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TFT-LCD Displays

(Thin Film Transistor Liquid Crystal Displays). 

TFT-LCD Displays are a cagegory of liquid crystal displays (LCDs) characterized by a Thin Film Transistor Layer deposited on the substrate for an active-matrix control design.  Traditional passive LCDs require the display primary controller to drive all individual pixels both with logical power for on/off state, but also for power drive.  The active-matrix element of the TFT-LCD separates the power from the logical components allowing the primary display controller to drive the logical state, while the transistors pull power directly seperately retaining power at all pixels simultaneously.  Active Matrix designs allow for higher contrast ratios with the direct power drive and ensure scalability to larger display sizes and resolutions such as monitors and televisions. TFT-LCD displays are suitable for most applications and are highly proven in the field in almost every industry.  


The conceptual technology behind TFT-LCD was derived by RCA engineers in the late 1950's for using metal-oxide semiconductor field-effect transistor, or MOSFET, as a drive mechanism for a liquid crystal display.  This was refined in the early 1960's to utilize a thin cadmium based film .  Refined by Westinghouse engineers to replace cadmium films with Indium based films.  Production realization was not possible in the 1960's due to limitations in the semiconductor substrate.  The breakthrough for production was in the turn of the 1980's as Amorphous Siilcon (aSi) by UK researchers and progressed by Japanese technology firms to refine the aSi substrate specifically for displays and products as they are used today.  The advances in production process in Japan resulted in first mass production products appearing in the early to mid-1980's from Japanese electronics corporations like Fujitsu and Toshiba.

Advances in performance have occurred since initial production including refinement of the substrate from aSi to Low-Temperature Poly-Silicon (LTPS) and similar processes.  LTPS-based TFTs are critical for applications utilizing high-resolutions like modern HD and 4K through process refinements and lower synthesis temperatures as implied by the name LTPS.  The substrate cost is lower cost to produce and ideal for cost-sensitive consumer applications like today's smart phones and high-resolution televisions.  aSi substrate is still widely utilized in the global capacity and focuses on more industrial applications that have more modest pixel densities.  

Advances in performance are also achieved with the liquid crystal alignment.  Original TFTs were utilizing Twisted Neumatic, or TN, device structure.  This structure companied with linear polarization on both sides of the glass (Top and Bottom) allows for controlling the passage of light.  The consequences of linear polarization results in greyscale 'inversion' at an angle defined by the alignment of the polarizers and the starting angle, or rubbing angle, of the substrate that the liquid crystal rotates upon.  TN fluid rotates 90-degrees, and the rubbing direction in process defines this starting point of this rotation.  The resulting technology results in one off-axis direction having inferior performance to the other angles.  In many applications this is not critical, so  TN LCD is still widely utilized globally.  However, new technologies in vertically-aligned liquid crystal fluids have made possible symmetrical viewing performance for a view angle free experience.  There are multiple vertically-aligned technologies and patents including IPS and MVA which fall under this approach of vertical shuttering.  VA alignment rotates perpendicular to the plane of glass and polarizers, compared to TN which rotates on the same plane as the glass and polarizers. Vertically-aligned devices still utilize polarization, but may use circular polarizers, density filters, or other thin films to enhance the contrast and optimize the optical performance.

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