HTPS-based systems
The structure of a polycrystalline silicon LC panel is very similar to that of the active matrix LCD (AMLCD) commonly used for television screen and monitor screens described in Chapter 12. The electron/hole mobility of poly- crystalline silicon is, however, much higher than that of amorphous silicon used in AMLCD, allowing the size of a thin-film transistor (TFT) to be made much smaller. It is also possible to have driver and shift register IC packages embedded on the panel itself. Due to the good aperture ratio, high-brightness polysilicon LC projection systems are feasible. There are two methods of fabricating polycrystalline silicon: low (<600ºC) and high (>1000ºC) temperature. Because the low-temperature type suffers from higher leakage current, the HTPS is normally used offering good performance in terms of degree of miniaturisation, high definition (HD), response speed and reliability.
Colour filter HTPS
Figure 19.5 shows a colour combining x-cube assembly used in a transmis- sive HTPS light engine using a colour filter. White light from the light source assembly is directed to three HTPS LV panels, one for each primary colour. The LV is sandwiched between two sheet polarisers in the same way as the traditional AMLCD display with the exception that the LV pixels do not sub-divide into three cells. Pixels of a LV contain one cell
each. This reduces the number of data (column) connections by a third. The output light from each LV panel is spatially modulated by their individual red, green and blue data coming from the video processing and for- matting board. At this point, all three images are monochrome. They are turned into RGB images by the following red, green and blue filters. The three images converge into a full colour picture by a solid glass ‘x-cube’ dichroic beam combiner. The colour picture is then projected on a screen via a telecentric projection lens. A typical 3-panel light engine assembly is shown in Figure 19.6. Three flexible tape carriers are used to feed video data as well as line select signals from the video and control boards to the LV panel. Shift registers are embedded with the LV driving circuitry to demultiplex the video data using a shift register and drive the panel.