a-Si-TFT
Amorphous silicon has been the go-to material for backplane technology for many years, and comes in a variety of different manufacturing methods, to improve its energy efficiency, refresh speeds, and the display’s viewing angle. Today, a-Si displays make up somewhere between 20 and 25 percent of the smartphone display market.
A spec comparison of common TFT types.
For mobile phone displays with a pixel density lower than 300 pixels per inch, this technology remains the preferable backplane of choice, mainly due to its low costs and relatively simple manufacturing process. However, when it comes to higher resolution displays and new technologies such as AMOLED, a-Si is beginning to struggle.
AMOLED puts more electrical stress on the transistors compared with LCD, and therefore favours technologies that can offer more current to each pixel. Also, AMOLED pixel transistors take up more space compared with LCDs, blocking more light emissions for AMOLED displays, making a-Si rather unsuitable. As a result, new technologies and manufacturing processes have been developed to meet the increasing demands made of display panels over recent years.
LTPS-TFT
What is LTPS
LTPS (Low Temperature Poly-silicon) is a Si-based material that consists of numerous crystalline silicon clusters of 0.3 up to several microns. In the semiconductor manufacturing industries, poly-silicon is usually formed by solid phase crystallization (SPC) which involves annealing at a temperature above 900°C. However, since the deformation temperature of glass is only 650°C, the SPC method is not suitable for the flat panel display manufacturing industries. As a result, the LTPS technology is a poly-silicone technology dedicated to applications for flat panel displays.
Illustrations Structures of silicon and mobility schematic
Advantages of LTPS Technology
The process of LTPS is far more complex than a-Si. On the other hand, the carrier mobility in LTPS TFT is 100 times (>100 cm2/V‧s) higher than that in a-Si TFT. In addition, LTPS allows CMOS processes directly on the glass substrate. The following are several aspects that p-Si is superior to a-Si:
Slim border: Conventional amorphous silicon display structure requires two or three edges on the frame to accommodate the driver IC, which makes slim border difficult. On the other hand, LTPS can directly integrate the drive circuits onto the glass substrate, which facilitates a slim border and high picture quality for the panel. In addition, the integration circuit of LTPS requires fewer number of external signal connections, which reduces the module components by 40% and effectively lowers the cost.
Compact module: Since part of the drive circuit can be fabricated on the glass substrate, the PCB circuit can be relatively simple and saves more PCB area.
High aperture ratio and high resolution: High mobility means smaller geometry of transistors are capable of providing sufficient charging power and higher capacitance than conventional amorphous silicon, which translates to a larger effective area transmitted by light. Take the 2-inch QVGA by AUO as an example, since its aperture ratio is as high as 58%, the number of backlight LEDs can be reduced, enabling cost reduction and energy saving, which is suitable to applications for cell phones or mobile devices.
Vehicle for OLED: Organic Light-Emitting Device (OLED) display has advantages such as fast response time, light weight, low power, and wide viewing angle. Compared to a conventional LCD, the drive method of OLED is a special current drive structure. In addition, the compensation circuit designed to overcome gray scale and achieve panel uniformity requires using two to six TFTs in one pixel. With the high-density layout characteristic of LTPS, an OLED display panel with high brightness and high picture quality can be easier to achieve. Meanwhile, the lifetime of OLED can be extended. High mobility means more current can be provided to the OLED device, which is more suitable to be used as the substrate of active OLED display.
InGaZnO-TFT
Currently, a-Si and LTPS LCD displays make up the largest combined percentage of the smartphone display market. However, IGZO is anticipated as the next technology of choice for mobile displays. Sharp originally began production of its IGZO-TFT LCD panels back in 2012, and has been employing its design in smartphones, tablets and TVs since then. The company has also recent shown off examples of non-rectangular shaped displays based on IGZO. Sharp isn’t the only player in this field — LG and Samsung are both interested in the technology as well.
Smaller transistors allow for higher pixel densities
The area where IGZO, and other technologies, have often struggled is when it comes to implementations with OLED. ASi has proven rather unsuitable to drive OLED displays, with LTPS providing good performance, but at increasing expense as display size and pixel densities increase. The OLED industry is on the hunt for a technology which combines the low cost and scalability of a-Si with the high performance and stability of LTPS, which is where IGZO comes in.
Why should the industry make the switch over to IGZO? Well, the technology has quite a lot of potential, especially for mobile devices. IGZO’s build materials allow for a decent level of electron mobility, offering 20 to 50 times the electron mobility of amorphous silicon (a-Si), although this isn’t quite as high as LTPS, which leaves you with quite a few design possibilities. IGZO displays can therefore by shrunk down to smaller transistor sizes, resulting in lower power consumption, which provides the added benefit of making the IGZO layer less visible than other types. That means you can run the display at a lower brightness to achieve the same output, reducing power consumption in the process.
One of IGZO’s other benefits is that it is highly scalable, allowing for much higher resolution displays with greatly increased pixel densities. Sharp has already announced plans for panels with 600 pixels per inch. This can be accomplished more easily than with a-Si TFT types due to the smaller transistor size.
Higher electron mobility also lends itself to improved performance when it comes to refresh rate and switching pixels on and off. Sharp has developed a method of pausing pixels, allowing them to maintain their charge for longer periods of time, which again will improve battery life, as well as help create a constantly high quality image.
Smaller IGZO transistors are also touting superior noise isolation compared to a-Si, which should result in a smoother and more sensitive user experience when used with touchscreens. When it comes to IGZO OLED, the technology is well on the way, as Sharp has just unveiled its new 13.3-inch 8K OLED display at SID-2014.
Essentially, IGZO strives to reach the performance benefits of LTPS, whilst keeping fabrications costs as low as possible. LG and Sharp are both working on improving their manufacturing yields this year, with LG aiming for 70% with its new Gen 8 M2 fab. Combined with energy efficient display technologies like OLED, IGZO should be able to offer an excellent balance of cost, energy efficiency, and display quality for mobile devices.
匿名