Emotion chip

The emotion chip was a device designed by Dr. Noonien Soong, to allow his creation Data to experience human emotions. Soong finished the chip in 2366 after twenty years of work, and summoned Data to his lab on Terlina III to install it. However, Lore also arrived, and stole the chip by masquerading as Data. (TNG: "Brothers")

In 2369, Lore used the emotion chip to trigger certain negative emotions in Data over a carrier wave, seeking to turn Data to his side. Lore's scheme failed after Enterprise-D personnel re-activated Data's ethical program. The chip was removed from Lore after he was dismantled, but Data did not install it, believing it to be too dangerous. Lore had revealed that the chip also contained memories, from Data's early days at Omicron Theta. (TNG: "Descent, Part II")

Geordi La Forge agrees to install Data's emotion chip in his neural net.

Data finally installed the chip in 2371, to further his growth as an artificial lifeform. Unfortunately, the chip overloaded his positronic relays, causing him to experience erratic emotions. The chip also became fused to his neural net, making it impossible to remove. Data was eventually able to control the emotions. (Star Trek: Generations)

By 2373, Data was able to turn the emotion chip on and off at will. The Borg Queen activated the chip during their effort to assimilate Earth, in an unsuccessful attempt to use Data's emotions to manipulate him. (Star Trek: First Contact) A way was eventually found to remove the chip, and Data left it behind on the USS Enterprise-E prior to his mission to Ba'ku in 2375. (Star Trek: Insurrection)

The chip changed in appearance in between "Descent, Part II" and Star Trek: Generations, though the chip was damaged so perhaps its appearance changed as part of the repairs.

The fate of the emotion chip is unknown, but it may have been destroyed when Data was killed in Star Trek: Nemesis.