Active

Active (diffusion, OD, RX — shops rename it) is the silicon island where a transistor lives, drawn as an opening in a thin skin of oxide.

The wide island is the silicon where a device lives. A gate crosses it.

In a PDK, that island is a layer, and it is not metal. Poly crossing active makes a gate. A contact on active reaches the island, and implant flavors it. A well usually surrounds it. Width and spacing on active are among the first rules people fail. A 0.22 micrometer-wide active strip is a skinny transistor; a 10 micrometer-wide strip is a fat one. In a PCell you set width and length, and active and poly rebuild. On the screen the island is a green or yellow rectangle under a poly stripe, with contacts on both sides.

Active also hosts diodes, bipolar emitters, and substrate taps, not only MOS islands. A tap is a small active square with a contact that ties a well or the substrate to a supply net. A foundry deck demands a tap every 20 or 30 micrometers along a well strip. Forget the taps and latch-up rules fail even when every transistor looks pretty. Drawing active as a wire you can snake across the chip is a mistake, because long active runs have resistance and latch-up stories. Use metal to travel.

The sheet, the difference between process layers and drawing layers, and the plugs that land on the island are covered in what a layer is for, process layers vs drawing layers, and vias and contacts.