@User: There are a few layers that the end user is "discouraged" from using because they represent devices that only exist for approved SkyWater cells. There are ways to work around this; e.g., you can just do "tech unlock *" and then you can paint the "mvnfetesd" layer over an nFET gate and make it an ESD device. But then it isn't an "approved" device. But consider that the only difference between the nFET device and the ESD nFET device is that one of them has an "ESD" marker layer over it. It's not an ESD FET because it has any kind of special mask layers; it's an ESD FET because it has a specific layout and is approved for use as an ESD device by SkyWater.

The sky130 PDK page says that "The ESD FET’s differ from the regular NMOS devices in several aspects, most notably: • Increased isolation spacing from contacts to surrounding STI • Increased drain contact-to-gate spacing • Placement of n-well under the drain contacts "