Why can 90-degree twisted-nematic passive-matrix displays show only a limited number of pixel rows?
In nematic arrangements the crystals are randomly positioned with all their long axes pointing in essentially the same direction, given by a vector known as the director. In smectic arrangements the crystals are further constrained to parallel planes; in smectic A the director is perpendicular to these planes, while in smectic C the director is at some other angle to the planes.
In 90-degree twisted-nematic (TN) passive-matrix displays, the limited number of pixel rows—typically around 20—is primarily due to the display's optical response characteristics when using passive-matrix addressing.1
In passive-matrix systems, pixels are activated by applying voltage to specific rows and columns. A key challenge in this process is "cross talk," where the voltage applied to activate a specific pixel inadvertently illuminates or affects neighboring, unwanted pixels.2
The 90-degree TN configuration lacks a sufficiently sharp change in optical transmission relative to the applied voltage. Because the optical response is not steep enough, it becomes difficult to distinguish between the "on" and "off" states as the number of rows increases, leading to excessive cross talk. To overcome this limitation, developers introduced supertwisted nematic (STN) displays, which use a higher twist angle (180–270 degrees) and a chiral dopant to create a sharper transmission-voltage curve, thereby allowing for a much larger number of addressable pixel rows.2