TFT LCD - Construction

Construction

Liquid crystal displays as used in calculators and devices with similarly simple displays have direct driven image elements – a voltage can be applied across one segment without interfering with other segments of the display. This is impractical for a large display with a large number of (colour) picture elements (pixels), since it would require millions of connections – top and bottom connections for each one of the three colors (red, green and blue) of every pixel. To avoid this issue, the pixels are addressed in rows and columns, reducing the connection count from millions to thousands. The column and row wires attach to transistor switches, one for each pixel. The one-way current passing characteristic of the transistor prevents the charge applied to the pixel from draining between refreshes to the display image. Each pixel is a small capacitor with a layer of insulating liquid crystal sandwiched between transparent conductive ITO layers.

The circuit layout process of a TFT-LCD is very similar to that of semiconductor products. However, rather than fabricating the transistors from silicon formed into a crystalline silicon wafer, they are made from a thin film of amorphous silicon deposited on a glass panel. The silicon layer for TFT-LCDs is typically deposited using the PECVD process. Transistors take up only a small fraction of the area of each pixel; the rest of the silicon film is etched away to allow light to pass through.

Polycrystalline silicon is sometimes used in displays requiring higher TFT performance. Examples include small high-resolution displays such as those found in projectors or view finders. Amorphous silicon-based TFTs are by far the most common due to their lower production cost, whereas polycrystalline silicon TFTs are more costly and difficult to produce.