Oxide TFTs: The Future of AMOLED Displays

By Rama Krushna Mahanty

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However, mobility and uniformity play a significant role in large-sized AMOLED display. So, There was a need for the material in the display industry, which can produce higher mobility and better uniformity with lower production cost. At the beginning of 20th century, several studies reported regarding transparent conducting oxide (TCO) and transparent semiconductor oxide (TSO). They alter their conductivity according to the amount of oxygen present in the thin film content. Apart from being an excellent semiconductor material, they also exhibit optical transparency. Thus, several transparent semiconductor materials like SnO2, ZnO, and IGO are studied as a channel material for transparent TFTs. Unlike other semiconductor materials, these oxide semiconductors can be fabricated at room temperature.

These materials are extensively studied on the glass and flexible plastic substrate owing to their low-temperature fabrication and post-deposition treatment. Because of room temperature fabrication, these materials can retrain their amorphous structure. So, They won’t suffer from low carrier mobility caused by poly-crystalline structure. The amorphous structure provides a smoother oxide-semiconductor interface with the lower amount of charge trapping site. Lower charge trapping provides better field-effect mobility, which results from a better on-current in the TFTs. Moreover, OLED pixels need high current injection to emit light. Current generation oxide transistor shows up to 40 times higher electron mobility as compared to conventional amorphous silicon transistor. Higher electron mobility causes higher on the current in oxide-based transistors. Thus, oxide TFTs with high on-current is strongly recommend for AMOLED display. Oxide-based TFTs can be miniaturised in size because of their higher mobility and faster switching capability. This means more pixel can be packed in the defined panel area resulting in higher pixel density. as compared to conventional amorphous silicon, most of TSOs are having smaller subthreshold voltage swing because of lower defect density. Lower subthreshold voltage swing makes oxide-based TFTs a suitable candidate for lower voltage operation device. However, for industrial application TSOs need to meet a certain level of manufacturability and reliability.

The material should hold its characteristics in extreme operating condition. Moreover, it should not show any offset in its characteristics with time. Keeping all those things in mind, TFT manufacturers focused on Indium Gallium Zinc Oxide (IGZO) as compare to other TSOs. A good quality of IGZO can be fabricated at room temperature using sputtering technique. Several research group have exhaustively tested IGZO with different substrate and dielectric layer. As compare to other TSOs, IGZO provides better current density during on condition and very low leakage current during off period. Some research group have reported on/off current ratio of 109 in IGZO TFT. The intensity of leakage current plays a critical role in large flat panel display. While running screensaver or browsing photos, all of the pixels hold the same color for a long duration of time. High leakage current causes TFTs to drain their charges. Thus, they need to be continually refreshed after a certain period of time. Unlike other TFTs, IGZO TFTs can retain their active state for a longer time because of their low leakage current. So, a great deal of power can be saved on mobile devices with IGZO TFTs. Along with that, IGZO is also able to take tensile stress on it. . Which makes it a suitable candidate for flexible display material.

Despite having several advantages, oxide-based TFTs are having some drawbacks. The major drawback of oxide-based TFTs is mobility uniformity. Although it shows better mobility uniformity as compared to LTPS, still lower uniformity is observed as compared to a-Si panels. This causes different TFTs having different on current across the panel. Several research groups have also reported a change in threshold voltage owing to charge trapping at the oxide-semiconductor interface. Recently, researchers are putting their efforts into studying the reason behind device instability and to improve long-term reliability. Different TFTs are being subjected to different stress conditions such as positive and negative biasing stress, temperature stress, and humidity stress. Unlike a-Si, oxide-based TFTs uses several expensive materials like Indium and Gallium. So, manufacturers don’t want to take a chance because of poor yield caused by poor stability. Several independent researchers and manufactures aggressively trying to address all these issues with choosing the different High-k dielectric material and with different pre and post process treatment.

Some giant flat panel manufacturers claim that they are ready for mass production of IGZO based oxide-display. So, in a couple of years, we can see next generation of the display which can turn our imagination into reality.


 

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