Thin-film transistor (TFT) LCD panels based on amorphous silicon rule the notebook display market and are making inroads on the desktop, but a brighter, thinner, lighter, and more reliable TFT technology, based on polycrystalline silicon (also known as polysilicon), is undergoing some significant developments.
Toshiba is the first to develop a process for making large-format polysilicon panels. Starting with 8.4- and 10.1-inch polysilicon displays, the company recently announced a 15-inch polysilicon panel that offers major advantages over its amorphous cousin.
One major advantage is polysilicon's higher electron mobility, which means that less power is needed to run an electrical current through the silicon layer. As a result, creating driver circuitry right on the edge of the panel--rather than on separate semiconductor components--becomes practical, making the display thinner and lighter. This also reduces the number of electrical contacts to the panel by as much as 95 percent. Since most notebook failures are caused by problems with panel connections, a polysilicon panel should be much more reliable.
The increased electron mobility also creates the possibility for the active matrix portions of each LCD cell to be smaller, which in turn lets more light through the display. As a result, the display is brighter and consumes less power. And the smaller LCD cell size also allows for very finely spaced LCD cells that offer a smaller-pixel pitch than that of a typical CRT display.
The production of polysilicon panels has been more difficult--and more expensive--than that of amorphous silicon. Amorphous silicon technology coats a glass substrate with a thin layer of silicon. To create a polysilicon display, this layer is heated and transformed into a layer of tiny crystals. One problem is that this process requires high temperatures--hot enough to melt glass substrates. Advances in polysilicon technology have led to a low-temperature process that uses a laser beam to heat the thin silicon film at the surface of the glass substrate while the glass itself remains at room temperature.
