Structure of Bipolar junction transistor:
A BJT contains three differently doped semiconductor regions that are: emitter region, base region and collector region. These regions are p type, n type and p type correspondingly, in a PNP and n type, p type and n type correspondingly, in a NPN transistor. Every semiconductor region is connected to a terminal, properly entitled as: emitter (E), base (B) and collector (C).
The base is physically located among the emitter and the collector and is created from lightly doped and high resistivity material. The collector that surrounds the emitter region, creating it almost not possible for the electrons injected into the base region to escape being collected, so making the resulting value of α very close to unity, and so, providing the transistor a large β. A cross section view of a BJT points out that the collector-base junction has a much larger area than as compared to emitter-base junction.
The bipolar junction transistor, different from other transistors, is generally not a symmetrical device. Here this means that interchanging the collector and the emitter makes the transistor leave the forward active mode and begin to operate in reverse mode. Because the internal structure of transistor is generally optimized for forward-mode operation, interchanging the collector and the emitter makes the values of α and β in reverse operation much smaller than as compared to those in forward operation; frequently the α of the reverse mode is lower than 0.5. The lack of symmetry is primarily because of the doping ratios of the emitter and the collector. The emitter is heavily doped, whereas the collector is lightly doped, permitting a large reverse bias voltage to be applied before the collector-base junction breaks down. In normal operation the collector-base junction is reverse biased. The cause the emitter is heavily doped is to increase the emitter injection efficiency: the ratio of carriers injected via the emitter to those injected by the base. For high current gain, most of the carriers injected into the emitter-base junction have to come from the emitter.