In 2012, two new approaches to TV have come to the forefront: large-sized OLED and 4K x 2K LCD TVs. Both involve technical challenges, and in their first versions are likely to carry price tags in the $10,000-and-up range. The path to mass adoption is not yet clear. However, the technology called oxide-TFT technology could play a key role in the development of either or both of these approaches.

by Paul Semenza

IN RECENT YEARS, new features in flat-panel TVs have included high color gamut, 3-D, LED backlights, thin, ultra-slim bezel, 120–480-Hz frame-rate driving, fast response time, and high brightness. The majority of TV households around the world have switched to flat-panel TVs as their national broadcast systems have converted from analog to digital and to HD (high definition). In some cases, the above features have been instrumental in selecting a TV, but not in driving new or additional TV purchases. So with sales growth at a standstill, the industry is searching for the next new technology that will drive it forward. During the past year, OLED and 4K x 2K TVs have emerged as the leading candidates. While there have been small (less than 20 in.) OLED TVs produced in the past, the current focus is on 55-in. screen sizes, in HD format (1920 x 1080). The 4K x 2K sets currently coming to market all are LCDs; while it is possible to produce 4K x 2K OLED TVs, existing manufacturing techniques are limited to HD.

For large-sized OLED TVs, the promise is in a thinner set with image quality superior to today's LCD TVs. However, panel makers are struggling to enter into mass production; despite having made large investments in cutting-edge factories, significant manufacturing challenges have yet to be solved for large sizes. NPD DisplaySearch estimates that the manufacturing cost for a new OLED TV is 10 times that of a high-end LCD TV of the same size. Thus, despite the great promise of OLED TV, manufacturers have delayed the start of mass production while they improve manufacturing processes.

The 4K x 2K (specifically 3840 x 2160 pixels) format, also referred to as ultra-high definition (UD), has four times the number of pixels of full HD displays, currently the state of the art in resolution for TVs. To many, this enhanced resolution gives a level of reality that surpasses any 3-D effects, without artifacts. (Some would go further, suggesting 8K x 4K is ideal.) As higher resolution becomes standard in mobile devices such as smart phones and tablet PCs, there is greater consumer awareness, and, unlike for other new features (such as smart, connected, 3-D TV, and LED backlights), viewers can easily observe the effect of higher resolution. For TVs greater than 60 or 70 in., some argue that full HD is not good enough, as the pixel density is quite low. Thus, LCD-TV panel makers have developed many 4K x 2K panels.

At the IFA consumer electronics trade show in Berlin last September, Samsung and LG competed once again for the position of leading OLED-TV developer, although the commercial availability was left unclear. Other TV brands, which are not currently able to develop OLED TVs, responded with 4K x 2K TV announcements, seeking to jump ahead in the large-panel commercialization race (Table 1). Chinese, Japanese, and also Korean brands introduced a range of 4K x 2K TVs and indicated that they plan to come to market in 2012. While there are only two OLED-TV panel suppliers at present, there are several 4K x 2K panel suppliers.

Oxide TFT: Filling the Gap Between High Performance and Low Cost

Many producers of OLED TV and 4K x 2K panels have been investigating oxide-TFT technology as a candidate for the active-matrix backplane. For OLED TV, a-Si TFTs have not been shown capable of providing the current required to drive large arrays of OLEDs (as opposed to LCDs, which are driven by voltage). For 4K x 2K panels, a key issue is the ability to drive millions of pixels at high frame rates (limited by RC time delays). While, at present, nearly all 4K x 2K panels are a-Si TFT-LCDs, many panel makers expect that oxide TFT will be a more economically feasible approach to producing such panels.

The most common oxide-TFT technology is IGZO (indium gallium zinc oxide). The most important characteristic of oxide TFTs is electron mobility. Traditional a-Si has an electron mobility of 0.5 cm²/V-sec, while IGZO is 10–30 cm²/V-sec, and LTPS is the highest at 50–150 cm²/V-sec, depending on the operating temperature. However, LTPS requires eight or nine photomask steps, including laser annealing and ion implantation, which are complex steps. IGZO is similar to a-Si, as the TFT structure of IGZO is bottom-gate inverted staggered, just like a-Si; IGZO does require one additional mask step compared to a-Si, for a total of six. In general, IGZO appears to be the easiest approach for an existing a-Si panel maker to move to high-mobility panel production.

There are disadvantages to IGZO. Because it is a metal oxide, oxygen, a very active material, is used in the process, and threshold-voltage stability is very low. Also, the uniformity of the electron mobility is lower than that for a-Si or LTPS. Finally, IGZO uses rare-earth metals (indium and gallium), which introduces risk in procurement and cost increases.

Panel Makers' Strategies for Oxide TFT

In Taiwan, AU Optronics Corp, (AUO) is converting some Gen 6 and 8 capacity to oxide TFTs, mainly for AMOLED backplanes. In addition to shifting some Gen 6 capacity to oxide TFTs, BOE is planning on implementing oxide TFTs at its Gen 8 fab as well as building a new Gen 8 IGZO fab in Beijing. Chimei Innolux is less enthusiastic because it does not have a concrete plan to develop large-sized AMOLED TVs. Its parent, Foxconn, is building a Gen 6 LTPS fab in Chengdu, where it plans to implement oxide TFTs for AMOLED backplanes. While it is expected that the same line could produce both IGZO and a-Si TFTs for LCD backplanes, it is not anticipated that lines will be able to switch between IGZO TFTs for OLED backplanes and IGZO or a-Si TFTs for LCD backplanes.

LG Display has shifted some capacity to IGZO in its Gen 8 fabs, which will be used to produce 4K x 2K LCD panels and AMOLED backplanes. The company is also shifting some capacity at its Gen 5 and 6 fabs. Samsung Display has only shifted some Gen 5 capacity to oxide TFTs, but its strategy may be changing.

Sharp has been the most aggressive at moving to oxide TFT. Since 2011, Sharp has begun transforming its Gen 8 to IGZO to produce high-resolution LCDs for tablet PC panels. In 2013, Sharp plans to start changing its Gen 10 gab over to oxide TFTs for the production of 4K x 2K LCD-TV panels.

In the future, new fabs will be designed to be IGZO compatible, but most current IGZO capacity is being converted from a-Si (Table 2). Because IGZO production is more complicated than a-Si, there are yield and cost tradeoffs. Just because a-Si capacity is converted to IGZO does not mean it will produce only oxide FPDs; the production mix will depend on costs and the market. Most panel makers are looking to utilize oxide TFTs for AMOLED TVs or 4K x 2K LCD-TV panels, although as mentioned, most 4K x 2K production has used a-Si, and the leader in oxide TFT, Sharp, is focused on LCDs for tablet PC panels.

4K x 2K: Technically Feasible, but Infrastructure-Limited?

One significant difference between 4K x 2K and OLED TV is that 4K x 2K requires changes in the production chain – from TV cameras and studios, through content distribution and transmission to the set at home – that are more like the HD and 3-D transitions. The key issues are the compression of video files to manageable sizes, delivery of video files, playback, and connection of the player to the display. Current compression technology (H.264/MPEG-4) results in very large amounts of data for 4K x 2K content; not only does doubling the resolution produce four times the pixels, movies are also transitioning from the standard 24 fps to 48 fps, an additional doubling of the data. As a result, movie files can be in the terabytes. A new compression standard – HEVC (high efficiency video coding, also referred to as H.265) – is being created, but it is not yet mature.

Early encoders will be large and relatively inefficient; similarly, HEVC decoder ICs will be few and costly at first. In decoding, 4K x 2K will require extra memory. Six frames will be necessary in storage for HEVC decoding and such large volumes of data and high frame rates will put a strain on memory bandwidth.

Even with HEVC, the problem of delivery remains. Internet speeds and content-delivery networks are not good enough to support such data traffic. For disc formats, 4K x 2K would require a re-write of the Blu-ray specification to support the standard and a new generation of players with 4K x 2K decoding and output. In the short term, 4K x 2K playback Blu-ray players that up-scale are appearing. While the marketing benefit of a player with 4K x 2K output is obvious, it is less clear why such up-scaling should be done in the player and not the TV set. The display connection is actually fairly simple, with HDMI able to handle the data within the 1.4a specification.

For broadcasters, the problems are more complicated. In satellite and cable, 4K x 2K could be transmitted today, assuming that decoder devices were available. However, the increase in bandwidth would require extra channels and thus extra satellite transponders. Since 4K x 2K will cost roughly 4 times that of an HD channel, it would need to provide increased revenues. For terrestrial broadcasters, spectrum is scarce, and it is likely that terrestrial 4K x 2K would not be feasible until HEVC becomes mature and is similar to MPEG-4 HD. It is worth noting that even today, 1920 x 1080p broadcasts are rare; most are interlaced, with many broadcasters actually downscaling to 1440 x 1080i and then stretching in decode.

As in 3-D, 4K x 2K display technology is ahead of the rest of the content chain. HD broadcast took more than 20 years to bring to maturity. NHK and the BBC have indicated that they make significant changes every few decades and appear to be focusing on what they consider achievable in that time scale – 8K x 4K, not 4K x 2K. Any product launches in the meantime will have to survive incomplete content-delivery infrastructures. Therefore, at this time, 4K x 2K does not appear to have strong interest from broadcasters.

However, there are other possible applications, such as a family communication screen that combines conventional TV tasks with others currently done by such devices as noticeboards, refrigerator doors, etc. These would be used at far closer ranges, where such resolution would be perceivable and valuable. The PC industry is also seeing a resurgence in increased resolution; the latest MacBook Pro has 2880 x 1800 pixels and Apple's competition is sure to respond. 4K x 2K could be the start of a broader rethink of TV's function as the best screen in the house, encompassing new applications.

Let the Market Decide

At the beginning of 2012, it appeared that OLED TV would be the leading new TV technology. However, manufacturing challenges have slowed the launch. Meanwhile, other panel makers have jumped into the fray with high-resolution LCD TVs. Both of these technologies are expected to be very expensive when they first become commercially available – perhaps just under $10,000 for a 55-in. OLED TV and over $20,000 for an 84-in. 4K x 2K LCD TV. (See this issue's Industry News for an overview of the 84-in. 4K TVs recently introduced by both Sony and LG.) Whether either crosses the chasm to mass adoption will depend on the perceived benefit, in terms of design and performance, as well as in functionality – can consumers use these sets for things that "old-fashioned" HDTVs or smaller OLEDs cannot do? •