What is BJT


Transistors are active electronic semiconductor components that are 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 component whose current flow can be controlled. Semiconductors with various doping levels 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 a control electrode, the base. Boundary layers exist between the emitter and the base and between the base and the collector.

The emitter is the current generating electrode, the collector is the current receiving and the base is the current controlling electrode. The current flows from the emitter to the collector and its strength is controlled by the base.

The doping of the transistors

Depending on whether the doping has an excess of electrons or holes, one speaks of negatively (n) or positively (p) doped material. In the narrow boundary layer between positively and negatively doped material, the pn junction or np junction, which forms the space charge zone, all of the processes that are decisive for the migration of holes and electrons take place. If an n-doped area is influenced with a negative charge, the electrons migrate to the boundary area and thus establish the conductivity between the two materials. It is different when the n-doped region is influenced by a positive voltage. Then electrons migrate from the border area, which means that there is no longer any conductivity. The bipolar transistor (BJT) is derived from the names of the two boundary layers: PNP transistor or NPN transistor. With the BJT transistor, the PN junctions are homogeneous. In contrast, they are heterogeneous in the case of the heterojunction transistor (HJT). In addition to these bipolar transistors, there are also unipolar transistors, known as field effect transistors.

The evolution of the transistor

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

Transistors are located in transistor housings that protect them from damage and impairment. Corresponding transistor housings are available for printed circuit boards with through-hole technology and also for SMT technology.