An LTPS display represents the use of a backplane technology made of a silicon-based material called low-temperature polysilicon. This material is responsible for turning individual pixels on and off. Several manufacturers have used this material in liquid crystal displays because of its advantages over other backplane technologies based on amorphous silicon or a-Si and indium gallium zinc oxide or IGZO. Some manufacturers have also been experimenting with using this material on organic light-emitting diode or OLED displays.
Pros: Advantages of LTPS Display and Applications
Older liquid crystal displays use amorphous silicon. Manufacturers of high-end displays have switched to low-temperature polysilicon because of its overall superiority. An LTPS display has inherent advantages over an a-Si display and even an IGZO display.
1. Better and Faster Electron Flow
A notable advantage of LTPS display is that it has larger and more uniform grains of polysilicon. Note that a-Si LCD has random-sized grains. Hence, in low-temperature polysilicon, electrons flow 100 times faster than in amorphous silicon. IGZO, on the other hand, has 30 to 40 times more electron mobility than a-SI. It is in this regard that LTPS remains inherently better than both amorphous silicon and indium gallium zinc oxide.
2. Higher Pixel Density and Resolution
The faster electron flow or better electron mobility translates further to higher pixel density and higher resolution. Manufacturers can produce display panels with better visual characteristics using low-temperature polysilicon than using amorphous silicon. The same advantageous electron flow translates to faster pixel response time. This improves the screen refresh rates of advanced LCD technologies such as in-plane switching or IPS LCD.
3. More Efficient Power Consumption
It is also the most power-efficient material for backplane technology. IGZO ranks second while a-Si ranks third. This lower power drive comes from its superior electron mobility and the fact that the number of components in the display module can be reduced by up to 40 percent by integrating row or column drivers onto the glass substrate. This makes LTPS ideal for mobile consumer electronic devices and other portable devices.
4. Flexible Display Application
A probable application of LTPS is in consumer electronic devices with flexible or bendable display panels. Its capacity to support high resolution and stable reliability make this material an ideal candidate for small-sized panels over other semiconductor materials. It is worth mentioning that flexibility is a critical issue in manufacturing small-sized consumer electronic devices such as smartwatches or other devices with small screens.
Cons: Disadvantages of LTPS Display and Limitations
1. Notable Manufacturing Challenges
Two of the critical disadvantages of LTPS center on its more complicated manufacturing process and higher material costs than amorphous silicon. A backplane technology based on this material is more expensive this produce. Take note that a 1080p low-temperature polysilicon TFT LCD would cost about 12 to 14 percent more than an a-Si TFT LCD. This drives up the overall costs and end-user price of consumer electronic devices.
2. Faster Aging and Degradation Rate
Another disadvantage of LTPS display panel is that it has a shorter lifespan than display panels based on either a-Si or IGZO. This material is more susceptible to heating issues from the operations of the transistors. Uncontrolled temperature leads to subsequent damage due to overheating. High temperature breaks down its silicon-hydrogen bonds. The quality of the panel decreases over time due to repeated exposure to thermal stress.
3. Advantages of IGZO and LTPO
It is also worth mentioning that IGZO has an electron mobility that is nearly as high as LTPS on top of a lower leakage current. LTPS and a-Si have high leakage current that necessitates continuous pixel refresh when displaying a still image. IGZO displays retain their active state longer than the two materials. A newer material called low-temperature polysilicon oxide or LTPO combines the advantages of LTPS and IGZO.
A Note on the Applications of Low-Temperature Polysilicon
It is important to reiterate that low-temperature polysilicon or LTPS is a backplane technology that can be applied not only in LCD but also in other display technologies. It has been used to improve the quality of IPS LCD. It is also applicable in OLED or AMOLED display technology. Researchers and manufacturers are also opting to use this material for use in emerging display technologies such as LCD with Mini-LED and MicroLED display technology. Apple has developed a new material that combines silicon-based and oxide-based materials. This is called low-temperature polysilicon oxide or LTPO.
FURTHER READINGS AND REFERENCES
- Bo, X-Z., Yao, N., Shieh, S. R., Duffy, T. S., and. Sturm, J. C. 2002. “Large-Grain Polycrystalline Silicon Films with Low Intragranular Defect Density by Low-Temperature Solid-Phase Crystallization Without Underlying Oxide.” Journal of Applied Physics. 91(5): 2910-2915. DOI: 1063/1.1448395
- Chang, T-C., Tsao, Y-C., Chen, P-H., Tai, M-C., Huang, S-P., Su, W-C., and Chen, G-F. 2020. Flexible Low-Temperature Polycrystalline Silicon Thin-Film Transistors. Materials Today Advances. 5: 100040. DOI: 1016/j.mtadv.2019.100040
- Konsyse. 2020. “What is LTPS LCD? What are the Pros and Cons?” Konsyse. Available online
- Miyata, Y., Furuta, M., Yoshioka, T., and Kawamura, T. 1992. “Low-Temperature Polycrystalline Silicon Thin-Film Transistors for Large-Area Liquid Crystal Display.” Japanese Journal of Applied Physics. 31(P. 1, No. 12B): 4559-4562. DOI: 1143/jjap.31.4559
