The Base layer is thinner and lightly doped so as to reduce the number of recombinations in the base layer, ensuring electrons/holes reach the CB junction. When forward biased the electrons (in case of NPN) and holes (in case of PNP) get thermally excited causing them to break the depletion layer and enter into the Base layer and recombine with holes or electrons depending upon the type of transistor. In a typical operation, the BE-junction is forward biased, which means that the P-type layer is at a more positive potential than the N-type layer, and the BC-junction is reverse-biased. The Emitter terminal emits charge carriers, which are electrons in the case of NPN type and holes in the case of PNP type BJT.Ĭharge flow in a BJT is due to the diffusion of charge carriers across a junction between two regions of different charge carrier concentrations. The Emitter region is highly doped while the Collector has balanced doping. The Base region is slightly doped and is thinner as compared to other regions. The PN junction that connects the base and emitter region is called the BE-junction while the junction connecting the base and collector is called the BC-junction. Two PN junction diodes are sandwiched together to form a BJT. In other words, a BJT is formed by a 'sandwich' of extrinsic semiconductor materials placed back to back.

Hence they are widely used as amplifiers and switches in electronic equipment such as mobile phones, industrial control, television, and radio transmitters.Ī bipolar junction transistor is created by combining two doped semiconductor materials that are back-to-back.

Another important application of BJT is switching circuits. The primary function of a BJT is to amplify current, which allows BJTs to be used as amplifiers. The three terminals of the BJT are the base, the collector, and the emitter. What is a BJT?Ī Bipolar Junction Transistor (BJT or BJT Transistor) is a three-terminal semiconductor device composed of two P-N junctions that can amplify or magnify a signal. In the case of a BJT, both electrons and holes could be used as charge carriers.

If an N-type semiconductor is used the charge carriers are electrons while if a P-type semiconductor is used the charge carriers are holes. Unipolar transistors or Field Effect Transistors (FETs), as the name suggests use only one type of charge carrier, either electrons or holes depending on the type of semiconductor material used.

Npn transistor base collector emitter