Transistors

When polysilicon appears to cross over an area of diffusion, a MOS transistor is created. The following image shows a single poly line crossing four different diffusion areas thus creating four transistors (circled in red):

This poly line must extend a short distance beyond the boundary of the diffusion area. This is to avoid the diffused area reaching around the end of the gate to short out the drain to source path. In the 6561 die shot, this extension of the polysilicon gate beyond the diffusion area is seen everywhere, as shown by the white circles in the image below. This extension is one of the easiest ways of spotting a transistor and it applies to all three types of transistor.

In some cases though, the gate continues on to become the gate to more transistors. The image at the top of this page shows four such transistors. There are also a few not highlighted in the image above.

There are three types of transistor used in the 6561. These are:

• • Depletion mode pull up transistor.

  • Enhancement mode pull down transistor.

  • Enhancement mode pass transistor.

These three types are discussed in more detail in the sub pages below.