For displays of high information content, such as large dot matrix panels, the viewing angle and contrast problems of directly multiplexed displays would disappear if each element were directly addressable. Because of geometry and space limitations, this is not possible by electrode design. The active matrix approach addresses this problem by incorporating a switch at each picture element (pixel).

An active matrix display is constructed by building a grid of switching elements on the inside surface of the glass substrate. The switching elements may be two terminal devices, such as diodes, or three terminal devices, such as transistors. Each display pixel has a switch located at the pixel. The switches/pixels are arranged in rows and columns, with the gates of each switch of one row tied together for connection to an external voltage source. The sources of each column of switches are also tied together for external addressing. When voltage is applied to a row of the matrix, the gates of that row allow data to be entered at the sources. The voltage at the source either turns the elements on or not. The voltage is then removed from that row, closing those switches, and the next row is addressed.

This cycle is applied to each row, then repeated, starting with the first row. Each pixel, therefore, is individually addressed.

The advantage to this method is that maximum contrast is achieved for the LC material, without increased viewing angle dependence. Also, voltages which only partially turn on the pixels may be used to achieve grey scale effects.

Whitepaper: Active Matrix LCD with Integrated Optical Touch Screen (PDF)