2.1 The Case for Active Matrix

Passive matrix LCDs are relatively easy to manufacture and do not require very large investments. An active matrix LCD, on the other hand, consists of thin film semiconductor circuitry on a glass substrate and shares some of the manufacturing complexity of integrated circuits on semiconductor wafers.

The efficient production of active matrix LCDs requires large capital investments at the same level of wafer fabs for IC manufacturing. The incentive for an LCD producer to make this investment rather than sticking with passive matrix LCDs is simple: image quality, including resolution, maximum size, contrast ratio, viewing angle, gray scale, and video performance is much better in AMLCDs than in passive matrix LCDs. The resolution limitation of the passive matrix was addressed in the previous chapter. Another drawback of passive matrix addressing is that the capacitance of the select and data buslines increases with the square of the diagonal size of the LCD. Since the buslines in passive matrix LCDs consist of transparent conductors with relatively high resistance, the RC delays and power consumption in large passive LCDs become unmanageable.

By adding a semiconductor switch at each pixel, the drawbacks of multiplexing in a passive matrix are eliminated. In the active matrix configuration, the voltage at each pixel and therefore its transmission and gray level can be accurately controlled.

A conventional semiconductor process on crystalline Si wafers could, in principle, be used for AMLCDs. In fact, this has been successful in reflective microdisplays for projection and...