Overview

Transistor is a semiconductor device widely used in amplification and switching circuits. Its working principle is based on the movement of charge carriers (electrons and holes) in semiconductor materials. Transistors usually have three layers of semiconductor materials, namely emitter, base and collector. There are two main types of transistors: NPN and PNP. The following takes the NPN transistor as an example to introduce its working principle.

The structure of the transistor

The NPN transistor is composed of three layers of semiconductor materials:

  • Emitter: usually doped with more impurities to form an N-type semiconductor.
  • Base: doped with less impurities to form a P-type semiconductor. The base is usually very thin.
  • Collector: doped with more impurities to form an N-type semiconductor.
  • A PN junction is formed between the base (P-type) and the emitter (N-type), and a PN junction is also formed between the base (P-type) and the collector (N-type).

Working principle

The working principle of the transistor is to amplify the signal by controlling the current. Its basic working process includes the following aspects:

  • Forward bias of the emitter-base junction (BE junction): When the transistor is working, the PN junction between the emitter and the base is usually forward biased, that is, the emitter voltage is higher than the base voltage. Due to this forward bias, the electrons in the emitter (for NPN transistors) are injected from the emitter to the base.
  • Reverse bias of the base-collector junction (BC junction): The PN junction between the base and the collector is usually reverse biased, that is, the collector voltage is higher than the base voltage. This allows the electrons injected from the base to move quickly to the collector.
  • Injection and amplification of carriers:
    Electrons in the emitter are injected into the base, but the area of ​​the base is very thin, and most of the electrons do not combine between the base and the base, but directly enter the collector.
    Because the base is very thin and the doping concentration is low, the base will only consume very few electrons, and most of the electrons will continue to pass through the base and reach the collector.
    In this process, the base current (I_B) controls the current (I_C) flowing between the collector and the emitter. The collector current (I_C) is the amplification factor of the base current (I_B), and the amplification factor is determined by the β (Beta) parameter of the transistor, that is:IC​=β⋅IB​Therefore, the transistor can amplify a smaller current (base current) into a larger current (collector current).
  • Current amplification effect: Since the collector current is much larger than the base current, the transistor plays the role of current amplification. This feature enables the transistor to play an important role in signal processing, such as in audio amplifiers and RF amplifiers.

Summary

Forward bias the emitter-base junction, and electrons are injected from the emitter to the base.
Reverse bias the base-collector junction, and the injected electrons flow to the collector.
The base current I_B controls the collector current I_C, and I_C is much larger than I_B, which plays the role of current amplification.
The transistor can operate in different working regions, such as saturation region, amplification region and cutoff region. These working states determine the behavior of the transistor as a switch or amplifier.

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