Saturday, May 18, 2024

Oxide TFTs: The Future of AMOLED Displays

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Mobile technology has become a vital part of our life within last few decades because of there auspicious effect on our lifestyle to make it easier. The technology that drives mobile technology has improved a lot in last decade making all of the mobile devices thinner, smaller, efficient and more powerful. Mobile devices are not only limited to having voice calls, text message, reading e-mails, but also then can provide high end stuffs like photos, documents editing, and navigation. all these things possible because of the amelioration caused by the mobile device technology, which brings the whole world immaculately to our fingertip. However, the display technology also deserves improvement over time, as it is the most dominant interface between the mobile devices and user.

Display manufacturers were earlier concerned about better color contrast, brightness, and higher resolution. Nowadays display technology has achieved a lot of improvement with better viewing angle and display quality, which draws consumers interest for bigger and better display type. As mobile devices are equipped with limited power supply, so they will suffer from lower battery life with a bigger display. manufactures should be concerned about power consumption, refresh rate for bigger and high-resolution display to prepare next generation display.

In display industry, the organic light emitting diode (OLED) successfully replaced light emitting diodes (LED) because of its lower power consumption, better picture quality, better durability. Passive matrix (PM) was the first driving principle to run OLED display panels. Passive matrix addressing system uses conduction grid to send the activation signal to the desired pixel using horizontal and vertical grid line. It was popular because of its simple working principle and easy to fabricate. However, increasing display size will increase the control line to drive the display.

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Higher voltage is required to achieve higher brightness and resolution for the passive matrix with bigger size display. High voltage operation of the will reduce the lifespan of display owing to several reliability issues occurred in the devices. Power consumption of the driving circuits will also increase exponentially with increased size of display with passive matrix. The above reasons limit the size of passive matrix display only up to 3 inch. so, passive matrix displays are limited to applications like mp3 players, digital watches, calculators, personal digital assistant (PDA) and driver information system (DIS) display used in the car. The above limitation encouraged the emergence of the active matrix for bigger size display. Active matrix addressing system includes a storage capacitor along with the thin film transistor (TFT) to address each pixel. The inclusion of storage capacitor enables the pixel to hold the charge for a limited period of time. so, the addressing driver need not to send continuous signal to keep the pixel turn on for a longer time.

Only the desired pixel receives a signal while switching off the TFT for the turned on pixel can hold the previous signal in storage capacitor until next refresh cycle. Active matrix addressing system provides faster, brighter and more colorful images comparing to the passive matrix, owing to their improved technology. Usually, active matrix addressing uses more than one TFTs to drive each pixel. some of them used to switch the storage capacitor, while other transistors used to provide the voltage source to the pixel. such addressing principle dramatically reduces the driving current of the pixel compared to the passive matrix. Implementation of active matrix addressing principle to drive OLED display opens the gate for new era called, an active-matrix organic light emitting diode (AMOLED). AMOLED display provides deeper contrast, wide viewing angle, richer color as well as consumes lower power. If a normal display screen is turned on, then its backlight is turned on.

Irrespective of the content on the display, power consumption will be same. power consumption is mostly dependent on brightness of the display. The user needs to set the brightness to an acceptable level to obtain an optimized battery life. Whereas in AMOLED display no backlight is used and the pixel is the source of light itself. If a pixel needs to show black color, it simply turned off and consume no power. So, power consumption is less for a darker image as compared to a brighter image. Several portable electronics companies provide always on display(AOD) to show some vital information like date, time and it consumes considerably less power compare to normal display. In current generation AMOLED display, either Low-temperature polysilicon (LTPS) or amorphous silicon (a-Si) TFTs are used as switching element. In a-Si TFTs, amorphous structure causes carrier trapping and dramatically reduces the mobility of the transistor. The LTPS type of TFT panel stuffers from poor uniformity.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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