With the launch of the new Apple 14 line of phones, the pressure is once again on providing the LTPO-based displays for the Pro and Pro Max, called Pro Motion displays. At this point, only SDC has been able to meet Apple’s requirements, as well as the single player shipment. LGD should soon become a supplier. But for now, that leaves SDC the position of Apple’s high-end monopoly supplier.
But what got us to this point? What is LTPO technology and why is it so difficult to produce and what are the alternatives?
LTPO (Low temperature Poly Oxide) is an Apple invention aimed at providing the ability to control the refresh rate of high-end AMOLED displays. It uses oxide process for switching transistors and LTPS for driving transistors. Figure 1 shows Samsung’s implementation of LTPO (Source OMDIA)
Theoretically, the pixel circuit can take advantage of the stability and uniformity of the LTPS transistors to give uniform currents to the pixels on the screen, and at the same time benefit from the low leakage current (and therefore the ability to pass at low frame rates) of the oxide transistor.
However, while the theory is simple, the execution is not. Although there are possibilities to crush the mask steps in total so that the overall mask count is not much higher than in LTPS AMOLED, and most analysts believe that the oxide adder is of about 2 more masks, the reality is that the hydrogen leaking out of the LTPS treatment causes the oxide transistor to get fouled. Also, fitting all the elements of the complex pixel circuit into each sub-pixel was a bit difficult, especially for gamers who were unfamiliar with the oxide process, e.g. BOE.
The goal of the LTPO circuit then is to enable variable refresh which allows fast switching for games at the high end of the scale, but also low refresh as low or below 1Hz at the other end of the scale. scaling to reduce power consumption in standby modes, resulting in a power reduction of approximately 20%. The power reduction increases as the ability to achieve low frame rate increases. So nice in theory, but difficult to achieve in practice. So Apple ended up creating technology that only one or even two players have the technical skills to make.
Step into the high performance oxide circuitry of Amorphyx
So what is the alternative? Well, it’s the oxide transistors that provide the low leakage current needed to allow a low refresh rate. But oxide transistors in the past did not have enough mobility and stability to be used as driver transistors that carry current in an AMOLED display. If a higher mobility, higher stability oxide transistor were available, the pixel circuit could be realized in oxide transistors and potentially with a simplified compensation circuit.
Amorphyx Inc, based in Corvallis, Oregon, has been working on its proprietary AMeTFT (Amorphous metal TFT) for several years. The company took inspiration from FinFET developments in semiconductors to develop a very smooth and flexible amorphous gate metal, coupled with a high-k dielectric insulator. The combination allows higher field strengths through the IGZO material, which leads to greater mobility. This oxide transistor also meets the stability requirements necessary for AMOLED.
In this world, an oxide-only system is conceivable as the pixel backplane for AMOLED. This is already the case with WOLED (White OLED) for televisions. Such a system would have much lower complexity and cost, probably only 40% of the cost of an equivalent LTPS based system and furthermore it is likely that the inherent stability and performance would mean that the compensation circuits for AMOLED could be simplified.
Amorphyx has now released data to show that this oxide transistor also has very low leakage currents. So much so that in early data, an AMeTFT oxide transistor can hold voltage for 20 seconds, providing a refresh rate of 0.05 Hz. Figure 2 shows how the drain-source current created by the transistor decays very slowly .
Amorphyx also showed peak mobilities of 75 cm²/Vs and reliability statistics of PBTS and NBTS of amplitude <1.7 V after 7200 seconds at 30 V applied, and amplitude < 0.8 V after 7200 seconds at 20 V applied. This is a high performance oxide transistor.
The implications, of course, of an all-oxide high-performance drive capability is that it could displace some LTPS usage in displays. It will also change the nature of competition in displays, as few companies are currently world-class in producing IGZO (eg Sharp and LGD, while Samsung SDC struggles to master the oxide).
It’s clear that refresh rate is a foundation of product performance that’s here to stay. With implications for better high-end gaming and improved low-refresh battery, this is a key product differentiator. Apple chose a path that was theoretically sound but practically difficult. Amorphyx opens up another path and new strategic options for smartphone backplanes. (HI)
Thanks to Amorphyx for posting this article behind the Display Daily paywall so it doesn’t count as one of your free articles if you don’t have a paid subscription.
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