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bipolar junction transistor (BJT)

Transistors are active electronic semiconductor components used in amplifiers, repeaters, transponders and other network and transmission components; but also in microprocessors and semiconductor memories.

The basic principle of the transistor was developed by Bell Laboratories in 1948. A transistor is a semiconductor device whose current flow can be controlled. Semiconductors with various dopants are used as the base material in which free electrons and holes are controlled as charge carriers between two boundary layers. A transistor has three electrodes: the emitter, the collector and as control electrode, the base. Boundary layers exist between the emitter and the base and between the base and the collector.

Experimental setup of the first transistor by Shockley, Bardeen and Brattain, photo: IEEE.org

Experimental setup of the first transistor by Shockley, Bardeen and Brattain, photo: IEEE.org

The emitter is the current-generatingelectrode from which charge carriers are injected into the base. This current controls the current that flows through the collector. The collector is the current receiving electrode. It collects the charge carriers from the base, through which the current flow in the emitter region is determined. The base is the thin central area between the emitter and collector that controls the current flow. The current flows from the emitter to the collector and its strength is controlled by the base.

The doping of the transistors

Current flow in a NPN transistor

Current flow in a NPN transistor

Depending on whether the doping has an excess of electrons or holes, the material is said to be negatively (n) or positively (p) doped. In the narrow boundary layer between positively and negatively doped material, the pn-junction or np-junction, which forms the space charge region, all processes crucial for hole and electron migration take place. When an n-doped region is affected with a negative charge, electrons migrate to the boundary region, establishing conductivity between the two materials. The situation is different if the n-doped region is influenced with a positive voltage. Then electrons migrate away from the boundary region, which means that there is no longer any conductivity.

Transistors. Circuit symbol and example of a characteristic curve

Transistors. Circuit symbol and example of a characteristic curve

From the designations of the two boundary layers, the bipolar transistor, in English the Bipolar Junction Transistor (BJT), is derived: pnp transistor or npn transistor. In the BJT transistor, the pn junctions are homogeneous. In contrast, they are heterogeneous in the heterojunction transistor(HJT). In addition to these bipolar transistors, there are also unipolar transistors, known as field-effect transistors.

The development of the transistor

The historical development of the transistor to the integrated circuit( IC) ranges from the pointed transistor, where wire tips formed the emitter and collector, to the planar transistor with a planar junction between the pn junctions, to the planar transistor in 1959. This transistor is based on planar doping layers by diffusion. During diffusion, in which foreign atoms diffuse into the material, changes occur in the atomic structure that produce the actual transistor properties. This process technology, later applied to monolithic chips, forms the basic technology for the production of the integrated circuit (IC).

Transistors are housed in transistor packages that protect them against damage and deterioration. Corresponding transistor hous ings are available for printed circuit boards with through-hole technology and also for SMT technology.

Informations:
Englisch: bipolar junction transistor - BJT
Updated at: 02.01.2022
#Words: 518
Links: active, semiconductor, network, transmission, current
Translations: DE
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