BJT NPN Transistor

According to the different ways of controlling current, transistors can be divided into bipolar junction transistors (BJT) and field-effect transistors (FET). In comparison, BJT have lower input resistance, making them significantly advantageous in applications involving linear amplification, switching, and high gain in the low-frequency and mid-frequency ranges. These transistors, depending on different configurations, can further be classified into NPN and PNP types. In this article, TechSparks will explore the knowledge related to NPN BJT transistors including structure, working principle and application.

Structure of NPN Transistor

npn transistor symbol

An NPN-type transistor consists of three semiconductor layers, namely N-P-N, referred to as the negative-positive-negative configuration. Correspondingly, it has the emitter region, base region, and collector region, with the emitter-base junction between the emitter region and base region and the collector-base junction between the collector region and base region. Internally, it can be simplified as two back-to-back connected diodes: the base-collector diode and the base-emitter diode.

When designing the transistor symbol, it is depicted with a forward arrow for conduction. Observing the NPN transistor symbol, you’ll notice a triangular arrow pointing toward the emitter, indicating that the current flows from the collector to the emitter. Specifically, the emitter region injects electrons into the base region, then the electrons are attracted to the collector region to form a current and the current flows in the opposite direction to the electron flow.

PS: Although we use the analogy of two diodes for understanding, replacing a transistor with two diodes in an actual circuit is not feasible to achieve amplification.

Working Principle of NPN Transistor

First of all, it needs to be explained that the three regions of the transistor are formed through a doping process. To achieve functionality, these regions have different doping concentrations. The base region has a low concentration of boron, a trivalent element, forming a small number of holes. The collector region is doped with a medium concentration of phosphorus, a pentavalent element, forming an appropriate number of free electrons. The emitter region is heavily doped with phosphorus, forming a large number of free electrons. However, due to the presence of the junctions, free electrons are confined to their respective regions and do not flow into the base region.

To enable amplification, free electrons must move. So, How does a NPN transistor work?

Connect the NPN transistor to a circuit to form the circuit diagram below, which includes two circuits labeled as “be” and “ce.”

Working Principle of NPN Transistor

In the “be” circuit, the diode (mentioned earlier in the analogy) is in a forward-biased state. In this state, the external electric field can overcome the internal electric field of the emitter-base junction, allowing free electrons to diffuse from the emitter region to the base region. These free electrons diffusing from the emitter region exhibit two states:

  • A small number of free electrons combine with holes in the base region.
  • A large number of free electrons flow to the collector region.

As the number of holes in the base region decreases, the external circuit replenishes them, resulting in a current “ib.” The movement of free electrons creates a current “ic,” and “ic/ib” remains constant. Therefore, the working principle of an NPN transistor can be understood as controlling the current flowing between the collector and emitter by the base current. However, it must satisfy the following conditions:

  • The base current must be greater than 0.
  • There must be a voltage difference between the collector and emitter, greater than 0.2 volts.

Applications of BJT NPN Transistor

NPN-type transistors find widespread applications in electronic circuits, especially in the low-frequency and mid-frequency domains. Here are some common scenarios:

Amplifiers:

Used to amplify output current by adjusting input current, extensively applied in fields such as audio amplifiers, televisions, and communication devices.

Switches:

Can act as switches in digital circuits, toggling between states by adjusting voltage, commonly seen in logic circuits.

Oscillators:

Function as oscillators in radio and communication devices to generate oscillating signals at specific frequencies.

Temperature Sensors:

Due to the temperature-dependent characteristics between input and output currents, they can be used as temperature sensors, providing temperature information by observing current changes.

Transient Protection:

Serve as transient protection components to prevent damage to sensitive elements caused by sudden voltage or current changes.

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