LCDs, Aperture Ratios and Hummingbirds

Nov12012

Why is a smartphone like a hummingbird? Neither can go more than a few hours without refueling, or bad things happen. (If you answered that smartphone owners are often forced to flutter around looking for a power outlet, I’ll accept that, too.)

Do you know where the power goes? As the graphic shows, around half your battery is spent on the display alone. And of that, the vast majority is used simply to power the backlight that all LCDs need. (The situation for AMOLED displays is similar, but for different reasons. That’s a subject for a future blog.)

Clearly, improving the power efficiency of the display is a powerful way to improve battery life. Turning the brightness down isn’t a helpful strategy. Indeed, we expect displays to be brighter and richer all the time, but not at the expense of already limited battery life. (Bigger batteries would also work, but the market has spoken on that one. Slim is in!)

How is this to be accomplished?

A high-definition LCD has an array of six million or so solid-state “shutters” that transmit or block light from the backlight at each red, green or blue sub-pixel. Each pixel has a transparent electrode that controls the actual liquid crystal material. Each electrode is controlled by, and shares real estate with, a thin-film transistor (TFT). The TFT blocks part of the light. The area ratio of the opaque transistor and the transparent electrode is a key metric called the aperture ratio.

So let’s make the transistors smaller. Obvious, right? Easier said than done. A smaller transistor will be slower, slowing the response time. The trend towards higher resolution makes things worse. The transistor size is effectively fixed, so only the electrode shrinks and so the aperture ratio actually goes down, as you can see from the graphic. This makes the display less energy efficient.

The solution is to build the transistor from better materials such as metal oxide or polycrystalline silicon (usually known as low-temperature polysilicon because of the way it’s made). These new materials have much higher electron mobility than ordinary amorphous (i.e. non-crystalline) silicon, allowing the transistor to be smaller, faster and more power-efficient. We’ve blogged about this in more detail before. See the links in the sidebar.

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Simply by replacing conventional amorphous silicon transistors with metal oxide ones, the aperture ratio can be doubled, cutting the necessary backlight power in half for the same brightness. Stepping up to LTPS provides further improvement. That means battery life could go up by a full 25%, which could be the difference between wondering if your smartphone will last the day and knowing it will.

The benefits of metal oxide and LTPS have been known for a while, but bringing them to mass production has been a challenge. For metal oxide, a key challenge has been depositing the transistor channel material, indium gallium zinc oxide (IGZO) with uniformity and stability high enough to achieve good production yield. Until now, LTPS has been held back by the lack of equipment capable of handling the really big glass sheets that bring economies of scale.

Earlier this week, we announced new technology to help move both metal oxide and LTPS into the mainstream (press release). The Applied AKT-PiVot™ PVD and Applied AKT-PX PECVD thin film deposition systems provide display manufacturers the tools they need to usher in the next era of ultra-high definition (UHD) televisions and high pixel density screens for mobile devices.

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