Two bits per transistor: high-density ROM in Intel's 8087 floating point chip
Note this gem in the footnotes:<p>&gt; Ion implantation was also used in the Z-80 microprocessor to create some transistor &quot;traps&quot;, circuits that looked like regular transistors under a microscope but received doping implants that made them non-functional. This served as copy protection since a manufacturer that tried to produce clones on the Z-80 by copying the chip with a microscope would end up with a chip that failed in multiple ways, some of them very subtle.