20
A Transistor for the Active Matrix
With supply burgeoning, the prices for liquid crystal display calculators and watches began a precipitous decline, and in order to make any profit at all, companies raced to upgrade existing products and develop completely new products. The new active matrix liquid crystal display (AMLCD) could provide the resolution and speed necessary for the new products, but although the concept of the active matrix was clear, the engineering problem was to find a suitable transistor to drive the liquid crystal.
Peter Brody’s early research on thin-film transistors (TFTs) at Westinghouse used a cadmium selenide (CdSe) transistor, and although workable, it was extremely sensitive to variations in composition and impurities and highly toxic to boot, making it unsuitable for consumer products. Sharp later developed a tellurium (Te) TFT that could be deposited directly on the glass substrate, but its electron mobility—the average speed of electrons in an applied electric field—was so high as to have a serious current leakage problem, which caused the on/off current ratio to be far from the 106 requirement for display use (see Chapter 22).
Because silicon semiconductors by this time had been extensively studied for use in transistors, it was the favorite of electronics engineers everywhere, so it was natural for silicon to be a candidate for thin film transistors for liquid crystal displays. However, the transistors in calculators were made from pure crystalline ...
