A transistor is made by fabricating two closely spaced PN junctions on a semiconductor substrate. These two PN junctions divide the semiconductor into three parts: the middle part is the base region, and the two outer parts are the emitter and collector regions. There are two common configurations: PNP and NPN.
Electrons are drawn from these three regions: the base (b), the emitter (e), and the collector (c).
The PN junction between the emitter and base regions is called the emitter junction, and the PN junction between the collector and base regions is called the collector junction. The base region is very thin, while the emitter region is thicker and has a higher impurity concentration. In a PNP transistor, the emitter region emits holes, and their movement direction is the same as the current direction, hence the emitter arrow points inward. In an NPN transistor, the emitter region emits free electrons, and their movement direction is opposite to the current direction, hence the emitter arrow points outward. The direction of the emitter arrow also indicates the conduction direction of the PN junction under forward bias. Both silicon and germanium transistors come in PNP and NPN types.
Transistor Package Types and Pin Identification
Common transistor packages fall into two main categories: metal packages and plastic packages. The pin arrangement follows certain rules. In the bottom view, the three pins are positioned so that they form the vertices of an isosceles triangle, numbered e, b, and c from left to right. For small and medium power plastic transistors, place them with the flat side facing you and the three pins pointing downwards; the pins are then numbered e, b, and c from left to right.
Many types of transistors exist in China, and their pin arrangements vary. When using a transistor with an uncertain pin arrangement, it is essential to measure to determine the correct pin positions or consult the transistor's user manual for its characteristics, technical parameters, and relevant information.
