Reality → Tech → Chips → Transistor
The electric functions of a transistor (amplification or switching) are achieved through manipulation of electron energy levels in layers of highly purified semiconducting material doped with traces of a suitable substance to achieve desired electrical properties [1] . The first prototype transistor (a triode or amplifier) was developed in the late 1940s by a team at Bell Labs [2] . Transistors became a huge market success in the 1950s, when they were replacing vacuum tubes and making small, portable transistor radios possible [3] . More significantly, miniaturized transistors acting as electronic switches became the fundamental building blocks of logic gates that implement programmed computer commands. The flat, miniaturized transistor integrated with other components in a semiconductor chip is the revolutionary invention that set off unparalleled technological development [4] .
The band theory of solid state physics assigns valence and conduction bands to the electron energy levels of atoms bound in a crystal lattice. Semiconductors are characterized by a gap between the two bands that can be crossed by free electrons if external energy is applied, i.e., the conductivity of semiconductors increases with temperature (whereas metals become less conductive as temperature rises). The amount of free electrons, or the conductivity, can be greatly influenced by doping (e.g., a dopant concentration in the order of ppm (parts per million) can increase the conductivity million-fold). Dopants can be of n-type (electron donors) or p-type (electron acceptors), e.g., phosphorus (5 valence electrons) is an electron donor and boron (3 valence electrons) is an electron acceptor for silicon (4 valence electrons). If a layer of n-doped silicon is brought into contact with a p-doped silicon layer and a voltage is applied perpendicular to the layers, electrons can only flow from the n-layer to the p-layer, and not in the reverse direction (see p-n junction). A pn transistor functions as a diode (one-way valve); a npn or pnp transistor functions as a triode (amplifier).
In 1947, Bardeen, Shockley, and Brattain put together the first working transistor at Bell Labs, New Jersey. Their work was honored in 1956 with a Nobel Prize (though plans for a similar device were already patented in 1925 by Lilienfeld). The Bell team's device was a point-contact transistor that was quickly followed, in 1948, by Shockley's invention of the bipolar junction transistor (actually, both devices are bipolar, meaning they exploit p and n conduction, and both devices worked as 'crystal triodes').
The first transistor radio , marketed in 1954, was equipped with Texas Instruments bipolar junction transistors. The early bipolar (p and n conduction) transistors operated at 22.5 V, close to the breakdown voltage. In 1957, Sony entered the market with a standard-setting 9 V transistor radio.
The Metal Oxide Semiconductor Field Effect Transistor (MOSFET) is a unipolar (either p or n conduction) transistor whose conducting channel is controlled by the application of an electric field. The planar, integrated field effect transistor, as part of the power-saving and noise-immune Complementary Metal Oxide Semiconductor (CMOS) technology, became the choice fundamental building block of modern computer microprocessors.