Asimov remained vague about the technical details except to assert that the brain's substructure was formed from an alloy of platinum and iridium. Asimov relied on the reader's knowledge of the capacity of positrons and electrons to be formed in pairs and to annihilate each other, in order to convey the impression that such pair creation and destruction could serve as a metaphor for the evanescence of thought. The focus of Asimov's stories was directed more towards the software of robots (such as the Three Laws of Robotics) than the hardware in which it was implemented.
In the final days of the robot era, the gravitonic brain was invented. Robots with this brain could obey the Three Laws, or any other laws, or none. This brain was used to weaken the Three Laws into the Four Laws of Robotics; the original Three Laws had become inherent in the design of positronic brains over the centuries.
In Star Trek the character Lieutenant Commander Data and his evil brother Lore have positronic brains They are not, however, constrained by the same laws as Asimov's robots.