Oxide TFTs: The Future of AMOLED Displays

By Rama Krushna Mahanty

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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.

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.

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