With the launch of the new range of Apple 14 phones, the pressure is once again on delivering LTPO-based displays for the Pro and Pro Max, dubbed Pro Motion displays. At this point, only SDC was able to meet Apple’s requirements, as was the sole player shipment. LGD is expected to join as a supplier soon. But for now, SDC remains the monopoly supply position for the high end of Apple’s offering.
But what brought us to this point? What is LTPO technology and why is it so difficult to manufacture and what are the alternatives?
LTPO (Low Temperature Poly Oxide) is an invention by Apple that aims to provide the ability to control the refresh rate in high-end AMOLED displays. It uses an oxide process for the switching transistors and LTPS for the driver transistors. Figure 1 shows Samsung’s implementation of LTPO (Source OMDIA)
In theory, the pixel circuit can use the stability and uniformity of the LTPS transistors to deliver even currents to the pixels across the display while benefiting from the low leakage current (and hence the ability to go to low frame rates). of the oxide transistor.
But while the theory is easy, the execution is not. While there are ways to shred mask steps in total so the total mask count isn’t much more than the LTPS AMOLED, and most analysts believe the oxide addition is about 2 extra masks, the reality is that hydrogen is escaping the LTPS Processing causes fouling of the oxide transistor. Additionally, fitting all the elements of the complex pixel circuitry into each sub-pixel was a bit of a difficult task, especially for gamers who don’t master the oxide process, such as BOE.
So the goal of the LTPO circuit is to allow for variable refresh that allows fast switching for gaming at the high end of the scale, but also low refresh of low or less than 1 Hz at the other end of the scale to save power consumption to reduce standby modes, resulting in a power reduction of about 20%. The power reduction increases as the ability to achieve a low frame rate increases. So in theory it’s nice, but difficult to do in practice. So Apple ended up developing a technology that only one, maybe two players have the technical ability to produce
Enter high performance oxide circuits from Amorphyx
So what is the alternative? Well, it’s the oxide transistors that provide the low leakage current needed for a low refresh rate. But oxide transistors have historically not had enough mobility and stability to be used as driver transistors that carry the current in an AMOLED display. If a higher mobility, higher stability oxide transistor were available, the pixel circuitry could be fabricated in oxide transistors and possibly with simplified compensation circuitry.
Based in Corvallis, Oregon, Amorphyx Inc has been working on its proprietary AMeTFT (Amorphous Metal TFT) for a number of years. The company took inspiration from the development of FinFETs in semiconductors to develop a very smooth, flexible amorphous gate metal coupled with a high-k dielectric insulator. The combination allows for higher field strengths across the IGZO material, resulting in greater mobility. This oxide transistor also meets the stability requirements required for AMOLED.
In this world, an oxide-only system as a pixel backplane for AMOLED is conceivable. 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 on top of that there’s a chance that the inherent stability and performance would mean that compensation circuits for AMOLED could be simplified.
Amorphyx has now published data showing that this oxide transistor also has very low leakage currents. So much so that in early data, an AMeTFT oxide transistor can hold a voltage for 20 seconds, giving a 0.05Hz refresh rate. Figure 2 shows how the drain-source current generated by the transistor decays very slowly.
Amorphyx has also demonstrated peak mobilities of 75 cm²/Vs and reliability statistics of PBTS and NBTS of <1.7 V after 7200 seconds with 30 V applied and <0.8 V after 7200 seconds with 20 V applied. This is a very powerful oxide transistor.
Of course, the implications of an all-oxide high-performance drive capability are that it could crowd out some of the use of LTPS in displays. It will also change the nature of competition in displays, as currently few companies are truly world-class in IGZO production (e.g. Sharp and LGD, while Samsung SDC is scrambling to dominate Oxid).
It’s clear that refresh rate is a foundation for product performance that’s here to stay. With implications for better high-end gaming and an improvement in low refresh rate battery, this is a key differentiator of the product. Apple chose a path that was strong in theory but difficult in practice. Amorphyx opens another path and new strategic options for smartphone backplanes. (I H)
Thanks to Amorphyx for putting 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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