Even though electronic devices require matching transistors, in reality it is impossible to manufacture as few as two completely identical transistors, especially for nanometer scale transistors. Because of quantum mechanical effects and the randomness of transistor dopant arrangement, every transistor on a die differs slightly from each other, even if they are spaced only a few nanometers apart. This problem is even more acute when trying to replicate performance of widely spaced transistors that may be tens of thousands of nanometers apart on the same die, transistors on neighboring die in the same wafer, transistors on different wafers, or even transistors manufactured at different fabricating facilities. Variations can occur due to process differences resulting in line edge variation, to other unwanted patterning effects that change channel, gate, or spacer size, to effective work function variation due to composition or crystal formation differences in the gate; or at the atomic scale, to random dopant fluctuations in quantity and spatial positioning of individual dopants in or near the transistor channel.
Transistor matching issues generally increase in significance as transistors are decreased in size. For typical transistors, transistor width and length mismatch typically increases inversely proportional according to the square root of the transistor area. For certain transistor attributes such as off-state current or threshold voltage variation, the matching variation in nanometer scale transistors can be great enough to create an unacceptable die, or result in high device failure rates.