LTPS or low-temperature polysilicon is a silicon-based material used in semiconductor components and devices. In liquid crystal or LCD technology, it is specifically a backplane technology and a major component of thin-film transistors or TFT responsible for turning individual pixels on and off.
Manufacturers have been using LTPT LCD in some of their consumer electronic devices because of its advantages over other backplane technologies based on amorphous silicon or a-Si and indium gallium zinc oxide or IGZO.
The Pros: Advantages and Applications of LTPS LCD
Conventional LCDs use amorphous silicon. However, one of the reasons why manufacturers are switching to low-temperature polysilicon is overall superiority. More specifically, LTPS LCD has inherent advantages over a-Si LCD and even IGZO LCD.
Better and Faster Electron Flow
A notable advantage of LTPS LCD 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. Thus, it is in this regard that LTPS remains inherently better than both amorphous silicon and indium gallium zinc oxide.
Higher Pixel Density
The faster electron flow or better electron mobility translates further to higher resolutions and faster pixel response time. Hence, manufacturers can produce LCDs with higher pixel density with low-temperature polysilicon than a-SI while improving the refresh rates of advanced LCD technologies such as in-plane switching or IPS LCD.
More Efficient Power Consumption
It is also the most energy efficient, with IGZO ranking in second and a-Si in third. This power efficiency comes from its superior electron mobility, as well as from the fact that the number of components in the LCD module can be reduced by up to 40 percent by integrating row or column drivers onto the glass substrate.
Flexible Display Application
A probable application of LTPS is in consumer electronic devices with soft and flexible displays. Furthermore, its capacity to support high resolution and stable reliability make this technology an ideal candidate for portable displays over other semiconductor materials. Note that flexibility is a critical issue in manufacturing small-sized portable displays.
The Cons: Disadvantages and Limitations of LTPS LCD
Two of the major drawbacks of LTPS is that it has a complicated manufacturing process and higher material costs than a-Si. Thus, backplane technology based on this material is more expensive to produce. A 1080p low-temperature polysilicon TFT LCD would cost about 12 to 14 percent more than an amorphous silicon TFT LCD.
Faster Degradation Rate
Another disadvantage is that LCDs based on this technology have a shorter lifespan than those based on a-Si and IGZO. The quality of LTPS LCD decreases over time due to overheating. Note that turning transistors on produces heat. Low-temperature polysilicon is susceptible to overheating. High temperature degrades the entire thin-film transistor by breaking the silicon-hydrogen bonds on the material.
Indium Gallium Zinc Oxide Advantages
Note that IGZO has an electron mobility nearly as high as low-temperature polysilicon. However, it has a lower leakage current. Both 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.
A Note on the Applications of Low-Temperature Polysilicon
It is important to highlight the fact that low-temperature polysilicon or LTPS is a backplane technology that can be applied not only in LCD but also in other display technologies. For starters, it has been used to improve the performance and quality of in-plane switching or IPS LCD. It is also applicable in organic light-emitting diode or OLED technology. Researchers and manufacturers are also opting to use this material for use in emerging display technologies to include mini-LED LCDs and microLED display technology.
A hybrid between IGZO and LTPS is possible. Apple Inc. demonstrated in its Apple Watch devices that it is possible to combine silicon-based and oxide-based materials with the so-called low-temperature polycrystalline oxide or LTPO display.
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