Tablet PC and Smartphone Displays Converge
The range between 5- and 9-in. screen sizes is emerging as the “sweet spot” for high-volume mobile devices. At the same time, display resolutions will continue to increase rapidly, with the mainstream pixel density for mobile phones moving above 300 ppi (pixels per inch) and above 200 ppi for tablet PCs. These trends provide significant opportunities for makers of small-to-medium high-resolution displays.
by Paul Semenza
THE SHIFT toward smartphones (mobile phones with processors capable of running operating systems and applications) has created significant demand for ever-larger screen sizes, which enhance the experience of games and other mobile applications, as well as usage of e-mail and the Internet. Starting with 5-in. displays in 2012, and followed by 6-in. displays in 2013, the mobile-phone market has shifted dramatically toward larger screen sizes. By 2018, 5-in. and larger displays will account for one-half of the mobile-phone market (Fig. 1).
Screen size is only part of the challenge of improving the mobile experience; for many tasks, information content and resolution are the key characteristics. The simplest measure in this regard is the number of pixels in the display. While there are many different pixel formats, the pixel counts on mobile-phone displays have increased dramatically. In 2012, three-quarters of mobile phones had less than a half-million pixels (equivalent to SVGA format); in 2014, a third will have 1 million pixels (HD) or more; and in 2015, a quarter will have 2 million pixels (FHD) or more. We can expect 4-million-pixel displays (QHD, WQXGA) to start shipping in 2014, and potentially 8-million-pixel displays (4K) sometime in the future.
The rapid shift to higher resolutions has outpaced the increase in screen sizes, leading to increasing pixel densities. As shown in Fig. 2, a key distinction is at 300 ppi, roughly HD resolution on a 5-in. display, with all the market growth taking place at or above this pixel density.
Figs. 1 and 2: At left, in Fig. 1, the mobile-phone market is in the midst of a significant transition to large screen sizes. At the same time, Fig. 2 (right) shows that as mobile-phone displays are getting larger, they are also rapidly shifting to higher pixel densities.
While Apple first drew attention to high resolution with its “Retina” display on the iPhone 4 at 330 ppi, it was quickly passed by higher pixel densities, including FHD displays on 5 in. (441 ppi) and then 4.5 in. (490 ppi), among others. The introduction of QHD and WQXGA displays this year is raising the bar to 500 ppi (QXGA on a 5-in. display) and above.
The shift to larger screen sizes and higher pixel densities has led to the possibility that smartphone displays could become available in the 4K format. Such a high pixel format requires new configurations of driver ICs (RAMless and ultimately compressed SDRAM), application processors, graphics processing units, and a high-bandwidth display interface (such as MIPI) in order to drive the display. Many of these components are in development, and, to some degree, content (such as 4K streaming from Netflix) and high-speed wireless networks (such as 4G LTE) are in development as well.
The question arises as to whether display technology is able to reach pixel densities in excess of 700 ppi at an affordable cost and level of power consumption. On a more practical level, such pixel densities are roughly twice the resolving power of the eye at typical viewing distances, leading some to wonder why manufacturers would suffer the cost and performance tradeoffs. While there have been some studies on the benefit of very high pixel densities on perceived image quality, there is relatively little basis for thinking that most viewers would perceive the difference between, for example, 600 and 700 ppi. One possibility is that the high pixel count could be utilized for other features, such as glasses-free 3-D.
The growth in high resolution has led to a shift in display technologies used for the largest (5 in. and larger) and highest resolution (FHD and higher) phones (Fig. 3).
Fig. 3: nbsp;Since the middle of 2013, the large-sized mobile-phone market has shifted toward higher-resolution technologies. As it has done so, LCD technologies have captured share from the AMOLED-display market.
Samsung was the first to move into this space with AMOLED displays, using its PenTile architecture to increase the effective resolution beyond the accuracy limitations of the organic material deposition process. In the past year, AMOLED displays have faced competition from LCDs in this space; from a-Si due to its low backplane cost and wide availability and from LTPS due to its ability to use smaller TFTs, enabling higher pixel densities with fewer LEDs, and thus lower backlight cost. While it is dominant in pixel densities up to 300 ppi, a-Si TFTs are limited in their ability to reach the highest resolutions, such as WUXGA and WQXGA, which are well above 300 ppi. At the same time, increased production of LTPS has enabled it to grow share. Leading LTPS suppliers LG Display, Japan Display, and Sharp have been increasing capacity, resulting in a doubling of input capacity between 2011 and 2013. Numerous suppliers are building LTPS capacity in China, including Tianma, Foxconn, BOE, and China Star, with the result that capacity is likely to again double between 2013 and 2016. Finally, oxide TFTs, in particular Sharp’s IGZO technology, have entered at the high end of the market.
Tablet PC Displays Get Smaller
While smartphones are driving the growth of mobile phones to larger than 5-in. screen sizes, the dynamics of the tablet PC market are driving growth to screen sizes below 9 in. As shown in Fig. 4, the tablet-PC market underwent a significant change in 2013, from one dominated by 9.7 in. to a bifurcated market with growth both above and below 9 in.
This has been due to a much higher rate of success for 7-in. devices, particularly those running the Android operating system. The success of these devices was one factor in Apple’s decision to develop the iPad mini, with a 7.9-in. screen. A particular area of growth has been the so-called “white box” market centered in China, in which companies with little brand or technology presence integrate “open cell” or unfinished TFT-LCDs with standard components into low-end devices running open-source operating systems. These and other smaller screen sizes are expected to grow faster than screen sizes above 9 in. and will approach one-half of the market within a few years.
While the pixel densities are not as high as in smartphones, tablet-PC resolutions are also increasing, with the key break point at 200 ppi, with displays above that expected to provide all of the unit growth starting this year (Fig. 5).
As stated above, a-Si TFT-LCDs can easily handle pixel densities to 300 ppi, so they have dominated the tablet-PC market. However, Sharp began producing IGZO tablet-PC panels in 2013, and with FHD and higher pixel formats moving from less than one-fifth of tablet PCs shipped in 2013 to one-half in 2016, there will be increased demand for LTPS TFTs as well as oxide TFTs. Finally, with continued development of PenTile technology as well as organic material deposition, AMOLED displays are also expected to play in the greater than 200 ppi tablet-PC panel market.
Figs. 4 and 5: Tablet PCs started out dominated by the original iPad 9.7-in. screen sizes, as shown in Fig. 4 at left, but, in the past few years, smaller screen sizes have grown much faster. At right, Fig. 5 shows tablet PCs are now at the beginning of a shift to higher pixel densities.
Is There Room in the Middle?
With the rapid growth of smartphones above 5 in. and tablet PCs below 9 in., one could reasonably expect overlap of these two device categories, something that can be seen in the “phablet” products, as well as cannibalization. But as shown in Fig. 6, for the next several years, there appears to be room for both types of device to grow, as many consumers are adopting multiple devices.
Fig. 6: Over the past few years, larger mobile phones and smaller tablet PCs have each been able to grow rapidly.
While key distinctions will remain – such as whether the primary connectivity technology is cellular or WiFi and whether the device is purchased with a contract from a telecommunications carrier – the design and characteristics of smartphones and smaller tablet PCs are likely to continue to converge. This is a very positive development for the companies that can provide displays from 5 to 9 in., particularly at high resolutions, and is driving rapid growth in the category overall (Fig. 7). •
Fig. 7: The rapid growth in smartphones and small tablet PCs, particularly in high-resolution formats, is driving growth in revenues for small-to-medium flat-panel displays.