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And each of them has two structural forms, NPN and PNP, but the most commonly used are silicon NPN and PNP transistors. Except for the power polarity, their working principles are the same. The following only introduces the current amplification principle of NPN silicon transistors.diode

Figure 1: Structure of NPN Transistor

Figure 1 is a structural diagram of an NPN transistor, which is composed of two N-type semiconductors sandwiching a P-type semiconductor. It can be seen from the figure that the PN junction formed between the emitter region and the base region is called the emitter junction, while the PN junction formed between the collector region and the base region is called the collector junction. The three leads are respectively called the emitter e, diode the base b, and the collector.diode

When the potential at point b is a few volts higher than the potential at point e, the emitter junction is in a forward biased state, while when the potential at point C is a few volts higher than the potential at point b, the collector junction is in a reverse biased state, and the collector power supply Ec is higher than the base power supply Ebo.diode

When manufacturing a transistor, it is conscious to make the majority carrier concentration in the emitter region higher than that in the base region, while making the base region very thin and strictly controlling the impurity content. In this way, once the power is turned on, due to the correct emitter junction, the majority carriers (electrons) in the emitter region and the majority carriers (control holes) in the base region can easily cross the emitter structure and diffuse in opposite directions. diode However, because the concentration of the former is higher than that of the latter, the current passing through the emitter junction is basically an electron current, which is called the emitter current Ie.diode