William W. Piper

The Demands On The a-Si FET As A Pixel Switch For Liquid Crystal Displays

The matrix-addressed liquid crystal display (LCD) has attracted considerable attention in the past few years as an alternative to the cathode ray tube (CRT). This type of display has been demonstrated to have stable performance over a long life as well as the attractive features of small volume, light weight, low power, good brightness and full color. Amor-phous silicon (a-Si) is currently the preferred material for switch devices and plasma enhanced chemical vapor deposition (PECVD)is the preferred deposition technology since the process can be carried out at low temperature, yields material with a low density of defect states and affords good step coverage. The electrical requirements for an a-Si field effect transistor (FET) used as a pixel switch for a LCD include switching time, on current and off current. These parameters depend on the instrinsic characteristics of the amorphous materi-als used, the overall display structure and the device geometry. Present and potential material and geometry limitations will be discussed for different display systems of current interest.

Amorphous Silicon FET Addressed Liquid Crystal Spatial Light Modulator

Advances in the technology of fabrication of large arrays of thin-film field-effect transistors for liquid crystal display applications is making available an important new component for many other applications such as spatial light modulators for optical signal processors. Hydrogenated amorphous silicon has a number of advantages over other semiconductors for such arrays relating primarily to cost and yield due to the simple, low temperature processing, and options offered by a transmissive device. There are also disadvantages stemming from the low electron mobility which is roughly 0.1% that of crystalline silicon. Still, sub-micro-second switching times are possible which is not only adequate for liquid crystal control within very large arrays but suitable for much of the row and column driving circuitry as well. In this paper we will describe the characteristics of a-Si devices, circuits, and LC cells controlled by them. We will also discuss the technology in the context of projecting what the capabilities of such devices could be for spatial light modulators and displays.