TFT LCD Technology

Table of Contents

A Thin-Film Transistor Liquid Crystal Display (TFT LCD) operates by controlling light transmission through liquid crystals to form images. Here’s a detailed breakdown of how a TFT LCD works:

TFT LCD Basic Structure

A TFT LCD consists of several layers:

  • Backlight: Usually composed of white LEDs, it provides the necessary light to illuminate the display.
  • Polarizers: Two polarizing filters are placed at the front and back of the liquid crystal layer. These filters control the direction of light.
  • Liquid Crystal Layer: This layer contains liquid crystals, which change their orientation in response to electric fields, thus controlling light transmission angle.
  • Thin-Film Transistor (TFT) Array: A matrix of tiny transistors, each connected to a pixel in the display. This array is crucial for precisely controlling each pixel.

TFT Display Working Principle

TFT LCD Backlight

The backlight provides uniform illumination across the entire screen. Since the liquid crystals do not produce light, the backlight is necessary for making the display visible. White LEDs are commonly used for this purpose, offering brightness and energy efficiency.

To improve efficiency, Pulse-Width Modulation (PWM) is used to drive LED backlight. it involves turning the LED ON/OFF at a specific rate that human eye perceives the LED being ON continuously but at a reduced brightness. The amount of the dimming is controlled by the duty cycle. Typically the LED is turned ON/OFF between 60 to 240 times a second (Hz). Any slower than 50 Hz and the eye perceives flicker.

Polarization of Light

Light from the backlight passes through the first polarizer, which allows only light waves oscillated in one direction to pass through. The second polarizer, placed at 90 degrees to the first one, blocks light unless the liquid crystals between them change light’s polarization.

Liquid Crystal Modulation

Liquid crystals used in displays (typically nematic liquid crystals) consist of long, rod-like molecules that can be aligned when an electric field is applied. These molecules twist or rotate when electric field is altered, which affects light’s angle and in turn how much light can pass through the second polarizer:

    • Applied voltage: The liquid crystals align in a way that rotates the light to match the second polarizer, allowing light to pass through and making the pixel appear bright.

    • No voltage: The liquid crystals stop rotating the light, so it is blocked by the second polarizer, making the pixel appear dark.

Why does light change its direction? The birefringence of liquid crystals is the key. Birefringence is the property of certain materials (like liquid crystals) to refract light differently depending on the orientation of their molecules. When the liquid crystal molecules twist, they change the optical path of the light by altering its direction as it passes through. This interaction allows the liquid crystal layer to control the polarization and therefore the brightness of the light exiting the LCD. This forms the basic mechanism behind how images are created on an LCD.

Pixel Control via TFTs

Each pixel in a TFT LCD is controlled by a corresponding transistor in the TFT array. A matrix of transistors is connected to the display’s pixels:

    • Active Matrix: This arrangement enables individual control of each pixel ON/OFF. When voltage is applied to a specific row and column, only the pixel at that intersection is activated.

    • Capacitor: A small capacitor maintains the pixel’s charge, ensuring that the pixel remains in its intended state until the next refresh cycle.

tft lcd working principle

Color Creation

Color TFT LCDs add a layer of color filters on top of the liquid crystal layer. These filters are arranged in red, green, and blue (RGB) subpixels:

    • Each pixel consists of three subpixels (red, green and blue).

    • By varying the intensity of the light passing through each subpixel, different colors are produced. When these subpixels are combined, they form the desired color for each pixel.

Refresh and Response Time

TFT LCDs refresh many times per second (typically 60Hz or higher) to update screen image. The response time refers to how quickly the liquid crystals can shift from one state to another, affecting how fast the display can change colors, which is crucial for motion clarity in fast-moving content (e.g., gaming or videos).

Viewing Angles

Since the liquid crystals’ alignment affects how light is transmitted, viewing angles in TFT LCDs is limited by how much liquid crystal can twist. However, advanced techniques such as In-Plane Switching (IPS) and Vertical Alignment (VA) improve viewing angles and color consistency by modifying the way liquid crystals are aligned and controlled.

TFT Display Categories

There are different terms in the market when referring to TFT LCD. Sometimes we hear TN, IPS or VA. The other time people say a TFT LCD is a-Si, LTPS or IGZO.

When people refer to TFT LCDs as TN, IPS, or VA, they are talking about the different types of panel technologies used in the construction of the display. Each type has its own unique characteristics in terms of how the liquid crystals are arranged and controlled, affecting the display’s color accuracy, response time, contrast, and viewing angles. Let’s break down each technology:

TN-VA-IPS

TN (Twisted Nematic) Panels:

    • How it works: The liquid crystals twist when an electric current is applied, controlling the passage of light. The “twisted nematic” refers to the 90-degree twist of the liquid crystal molecules in the off state.

    • Advantages:
        • Fast response times: TN panels are popular for gaming because of their quick pixel response, reducing motion blur.

        • Low cost: TN panels are typically the most affordable type of TFT LCD.

    • Disadvantages:
        • Narrow viewing angles: Colors and contrast shift dramatically when viewed from the side or above.

        • Poor color reproduction: Compared to other panel types, TN panels have inferior color accuracy and contrast.

    • Use cases: Gaming monitors, budget displays, general-purpose screens.

IPS (In-Plane Switching) Panels:

    • How it works: In IPS panels, the liquid crystals are aligned parallel (in-plane) to the glass layers and move horizontally when voltage is applied, rather than twisting as in TN panels. This keeps the light passing through the display more consistent, especially at wide viewing angles.

    • Advantages:
        • Wide viewing angles: Colors and contrast remain stable even when viewed from the sides or at steep angles (up to 178°).

        • Better color accuracy: IPS panels offer superior color reproduction and are preferred for tasks requiring precise color, like photo and video editing.

    • Disadvantages:
        • Slower response times: Compared to TN, IPS panels usually have slower response times, though modern versions have improved significantly.

        • Higher cost: IPS panels are generally more expensive to produce than TN panels.

    • Use cases: Professional monitors for graphic design, video editing, photography, high-end smartphones, tablets and medical screen.

VA (Vertical Alignment) Panels:

    • How it works: In VA panels, the liquid crystals align vertically when no voltage is applied and tilt when voltage is introduced. This vertical alignment allows VA panels to block more light, resulting in deeper blacks and higher contrast.

    • Advantages:
        • Superior contrast ratios: VA panels offer much better contrast than both TN and IPS, with deep blacks and bright whites, making them ideal for watching movies or working with high-contrast images.

        • Good color reproduction: Though not as good as IPS, VA panels offer better colors than TN panels.

    • Disadvantages:
        • Moderate viewing angles: Better than TN but not as wide as IPS. There can be a slight shift in color and contrast when viewed from extreme angles.

        • Slower response times: VA panels generally have slower response times than TN and IPS panels, which can cause motion blur, particularly in fast-moving scenes.

    • Use cases: TVs, monitors for entertainment and general use, displays requiring high contrast.

Summary of Differences:

Panel Type Response Time Color Accuracy Contrast Viewing Angle Cost
TN Fast Low Low Narrow Low
IPS Moderate High Moderate Wide High
VA Slow Moderate High Moderate Medium

Each of these panel technologies is built on TFT (Thin-Film Transistor) technology, which refers to how each pixel is individually controlled for better image quality and faster refresh rates. The difference lies in how the liquid crystals move or align, leading to different strengths and weaknesses for each type of panel.

On the other hand, a-Si (Amorphous Silicon), LTPS (Low-Temperature Polycrystalline Silicon), and IGZO (Indium Gallium Zinc Oxide) are different types of backplane technologies used in TFT LCD and OLED displays. The backplane is the layer that contains the transistors which control the pixels. These technologies differ in their electrical properties, efficiency, and performance, and they play a crucial role in determining a display’s resolution, power consumption, and overall quality. Let’s look at them one-by-one:

a-Si (Amorphous Silicon)

    • What it is: Amorphous silicon is the most common material used in the backplane of TFT LCDs. In an a-Si display, the transistors are made from amorphous silicon, which is a non-crystalline form of silicon.

    • Advantages:
        • Low cost: a-Si is cheaper to produce compared to other backplane technologies, which is why it’s widely used in mainstream and budget displays.

        • Mature technology: It has been in use for many years and is well understood by manufacturers, resulting in high production yields.

    • Disadvantages:
        • Lower electron mobility: This means it can’t switch pixels as quickly as more advanced technologies, leading to lower performance in terms of response time and power efficiency.

        • Lower resolution: a-Si is not ideal for high-resolution displays (like 4K or higher) because the transistors cannot be made small enough to control extremely dense pixels effectively.

    • Use cases: Most common in mainstream LCDs, televisions, and monitors where extreme resolution or efficiency is not a critical factor.

LTPS (Low-Temperature Polycrystalline Silicon)

    • What it is: LTPS refers to polycrystalline silicon formed at a relatively low temperature, making it more efficient and capable than a-Si. The transistors in LTPS displays are made from polycrystalline silicon, which has much higher electron mobility than amorphous silicon.

    • Advantages:
        • Higher electron mobility: LTPS transistors switch faster, allowing for higher refresh rates, faster response times, and better performance in high-resolution displays.

        • Better power efficiency: LTPS is more energy-efficient because it requires less power to operate each pixel, making it ideal for mobile devices and OLED displays.

        • Smaller transistors: LTPS allows for smaller transistors, which means higher pixel density and higher resolution.

    • Disadvantages:
        • Higher cost: The production process for LTPS is more complex and expensive than a-Si, which increases the overall cost of the display.

        • Heat management: Because LTPS transistors are more tightly packed, heat dissipation can be an issue in some cases.

    • Use cases: LTPS is commonly used in high-end smartphones, tablets, OLED displays, and 4K+ LCD screens where higher resolution and power efficiency are critical.

IGZO (Indium Gallium Zinc Oxide)

    • What it is: IGZO is a semiconductor material made from indium, gallium, and zinc oxide. It offers an alternative to silicon-based transistors in LCD and OLED displays and has unique electrical properties that make it more efficient than a-Si.

    • Advantages:
        • Extremely high electron mobility: IGZO transistors have much higher electron mobility than a-Si, allowing for faster pixel switching and better performance in high-resolution displays.

        • Low power consumption: IGZO can hold charge longer, reducing power consumption in static images (important for applications like e-readers or smartphones).

        • Transparent: The transparency of IGZO allows for better light transmission, improving the overall brightness and energy efficiency of the display.

        • Better for high-resolution displays: IGZO can support very high pixel densities (like 8K displays) because of its ability to make smaller and more efficient transistors.

    • Disadvantages:
        • Moderate cost: IGZO is more expensive than a-Si, though it is usually less expensive than LTPS.

        • Relatively new technology: IGZO technology is still being refined and has fewer manufacturers compared to a-Si or LTPS.

    • Use cases: IGZO is used in high-end displays like 4K and 8K TVs, high-resolution monitors, and OLED panels where energy efficiency and high resolution are needed.

 

Comparison of a-Si, LTPS, and IGZO:

Technology Electron Mobility Power Efficiency Cost Resolution Capability Use Case
a-Si Low Low Low Low Budget and mainstream displays
LTPS High High High Very High High-end smartphones, 4K+ LCDs
IGZO Very High High Moderate Very High High-resolution monitors, 4K/8K TVs

Summary of Key Differences:

    • a-Si is cheap and widely used in standard TFT LCD displays, but it has lower performance and is not suitable for high-end displays.

    • LTPS offers much higher performance and is widely used in premium smartphones and OLED panels but is more expensive to produce.

    • IGZO combines high efficiency with excellent resolution capability and lower power consumption, making it a great option for high-end displays, though it is less widely used compared to LTPS.

Summary

TFT LCD is the dominant display solution, with different technologies improve performance, efficiency, and resolution, making them ideal for different applications depending on the cost and desired display characteristics. It also provides multiple interfaces for user to use.

Putting these two categories of TFT LCD together:

Attribute TN Panel IPS Panel VA Panel
Transistor Type a-Si a-Si, LTPS, IGZO a-Si, IGZO
Color Quality Low High Moderate
Viewing Angle Narrow Wide Moderate
Response Time Fast Moderate Moderate
Contrast Low Moderate High
Cost Low High Moderate

Please feel free to contact us if you have any questions on TFT LCD or plan to start a project using LCD. Menco is an official agent of Tianma Microelectronics Co, which is a major TFT display panel manufacturer in China.