The Bipolar transistors normally made up of n-p-n or p-n-p junctions of either silicon (Si) or germanium (Ge) material. These junctions are, in fact, produced in a single slice of silicon by diffusing impurities through a photo graphically reduced mask. Silicon transistors are work better when compared with germanium transistors in the wide-spread majority of applications (mainly at high levels of temperature) and thus germanium devices are very rarely encountered in modern electronic equipment.The construction of typical n-p-n and p-n-p transistors is shown in diagrams 12.1 and 12.2. For conducting the heat away from the junction (important in medium and other high-power applications) the collector is allied with the metal case of the transistor.
The symbols and common simplified junction models for n-p-n and p-n-p transistors are presented in diagram below. It is essential to note that the base region (p-type material in the case of an n-p-n transistor or n-type material in the case of a p-n-p transistor) is tremendously narrow.