Cutting-Edge LCDs: What You Need to Know
The display industry is in the middle of one of the most significant technical transitions of the last 20 years and it is also mostly invisible to the average consumer. It has all to do with the advances taking place in transistors – the electronic switches that control the display’s picture, providing clarity and crisp imagery.
Every pixel on your LCD screen is turned on or off by a transistor. The complete array of pixel transistors is known as a backplane, as you can see in the graphic. Clearly, the performance of the backplane directly affects the quality of the display for your TV, smartphones and tablet PCs. How fast the switch can be turned on and off refers to the refresh rate and the total number of pixels on the screen equals picture resolution.
Today, there are three backplane technologies, which we call amorphous silicon (a-si), low temperature poly-silicon (LTPS) and metal oxide (MO). If you are buying a TV, should you care what transistor technology is in it?
Before we talk backplane technologies, let’s go into some definitions. Most consumers are interested in sharpness of picture, brightness of color, viewing angle, and no visual “jagging” of images in motion. Resolution is one key specification which most consumers are familiar with.
HD or high definition is the current standard for TVs and is the measure of the number of pixels that make up the screen image. HD TVs have 1080 rows x 1920 columns for a total of 2,073,600 pixels. (In fact, there are three times that many pixels because each one is broken down into red, blue and green sub-pixels.) But even HD resolution for a 55 inch TV is only 40 pixels per inch, or PPI. Smartphones and tablets are generally held much closer to the user’s eyes, so much higher PPIs are common. For example the Apple iPad 3 has an amazingly high 264 PPI resolution. However, as TVs go larger in size, ultra definition is on the roadmap to increase picture resolution.
Another key specification is refresh rate. 60Hz – meaning the picture refreshes itself 60 times per second – tends to be a minimum standard. To display fast-moving images more smoothly, many manufacturers have gone to 120Hz and 240Hz; it is mostly a matter of consumer preference as to whether you can discern the difference in picture quality to justify the price.
3-D content is a technology that requires at least a 120Hz refresh rate. The dominant 3-D technology used today creates the illusion of depth by actually displaying two different sets of images. 3-D glasses allow each eye to see only one set of images at a time and your brain then translates the dual image inputs into a single image with depth.
Higher resolution, faster refresh rates and 3-D capability are moving beyond the capacity of the a-Si transistor.
For most of the history of displays, a-Si has been the dominant material for the transistor backplane. A-Si thin film transistors are relatively inexpensive to manufacture, have a high manufacturing yield and it has been relatively easy to scale the process glass sizes up to a whopping 9m2. That’s a major reason why TFT-LCD displays have come to be the dominant technology as consumer have come to expect the cost reductions that this scalability has enabled.
The transistor’s objective is to charge the pixel to the operational voltage quickly and hold the charge until the pixel is refreshed. It turns out that a key property of the transistor is carrier mobility which is the speed at which electrons can move about. Slow electron mobility translates to sluggish response time. The carrier mobility of a-Si is no longer sufficient for high resolution and high response rate displays such as in the latest smartphones and tablet PCs.
The alternative choices are LTPS and MO. The best alternative will be a material with higher carrier mobility, low manufacturing costs, high manufacturing yield and the ability to scale the process to large glass (while maintaining good film uniformity) to take advantage of the reduction in cost per square meter of glass associated with manufacturing on larger size glass panels.
LTPS offers the highest available carrier mobility but is more costly to manufacture as it requires additional manufacturing steps. It’s also challenging to achieve good film uniformity and high manufacturing yield. In addition, the scalability of LTPS is limited by the laser annealing equipment that currently is only available up to medium size glass (less than 3m2 or so).
MO technology, using on indium gallium zinc oxide (IGZO) is the most promising alternative technology that can achieve a desired balance between high mobility and good uniformity on large area glass. (Today, in fact, we announced new PECVD technology for depositing advanced insulating films to enable MO transistor manufacturing on huge glass up to the 9m2 I mentioned above.) MO requires fewer manufacturing masking steps than LTPS and can be deposited at room temperature, meaning it is conceivable that the process could be used in the future to make flexible displays.
If the industry is successful in making the leap to new backplane technologies, the change will be cost effective and relatively transparent to the consumer. Hopefully this article will be helpful the next time you’re in a store trying to pick out the best choice when faced with a wall of shiny new televisions.