A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and electron holes as charge carriers. In contrast, a unipolar transistor, such as a field-effect transistor (FET), uses only one kind of charge carrier. A bipolar transistor allows a small current injected at one of its terminals to control a much larger current between the remaining two terminals, making the device capable of amplification or switching. BJTs use two p–n junctions between two semiconductor types, n-type and p-type, which are regions in a single crystal of material. The junctions can be made in several different ways, such as changing the doping of the semiconductor material as it is grown, by depositing metal pellets to form alloy junctions, or by such methods as diffusion of n-type and p-type doping substances into the crystal. The superior predictability and performance of junction transistors quickly displaced the original point-contact transistor. Diffused transistors, along with other components, are elements of integrated circuits for analog and digital functions. Hundreds of bipolar junction transistors can be made in one circuit at a very low cost. Bipolar transistor integrated circuits were the main active devices of a generation of mainframe and minicomputers, but computer systems now use complementary metal–oxide–semiconductor (CMOS) integrated circuits relying on the field-effect transistor (FET). Bipolar transistors are still used for amplification of signals, switching, and in mixed-signal integrated circuits using BiCMOS. Specialized types are used for high voltage and high current switches, or for radio-frequency (RF) amplifiers.
History The bipolar point-contact transistor was invented in December 1947 at the Bell Telephone Laboratories by John Bardeen and Walter Brattain under the direction of William Shockley. The junction version known as the bipolar junction transistor (BJT), invented by Shockley in 1948, was for three decades the device of choice in the design of discrete and integrated circuits.
Germanium transistors The germanium transistor was more common in the 1950s and 1960s but has a greater tendency to exhibit thermal runaway. Since germanium p-n junctions have a lower forward bias than silicon, germanium transistors turn on at lower voltage.
Early manufacturing techniques Various methods of manufacturing bipolar transistors were developed.
Point-contact transistor – first transistor ever constructed (December 1947), a bipolar transistor, limited commercial use due to high cost and noise. Tetrode point-contact transistor – Point-contact transistor having two emitters. It became obsolete in the middle 1950s. Junction transistors Grown-junction transistor – first bipolar junction transistor made. Invented by William Shockley at Bell Labs on June 23, 1948. Patent filed on June 26, 1948. Alloy-junction transistor – emitter and collector alloy beads fused to base. Developed at General Electric and RCA in 1951. Micro-alloy transistor (MAT) – high-speed type of alloy junction transistor. Developed at Philco in 1957. Micro-alloy diffused transistor (MADT) – high-speed type of alloy junction transistor, speedier than MAT, a diffused-base transistor. Developed at Philco in 1958. Post-alloy diffused transistor (PADT) – high-speed type of alloy junction transistor, speedier than MAT, a diffused-base transistor. Developed at Philips. Tetrode transistor – high-speed variant of grown-junction transistor or alloy junction transistor with two connections to base. Surface-barrier transistor – high-speed metal-barrier junction transistor. Developed at Philco in 1953. Drift-field transistor – high-speed bipolar junction transistor. Invented by Herbert Kroemer at the Central Bureau of Telecommunications Technology of the German Postal Service, in 1953. Spacistor – around 1957. Diffusion transistor – modern type bipolar junction transistor. Prototypes developed at Bell Labs in 1954. Diffused-base transistor – first implementation of diffusion transistor. Mesa transistor – initially developed at Bell Labs in 1955 and produced by Fairchild in 1958. Planar transistor – the bipolar junction transistor that made mass-produced monolithic integrated circuits possible. Developed by Jean Hoerni at Fairchild in 1959. Epitaxial transistor – a bipolar junction transistor made using vapor-phase deposition. See Epitaxy. Allows very precise control of doping levels and gradients.
Function BJTs exist as PNP and NPN types, based on the doping types of the three main terminal regions. An NPN transistor comprises two semiconductor junctions that share a thin p-doped region, and a PNP transistor comprises two semiconductor junctions that share a thin n-doped region. N-type means doped with impurities (such as phosphorus or arsenic) that provide mobile electrons, while p-type means doped with impurities (such as boron) that provide holes that readily accept electrons.
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