The emitter current is the sum of base and collector currents. In this manner, the entire emitter current flows in the collector circuit. The remaining (more than 95%) electrons cross over the base region and reach to the collector region to constitute the collector current (). Since the base is lightly doped and very thin, hence, only a small number electrons (less than 5%) combine with the holes to constitute the base current (). As these electrons flow through the p-type base, they tend to combine with the holes. With the forward-biased emitter-base junction and reverse-biased collector-base junction, it can be seen that the forward bias causes the flow of electrons from the n-type emitter into the p-type base. Therefore, the collector current depends upon the emitter current and nearly equal to the emitter current. The forward bias of the emitter-base junction causes the emitter current to flow and this emitter current entirely flows in the collector circuit. The emitter-base junction of BJT is forward-biased, whereas the collector-base junction is reverse biased. The doping concentration of the collector region is moderate. The base is lightly doped and very thin, hence it passes most of the charge carriers injected by the emitter to the collector. The emitter is heavily doped so that it can supply a greater number of charge carriers (electrons or holes) to the base. The resistance of forward-biased junction is very small as compared to that of the reverse-biased junction. In general, the emitter-base junction of the BJT is made forward-biased, whereas the collector-base junction is reverse-biased. The one section is called the Emitter, the other is called the Collector, and the middle section is called the Base and forms two pn-junctions between emitter and collector. During the transistor operation, a lots of heat is generated at the collector, hence the collector is made larger to dissipate the heat.Ī transistor has three sections of doped semiconductors. The base is much thinner than both emitter and collector. The collector region is wider than both emitter and base. There are two pn-junctions, hence a transistor may be regarded as a combination of two back-to-back connected diodes. For the pnp-transistor, the conventional current flows into the emitter as indicated by the inward arrow. Emitter and Collector are taken out from the two p-type semiconductor layers and the Base terminal is from the n-type semiconductor. For npn-transistor, the conventional current flows out of the emitter as indicated by the outing arrow.Ī pnp-transistor is composed of two p-type semiconductors which are separated by a thin layer of n-type material. In BJT symbol, the arrow on the emitter terminal indicates the direction of conventional current in the emitter with forward bias. Emitter and Collector are taken out from the two n-type semiconductor and the Base terminal is from the p-type semiconductor. NPN TransistorĪn npn-transistor is composed of two n-type semiconductor materials which are separated by a thin layer of p-type semiconductor. In this article, we will discuss in detail the working principle of both these types of BJTs.

A BJT can also be used as a solid state switch in electronic circuits. The primary function of BJT is to increase the strength of a weak signal, i.e., it acts as an amplifier. A Bipolar Junction Transistor (BJT) is a three-terminal device which consists of two pn-junctions formed by sandwiching either p-type or n-type semiconductor material between a pair of opposite type semiconductors.

Bipolar transistor operation modes