The designers of the Z-80 did the same thing (modifying dopant levels so transistors would look like normal transistors but function differently) in order to av

The designers of the Z-80 did the same thing (modifying dopant levels so transistors would look like normal transistors but function differently) in order to avoid competitors from cloning the chip. See the very interesting interview about the Z-80 (page 13): http://archive.computerhistory.org/resources/text/Oral_History/Zilog_Z80/102658073.05.01.pdf http://archive.computerhistory.org/resources/text/Oral_Histo... Faggin (Zilog CEO): "Yes, we were concerned about others copying the Z80. So I was trying to figure what we could do that that would be effective, and that’s when I came across an idea that if we use the depletion load the mask that doesn’t leave any trace, then I could create depletion load devices that look like enhancement mode devices. And by doing that we could trick the customer into believing that a certain logic was implemented, when it was not." Shima (Z-80 designer): "I placed six traps for stopping the copy of the layout by the copy maker. And one transistor was added to existing enhancement transistors. And I added a transistor looks like an enhancement transistor. But if transistors are set to be always on state by the ion implantations, it has a drastic effect" I think you could detect dopant tampering (and these traps) with scanning capacitance microscopy, but I don't have any actual experience with that.