TFT Technologies

TFT Display  (TN Film)

TFT (Thin-Film Transistor) Displays are active-matrix LCDs with full RGB color screens. These screens feature bright, vivid colors and have the ability to show fast animations, complex graphics and crisp custom fonts. TFTs are perfect displays for providing a rich user interface for all types of products. While typically used in consumer devices like personal DVD players and handheld devices, TFTs are also well suited for industrial application.

TFTs are Active-Matrix LCDs that have tiny switching transistors and capacitors. These tiny transistors control each pixel on the display and require very little energy to actively change the orientation of the liquid crystal in the display. This allows for faster control of each Red, Green and Blue sub-pixel cell thus producing clear fast-moving color graphics.

The transistors in the TFT are arranged in a matrix on the glass substrate. Each pixel on the display remains off until addressed by applying a charge to the transistor. Unlike conventional Passive-Matrix displays, in order to activate a specific pixel, the corresponding row is turned on and a charge is sent down the proper column. This is where only the capacitor at the designated pixel receives a charge and is held until the next refresh cycle. Essentially, each transistor acts as an active switch. By incorporating an active switch, this limits the number of scan lines and eliminates cross-talk issues.

The main problem with TN Film technology is that viewing angles are pretty restrictive, especially vertically, and this is evident by a characteristic severe darkening of the image if you look at the screen from below. Contrast and colour tone shifts can be evident with even a slight movement off-centre, and this is perhaps the main drawback in modern TN Film panels. Some TN Film panels are better than others and there have been improvements over the years to some degree, but they are still far more restrictive with fields of view than other panel technologies.    

MVA ― Multi-domain Vertical Alignment
 

MVA (Multi-domain Vertical Alignment) displays can offer wide viewing angles, good black depth, fast response times, and good color reproduction and depth. Each pixel within a MVA type TFT consists of three sub-pixels (Red, Green and Blue). Each of these sub-pixels is divided further into two or more sub-pixels, where the liquid crystals are randomly lined up due to the ridged polarized glass. When a charge is applied to the transistor, the crystals twist. With these crystals being randomly placed, it allows the backlight to shine through in all different directions keeping the intended color saturation retained while giving the display a 150deg. viewing angle.

How it works:

1. Light is generated from a backlight source, typically LED. Light is generated as close to white spectrum.

2. Driver ICs will logically control to activate pixels on or off.

  » Inactive LCD pixels will block light

  » Active pixels will open with the direction of the light to let it pass through.

3. Top Circular polarization is added to enhance contrast

4. Color is added through a color filter to all sub-pixels (R,G,B)

IPS ― In-Plane Switching

In-Plane Switching (IPS) TFTs were developed to improve on the poor viewing angle and the poor color reproduction of TN TFT panels at that time. The crystal molecules move parallel to the panel plane instead of perpendicular to it. This change reduces the amount of light scattering in the matrix, which gives IPS its characteristic wide viewing angles and good color reproduction. Because of its wide viewing angle and accurate color reproduction (with almost no off-angle color shift), IPS is widely employed in high-end monitors aimed at professional graphic artists.

The name In-Plane Switching comes from the crystals in the cells of the IPS panel lying always in the same plane and being always parallel to the panel’s plane (if we don’t take into account the minor interference from the electrodes). When voltage is applied to a cell, the crystals of that cell all make a 90-degrees turn. By the way, an IPS panel lets the backlight pass through in its active state and shutters it in its passive state (when no voltage is applied), so if a thin-film transistor crashes, the corresponding pixel will always remain black, unlike with TN matrices.

IPS (In-Plane Switching) displays provide consistent, accurate color from all viewing angles without blur or grayscale inversion. IPS displays show clear images with fast response time, and no halo effect is produced when touched. Each pixel within an IPS type TFT consists of three sub-pixels (Red, Green and Blue). Each sub-pixel has a pair of electrodes to control the twisting of the Liquid Crystals. Unlike TN type TFTs where the electrodes are on opposing plates, the electrodes in an IPS TFT are on only one of the glass plates (i.e. in the same plane). When voltage is applied to the electrodes, all the Liquid Crystal molecules align in parallel with that plane and allow light to pass through to the polarizers and RGB color filters. In effect, TN displays force the Liquid Crystal molecules perpendicular to the glass which blocks some light from coming out at wide angles, while IPS displays keep the Liquid Crystal molecules in line to allow light through at all angles.   
 


LTPS

Low-temperature polycrystalline silicon (LTPS) is polycrystalline silicon that has been synthesized at relatively low temperatures (~650 °C and lower) compared to in traditional methods (above 900 °C). LTPS is important for display industries, since the use of large glass panels prohibits exposure to deformative high temperatures. More specifically, the use of polycrystalline silicon in thin-film transistors (LTPS-TFT) has high potential for large-scale production of electronic devices like flat panel LCD displays or image sensors.

Polycrystalline silicon (p-Si) is a pure and conductive form of the element composed of many crystallites, or grains of highly ordered crystal lattice. In 1984, studies showed that amorphous silicon (a-Si) is an excellent precursor for forming p-Si films with stable structures and low surface roughness. Silicon film is synthesized by low-pressure chemical vapor deposition (LPCVD) to minimize surface roughness. First, amorphous silicon is deposited at 560–640 °C. Then it is thermally annealed (recrystallized) at 950–1000 °C. Starting with the amorphous film, rather than directly depositing crystals, produces a product with a superior structure and a desired smoothness. In 1988, researchers discovered that further lowering temperature during annealing, together with advanced plasma-enhanced chemical vapor deposition (PECVD),could facilitate even higher degrees of conductivity. These techniques have profoundly impacted the microelectronics, photovoltaic, and display enhancement industries.

LTPS is needed for:

1. Circuits on glass – integrated drivers and scanners and multiplexer – reduces use of external IC and connectors to panel

2. Much smaller TFT ⇒ larger aperture ratio – mobile applications

3. Much more stable than a-Si under high current loading (OLED needs current to drive)

Transflective mode

Transflective LCDs combine elements of both transmissive and reflective characteristics. Ambient light passes through the LCD and hits the semi-reflective layer. Most of the light is then reflected back through the LCD. However some of the light will not be reflected and will be lost. Alternately a backlight can be used to provide the light needed to illuminate the LCD if ambient light is low. Light from the backlight passes through a semi-reflective layer and illuminates the LCD. However as with ambient lighting some of the light does not penetrate the semi-reflective layer and is lost.

Transflective LCDs are used in devices that will operate in a wide variety of lighting conditions (from complete darkness to full sunlight). Under dim lighting conditions transflective LCDs offer visual performance similar to transmissive LCDs, whilst under bright lighting conditions they offer visual performance similar to reflective LCDs. However this performance is a tradeoff because the transflective mode is less efficient due to some light loss.

A simple transflective display is shown as below, in which there are two regions, T and R respectively. The cell gap in two regions are different, dT = 2*dR. This is to maintain the reflection and transmission from two regions are the same intensity, and give same color reproduction, because in the T region, light only goes through the LC layer once, while in the R region, light passes through twice.

Reflective film
Integrate such film (like polarizer) at the front and backside of TFT panel.

Full viewing angle
Full viewing angle film
Integrating the film on the front of TFT-panel

Sunlight readability
1.Pure transflective TFT panel integrating the reflective layer on the pixel, in-cell.
2.Transmissive TFT panel with high brightness-High brightness LEDs + Bright enhancement film.