iPhone 18 with LTPO+: Intensifying Technology Race Among Panel Makers
The iPhone 18 will feature a new type of OLED backplane technology called LTPO+. While existing LTPOs employed a hybrid structure using oxide semiconductors only for the switching TFT, LTPO+’s key feature is the switch to oxide TFTs for both the switching TFT and the driving TFT. This is believed to be Apple’s strategy to improve power efficiency in its next-generation OLED panels and to address brightness uniformity and image retention issues during extended use.
LTPO+ Compensation Circuit Patent (Source: Apple)
Conventional LTPS (low-temperature polycrystalline silicon)-based driving TFTs offer high mobility, making them advantageous for high-brightness operation. However, the numerous traps at grain boundaries result in high hysteresis and unstable current characteristics, making them prone to gradation errors and brightness unevenness over extended periods of use. In contrast, oxide TFTs boast low hysteresis and stable current characteristics, maintaining a constant current under constant gate voltage conditions. This reduces pixel-to-pixel current variation, improving brightness uniformity and color stability. Furthermore, residual charge accumulation is suppressed, reducing image retention.
Despite these advantages, many technical challenges remain for the application of oxide as a driving TFT. Oxide semiconductors have lower mobility than LTPS, making it difficult to secure sufficient driving current. This can lead to slower current response times at high brightness and refresh rates. Furthermore, ensuring stability under prolonged bias and thermal stress is essential. This is because electron trap accumulation during extended driving can lead to current reductions and subtle color shifts. Meanwhile, even in the LTPO+ structure, some circuit elements are still composed of LTPS. Since these LTPS elements are not as high-performance as the driving TFTs, securing cost-effective, low-cost LTPS manufacturing technology is crucial. Unlike high-quality driving LTPS, LTPS for peripheral circuits or sensing elements prioritizes yield, uniformity, and low-cost processes over high mobility. These process simplifications and cost-saving technologies enhance the competitiveness of LTPO+ mass production.
In other words, LTPO+ is a structure achieved through a balance between oxide and LTPS processes, with one key focus being high performance (oxide) and the other being low cost (LTPS).
From this perspective, the key challenges for oxide-driven TFTs can be summarized as four:
First, ensuring bias and thermal stress reliability – technology to suppress electrical degradation during long-term operation and minimize ΔVth (threshold voltage shift).
Second, integrating compensation circuits – designing a circuit-level compensation circuit to compensate for fluctuations in oxide device characteristics and ensure operational stability.
Third, securing large-area uniformity – a technology that minimizes current variations across the substrate to maintain luminance uniformity.
Fourth, appropriate subthreshold swing (SS) control – an excessively low SS can lead to sensitivity to threshold voltage variation and time variation (ΔVth), which can increase current dispersion. Therefore, SS optimization is required to balance power efficiency and operating stability.
Ultimately, the success of LTPO+ depends not only on the performance of the oxide driving TFT but also on the cost competitiveness of the auxiliary LTPS process. Apple will only be able to fully adopt LTPO+ for the iPhone 18 if it reaches target levels in mobility, reliability, uniformity, and manufacturing cost. The industry predicts that technological competition among existing iPhone panel suppliers will intensify, focusing on securing oxide TFT performance and developing low-cost LTPS processes. LTPO+ is expected to mark a new turning point for panel technology in the next-generation mobile OLED market.
Changwook Han, Executive Vice President/Analyst at UBI Research (cwhan@ubiresearch.com)
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