Inverter

The Inverter performs the NOT boolean operation, which is for the output to logically negate the input. In the 6561 die shot, an Inverter is created with a depletion mode Pull Up transistor and an enhancement mode Pull Down transistor.

The following image shows the same part of the die shot repeated twice, side by side. On the left hand side, it shows the section of the die shot with only a few labels added to show where VDD and VSS are, and also to identify the pull up transistors. The image is of two inverters, one on top of the other. The input for each inverter is labelled, as is the output.

On the right hand side of the image, the same image is repeated but with the addition of coloured edges showing where the diffusion, polysilicon, buried contacts, and metal contacts are. The green is the diffusion area, the red the polysilicon, the blue the buried contacts that connect polysilicon to diffusion, and the white the round metal to diffusion contacts.

What we can see is that in each case the input to the Inverter comes in from the right on a green diffusion line. It then connects to a red polysilicon line via the blue buried contact. The red polysilicon line then crosses over a diffusion area and then a bit beyond. This short overlap of the polysilicon beyond the diffusion is what identifies it as a transistor rather than a connection. Whenever polysilicon crosses over diffusion, a transistor is formed. The polysilicon is the gate for the transistor. It is right above VSS and so is a pull down transistor.

Above the pull down transistor, we can see another red polysilicon area. This is also over a diffusion area and overlaps, so also forms a transistor. It's fat rectangular shape identifies it as a depletion mode transistor. The VDD immediately to the right of it identifies that side of the depletion mode transistor as the drain, and the side where the blue buried contact connects it to the diffusion is the source. It is this buried contact that connects the depletion mode transistor's gate to it's source, which is characteristic of a depletion mode transistor.

So we have a pull down transistor and a pull up transistor. Between the two is an area of diffusion that is the output of the Inverter. A diffusion line leads off to the left carrying the output signal. Since this is an Inverter, that output signal will be the opposite of the input signal.

You will see inverters like this all over the surface of the 6561 die shot, although quite often it uses an inverting super buffer (which we discuss in another section) rather than a simple inverter. Logically they do the same thing though.