Flutterby™! : Doping transistors to make trojans

Stealthy Dopant-Level Hardware Trojans Becker, Regazzoni, Paar and Burleson (PDF):

Contrary to the majority of past works, in this paper, we investigate a new family of Trojans that do not need any extra logic resources but merely require a change in the dopant polarity of a few transistors. Hence, these Trojans add zero overhead in terms of additional transistors and metal wires. We show that such a change will not be detected by several of the common Trojan testing methods, including optical inspection.

Or: How deeply do you trust your supply chain?