temperature polysilicon (LTPS), high-temperature polysilicon (HTPS), continuous granular crystalline silicon (CG Silicon). Currently, a-Si, IGZO, and LTPS LCD are the most common ones in market.

TFT LCD's performance and efficiency depend heavily on the type of transistors used. Followings are the primary transistor technologies employed in TFT LCDs nowadays:

1.Amorphous Silicon (a-Si) TFTs
Amorphous silicon is the non-crystalline form of silicon. Because of its maturity, stability and low cost, a-Si technology can achieve high yields on mainstream display products of all sizes.
Advantages: Easy and inexpensive to manufacture, suitable for large-area displays.
Disadvantages: Lower electron mobility compared to other materials, resulting in slower response times.
Applications: Widely used in standard computer monitors, televisions, and embedded systems.

2.Oxide TFTs (IGZO – Indium Gallium Zinc Oxide)
IGZO is a newer material offering high electron mobility and low power consumption. It is a thin film transistor technology that refers to a layer of metal oxide on top of the active layer of TFT-LCD.
Advantages: Higher resolution, better transparency, and lower power consumption compared to a-Si.
Disadvantages: More expensive than a-Si, although manufacturing processes are improving.
Applications: Used in high-resolution displays, such as 4K monitors and high-end tablets.

3.Low-Temperature Polycrystalline Silicon (LTPS) TFTs
LTPS also refers to a physical transformation in which amorphous silicon irradiated by laser light, undergoes energy level transitions and transforms into a polycrystalline structure. It has higher electron mobility than amorphous silicon.
Advantages: Faster response times, higher resolution, and better power efficiency, dynamic and rich colors, fast response and less reflective.
Disadvantages: More complex and expensive manufacturing process.
Applications: Commonly used in high-end smartphones, tablets, and laptops where high performance is critical.

Putting them together, this table summarizes TFT LCD's variants.

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Common Interfaces Used By TFT Displays

Choosing the right TFT LCD module, interface is one of the key factors user should consider. It relates to application requirements, display size, panel resolution and host machine specification.

1.MCU (Micro Controller Unit)

Many MCUs have built-in LCD controller function. The interface uses a parallel bus to transfer data(8/9/16/18 bits), control and power signals. There are two types are popular in the market, 6800 and 8080. This interface is mainly used for smaller size screens.
Advantages: Simple, low power consumption, no need clock and synchronization signal.
Disadvantages: Limited bandwidth, resolution and color depth.

2.SPI (Serial Peripheral Interface)

SPI is a full-duplex synchronous serial communication protocol used for short-distance communication. SPI is commonly used in simple, low-resolution displays such as those found in microcontroller-based projects, wearables, and small industrial control panels.
Advantages: Low cost, low power consumption and easy to implement.
Disadvantages: Limited bandwidth, resolution and color depth.

3.RGB (Red Green Blue)

RGB is a transistor-transistor TTL interface running on a parallel bus. In terms of RGB mode, there are DE , SYNC, and SYNC-DE MODE. The RGB(16/18/24 bits) interface is commonly used in industrial control panels, automotive dashboards, and consumer electronics such as digital cameras and some monitors, where straightforward design and high-speed data transfer are essential.
Advantages: Higher bandwidth and color depth, easy to implement, with straightforward signal timing.
Disadvantages: High pin count, high power consumption, limited transmission distance and can create electrical noise.

4.LVDS (Low-Voltage Differential Signaling)

LVDS is a differential signaling system used to transmit data at high speeds over twisted-pair copper cables. It is a popular choice for large LCDs and peripherals in need of high bandwidth, like high-definition graphics and fast frame rates.
Advantages: Low noise, low power consumption, low Electromagnetic interference (EMI), and transmit farther distance.
Disadvantages: Requires differential pairs and proper impedance matching.

5.MIPI DSI (Mobile Industry Processor Interface Display Serial Interface)

MIPI uses similar differential signaling to LVDS, with a clock pair and one to eight pairs of data lanes. MIPI supports a complex protocol that allows high speed and low power modes, as well as the ability to read data back from the display at lower rates. It is widely used in high-end applications, like smart phone, tablet and laptop.
Advantages: High speed interface with high bandwidth, low power consumption and EMI.
Disadvantages: Complex interface with high pin count, requires sophisticated controllers and drivers.

6.eDP (Embedded DisplayPort)

eDP is a digital interface based on DisplayPort technology, optimized for internal display connections. eDP interface is commonly embedded in tablets, notebooks, monitors, and large-scale high-resolution displays.
Advantages: High speed interface with high bandwidth, low power consumption and EMI.
Disadvantages: More expensive and not as widely supported.

As general guide line, low-end applications may use MCU or SPI interfaces, mid-range applications may use RGB/TTL, and high-end applications can use LVDS, eDP and MIPI.

Conclusion

Understanding the differences between these TFT LCD variants, their driving methods, and the types of transistors used helps consumers and professionals make informed decisions based on their specific needs and applications. As technology continues to advance, we can expect further improvements and new variants that push the boundaries of display performance. As a company with over 25 years experience in LCD field, Topway Display is always ready to help you picking the right TFT LCD for your next project.