1 Photocapacitors

1.1 Conductor–insulator–semiconductor structures

A forensic examination of the inside of any silicon chip would reveal a miniature network of metal tracks criss-crossing on several levels, separated by insulating layers of silicon dioxide and periodically stitched down to the underlying tracks and the underlying silicon. Down in the silicon proper there is an intricate pattern of islands of p-type material in pockets of n-type material and vice versa. The precision and regularity of the patterns of different materials tells of a highly sophisticated design comprising the deployment of standard components that are, apparently, configured to bring about specific electronic functions.

We will now look at the triplet formed by a conductor, an insulator and a semiconductor. This can be arranged to provide an electronic switch, which is just the sort of thing that is needed to connect efficiently to individual pixels in an imaging sensor or a display and to address individual locations in an electronic memory. Furthermore, the same structure is as sensitive to light as a p–n junction and it provides the basis of a semiconductor memory. Thus, conductor–insulator–semiconductor structures are central to the story of the digital camera. We begin, therefore, with a closer look at this three-layer sandwich.