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Material Challenges for Flexible OLED Displays
Published in Zhigang Rick Li, Organic Light-Emitting Materials and Devices, 2017
LTPS is made by annealing and crystallizing a-Si:H, commonly using a scanning excimer laser [32]. (The a-Si:H films are deposited by a vacuum process—PECVD.) Composed of polycrystalline Si grains, LTPS transistors have a much higher mobility (30–300 cm2/V-s), are voltage-bias stable, and therefore are more attractive than a-Si:H for driving OLEDs. However, because of added process complexity, LTPS transistors are more expensive to manufacture than a-Si:H, and nonuniformity in transistor properties due to variation in grain size is an issue, especially over larger areas. Nonuniform mobility over a display can also cause an unacceptable variation in OLED light intensity. The higher mobility of LTPS, however, does allow integration of transistors on the same substrate for display driver/logic and pixel addressing, which is advantageous for smaller OLED displays, e.g., cell phones and digital cameras, where there is limited space to accommodate a separate driver circuit board. The mobility of a-Si:H TFTs is too low for OLED drive/logic circuits.
Liquid crystal displays
Published in John P. Dakin, Robert G. W. Brown, Handbook of Optoelectronics, 2017
Polycrystalline silicon (p-Si) was among the first semiconductors to be used for LCDs [80] and found in the first applications for TFT by Canon as the watch used in the 1983 film Octopussy and Sharp’s 1991 hang-on-the-wall TV [7]. The material has a high mobility of 200–400 cm2/V·s, which is intermediate between the 1.5 cm2/V·s of amorphous silicon and 1400 cm2/V·s for crystalline. Such high mobilities allow far smaller transistors, higher ON currents [particularly important for organic light emitting diode (OLED) displays], and potentially integrating the display drivers onto the glass itself. This latter advantage potentially leads to significant overall cost savings because the drivers would be produced in the same process steps as the pixel TFT. The problem with producing p-Si TFTs was the very high processing temperatures, requiring those early demonstrators to be produced on quartz substrates. In the mid 1980s [81], LTPS TFTs were fabricated using excimer laser annealing of the α-Si to form the polycrystalline structure while keeping the processing temperature to 260°C, equivalent to that used for α-Si. Today, many smart phones benefit from the excellent properties of LTPS, which allows resolutions above 400 dpi and better battery life due to the reduction in backlight power that the high aperture ratio allows. However, the cost of LTPS is high because the fabrication of the top-gate transistors required uses 9–11 critical mask steps: this typically adds about 20% cost to the panels over equivalent α-Si LCDs.
A high-gain two-stage amplifier using low-temperature poly-si oxide thin-film transistors with a Corbino structure
Published in Journal of Information Display, 2022
Dong-Hwan Jeon, Won-Been Jeong, Jeong-Soo Park, Hoon-Ju Chung, Seung-Woo Lee
Currently, various backplane technologies have been used in display applications. For instance, thin-film transistors (TFTs) made of n-type hydrogenated amorphous silicon (a-Si:H) have been widely used in liquid crystal display panels. TFTs made of low-temperature polycrystalline silicon (LTPS) exhibit high mobility, thereby allowing for high-speed operation. N-type amorphous indium-gallium-zinc oxide TFTs, also known as oxide TFTs, have extremely low leakage currents and uniform device performance. LTPS and oxide TFT technologies have been widely used in mobile and large area displays, respectively. Recently, low-temperature poly-Si oxide (LTPO) TFT technologies have been actively studied for mobile applications [1]. The low leakage current of oxide TFT contributes to the realization of a variable refresh rate and low power consumption in display applications. As a result, various studies adopting the LTPO technology have been conducted, such as pixel circuits for displays using organic light-emitting diodes [2–5], liquid crystals [6–9], and micro-light-emitting diodes [10]. In addition to pixel circuits, LTPO technology has been studied in display circuits, such as gate drivers [2,11–13], level shifters [14], and inverters [15–19].