transistor
A transistor is a tiny switch with no moving parts: a small signal at one terminal controls a much larger current flowing between the other two. Think of a faucet where a feather-light touch on the handle governs a powerful gush of water — except here the "hand" is itself electrical, so transistors can control other transistors. That self-controlling loop is the seed from which every computer, phone, and digital chip grows; it is the single invention that all digital electronics is built from.
The first transistor was built at Bell Labs in 1947. The version that runs modern chips, the MOSFET, came about a decade later: a voltage on its insulated gate builds an electric field that opens or pinches off a conducting channel between source and drain — switching current on (a 1) or off (a 0). The same device can also amplify, nudging a faint signal into a strong one. Transistors shrank from thumb-sized to a few nanometers, and a phone chip now packs tens of billions onto a sliver of silicon the size of a fingernail.
The 1947 Bell Labs device was a point-contact transistor; the more practical junction transistor followed in 1948, earning Bardeen, Brattain, and Shockley the 1956 Nobel Prize. The MOSFET that dominates today's chips came later, demonstrated by Atalla and Kahng at Bell Labs around 1959–1960. A common confusion: the gate of a MOSFET draws almost no steady current — it works by electric field, not by current flowing into the channel — which is exactly why CMOS logic sips so little power between switches.