What are active matrix LCD and TFT-LCD?

In light of display technology, LCD can be classified as Passive Matrix LCD and Active Matrix LCD, TFT-LCD is thin film transistors active display.

I.LCD display technology: Passive Matrix vs. Active Matrix

Liquid crystals (LCs) are a state of matter that has properties between those of conventional liquids and those of solid crystals, in which the constituent molecules tend to align themselves relative to each other. Liquid crystal displays (LCD) use the unique character of Nematic LC which are optically active and align themselves with an applied field.

The character of Nematic phase LC: The center of gravity of the molecules in the Nematic phase lacks positional order, allowing it to flow free and their center of mass positions are randomly distributed as in a liquid, but the directors of the molecules are spontaneously aligned with their long axes roughly parallel.

There are two ways to drive the working of LC in the electric field, either with a passive matrix or an active matrix grid. Therefore, LCD can be classified as Passive Matrix LCD (PMLCD) and Active Matrix LCD (AMLCD).

﹣Passive Matrix LCD (PMLCD)

Passive matrix display is the first commercialized LCD technology, which is designed with a simple grid of row and column electrodes respective on the top and bottom plates to address the pixels.

The working principle of the passive display is to use the input signal to drive the electrodes of each row in turn, so when a row is selected, the electrodes on the column will be triggered to turn on those pixels located at the intersection of the row and the column.

Disadvantages:

This method is relatively simple. However, its disadvantages are

● Crosstalk

Given the multiplexing structure, if there is a large current flowing through a segment, a nearby segment will be affected, causing ghosting.

● Poor contrast ratio

Thereby, the range of allowed voltage is limited, and the contrast ratio is reduced.

● Row limit and low response

While the rows and columns are added, the current goes low, consequently, the cell will turn on and off slowly.

It is usually limited to about 320 rows.

﹣Active Matrix LCD (AMLCD) / TFT LCD

AM display removes these multiplexing limitations by exploiting a switching element and a storage capacitor at every pixel of the display.

These switches are usually implemented through transistors, which are fabricated by using the current-carrying thin film (usually a film of silicon—Si) and therefore called thin film transistors, TFTs.

Active Matrix LCD is also known as TFT LCD.

A color display with 1024×768 resolution include 1024x3x768=2,359,296 sub-pixels

Similar to the passive matrix LCD, the upper and lower layers of the active matrix LCD are also arranged vertically and horizontally with transparent electrodes made of indium tin oxide (ITO).

The difference is that a small transistor is added to each unit, and the transistor controls the on and off of each pixel.

It allows the column voltages to be applied only to the row that is being addressed, while the storage capacitor maintains the pixel information for the whole frame also when the addressing signal is removed.

Thus, high contrast is possible and a fast LC mixture can be used, since the pixel no longer has to respond to the average voltage over a whole frame period, as in PMLCDs. For the same reason, the phenomenon of crosstalk is also minimized.

﹣Revolution of the LCD technology

▶ Passive LCD, from TN, STN, DSTN to FSTN

Early passive matrix screens relied on twisted nematic (TN) designs. The polarizing directions of the upper and lower polarizing plates are at 90°, so the liquid crystal in the middle is twisted at 90°.

The resulting LCD panels have low contrast and slow response times. This method works well for low-information displays but not for computer displays.

The Super Twisted Nematic (STN) method is improved by changing the chemical composition of LC so that the LC molecules are twisted more than once, therefore, the light twist reaches 180° to 270°, which can greatly improve the contract.

In the early 1980s, STN technology was very popular. But it comes along with a color shift in light, especially where the screen is off-axis. This is why early computer screens were always bluish and yellowish.

To solve the color shift problem, double-layer STN (DSTN) and further improved technology Film-compensated STN (FSTN) were developed with comparable display quality and low cost.

And the dual scan concept for FSTN was proposed in the early 1990s, to solve the ghost phenomenon and significantly improve contrast, and graphic quality, and shorten the response time. It is still widely used in low-priced computers.

▶ Active LCD, the advanced flat-panel display became mainstream in the late 90s

In comparison with the passive matrix, the active matrix is a much more advanced technology for flat-panel display.

But to compete with the CRT in the 90s, the picture quality of AMLCDs did not meet the CRT standard.

Drawbacks were the limited viewing angle and color gamut, poor black level, low peak luminance and slow response time of the LC-material, high cost, and low yield in production.

In the past 30 years, those problems have been addressed and solved largely.

● Since 2005, the picture quality of LCDs for TV surpassed that of CRTs, the milestone is Full High Definition (FHD) LC-modules with 1080 lines introduced into the market, which has good daylight contrast and high resolution.

● The viewing angle could be enlarged spectacularly by applying new LC structures like In-Plane Switching (IPS) and Vertically Aligned Nematic (VAN) LCs.

● The color gamut has been improved by applying new color filter materials and light-emitting diodes (LEDs) in the BLU.

● High dynamic contrast (including good black level) is achieved by adequate dimming techniques.

● Minimizing the thickness of the LC cell and applying new LC recipes have shortened the response time.

● Since the panel cost of a TFT LCD module is relatively high, decreasing the cost of manufacturing an LC panel is one of the main issues. The most effective way is to enlarge the mother glass or substrate of the back and front plates.

Until 2021, Gen11 is in production, which can process the size of glass substrate 3000*3320(mm). (In the industry, “Gen” stands for generation, which is used to indicate the size of the glass substrate for an LC panel. )

● With the production process improvement in high yield and cost efficiency, TFT-LCD has been taking the lead in the display market now.

﹣PMLCD vs. AMLCD

Even though AMLCD (TFT LCD) takes advantage of the display technology and display quality, the cost of some modules can compete with the similar size of passive LCDs.

PMLCD still occupies some specific markets with low cost and no high resolution required.

● Passive Matrix LCD

Working Principle: Pixels are addressed directly and they must retain their state between screen refreshes without the benefit of a steady electrical charge.

Further Classification/Technology Development: TN, STN, DSTN, FSTN with dual scanning.

Contrast Ratio: Lower

Response time: Slower

Color Saturation: Lower

Resolution: Lower

Grayscale: Very limited

Brightness: Lower

Power Consumption: Lower

Cost: Lower

Custom: Flexible with lower cost

Application: Numerical displays, mono color word processors, mainly low cost, low power monochrome display, and graphic display.

● Active Matrix LCD

Working Principle: A switch is placed at each pixel which decouples the pixel-selection function. Thin Film Transistor is the main technology of the AMLCD subgroup.

Further Classification/Technology Development: In light of viewing angle: TN, IPS, VAN; In light of the material used for its elaboration: amorphous silicon (a-Si), low-temperature polycrystalline silicon (LTPS), continuous grain silicon (CGS), indium gallium zinc oxide (l IGZO), high-temperature poly-silicon (HTPS).

Contrast Ratio: Higher

Response time: Faster

Color Saturation: True color

Resolution: Capable of much higher resolution, reach 8K resolution now.

Grayscale: Capable of high

Brightness: Higher

Power Consumption: Higher

Cost: Higher

Custom: Higher NRE cost on LCD panel, but flexible due to multi-options: size, resolution, BL, cover finish, etc.

Application: Mobile phones, computer screens, monitors, television, and mainstream color displays.

II. Key features of TFT LCD in the industry

With the benefit of the active matrix display technology and the technology development of related materials, components, and production in recent years, the TFT-LCD has attracted markets to adopt this technology.

● Flexible in size

In theory, size can have no limits.

In the market, the most common size in applications, cover less than 1 inch for wearables, and larger than 50 inches for television.

● High integration

With the revolution of the IC industry, TFT-LCD can provide high resolution with less thickness and size.

For instance, the 1.3-inch TFT can have a resolution of XGA (1024 x 768) containing millions of pixels, while the TFT film for 16.1-inch with a resolution of SXGA (1280×1024) is only 50 nm.

● High display performance

In TFT-LCD, each pixel on the liquid crystal display is controlled independently by a thin film transistor integrated behind.

By this means, the display can have a quick response and high accuracy in color, to provide high contrast, high brightness, and true color.

● Cost efficiency

From an industrial perspective, the application of glass substrates and plastic substrates fundamentally solves the cost problem of large-scale semiconductor integrated circuits.

And with the optimization of production and technology in the last 30 years, the yield rate has improved, average above 90%, much better than OLED of about 80%.

● Power consumption and lifespan

In comparison with CRT, TFT LCD consumes about one-tenth of the power of a CRT display, and the reflective TFT LCD is even only about one-hundredth of that.

The power consumption of the TFT display is mainly from the backlight, accounting for about 90%. Reducing the power consumption of BL is the focus of the future development of TFT liquid crystal display, as well as improving its lifespan.

Because the backlight is also the key factor that determines the service life of LCD. When the backlight turns dark while it reaches its maximum service life, the LCD screen will wear out. Even before then, the display quality will go down.

However, by comparison with OLED, TFT-LCD still has a longer service life, more than 30,000 hours.

III. Application of TFT-LCD

TFT-LCD is an interface between humans and machines, every industry still has its unique requirement for its terminal to work in a specific scene.

In light of the requirement in size and features for various industries, we have shared our experience and understanding of the Application segment on the site.

If you want to use TFT-LCD and Graphic LCD in your terminal, feel free to Contact Us. We supply small-and-medium sizes and could provide a custom solution for your application.