PCell

A PCell (parameterized cell) is a named drawing written as a recipe: numbers go in, and a script draws the shapes.

Four gates on one island, plugs at the ends, one metal strap. Numbers in a recipe drew this.

Transistors, resistors, MEMS springs, and even some via arrays should not be hand-drafted each time. Set width to 2.0 micrometers, length to 0.18, and fingers to 8, and the script paints active, poly, contact, and metal. Change a knob and the geometry rebuilds. Analog and MEMS live on this habit. On screen the result is a cell with a property form, not only a vertex list. Stretch a MOS and the contacts respawn. Flatten the cell and the ink stays while the knobs are gone.

PCells live in a library or a PDK. They still instance like ordinary cells. The scripts are why people write layout code. Editing the flattened rectangles, and expecting the next refresh to keep the tweak, loses the tweak. The recipe wins. Edit the parameters, or convert to a static cell on purpose.

A PCell is only as honest as its script. A width of 1.99 micrometers can draw a legal device while 2.00 micrometers snaps a contact off-grid. Read the property list after each change. If two people edit the same recipe in two libraries, matching pairs stop being twins. Lock the PDK version the same way a foundry deck is locked. The named drawing the recipe stamps is cells and instances. Why that recipe is a script at all is why people write layout scripts. The file those stamps land in is what is a layout.