A Bipolar Junction Transistor (BJT) is a three-terminal semiconductor device that amplifies and switches electronic signals. It is one of the most common types of transistors used in electronic circuits. There are two main types of BJT: NPN (Negative-Positive-Negative) and PNP (Positive-Negative-Positive). Here, I'll explain the operation of an NPN BJT.
The NPN BJT has three regions of doped semiconductor material: two layers of N-type material (the emitter and collector) separated by a thin layer of P-type material (the base). These regions create two P-N junctions: the base-emitter junction and the base-collector junction.
Base-Emitter Junction:
The base-emitter junction is a forward-biased P-N junction. It means that the N-type material at the emitter terminal is connected to the positive voltage (VBE) and the P-type material at the base terminal is connected to the negative voltage (VBB). This biasing arrangement allows current to flow from the emitter to the base.
Base-Collector Junction:
The base-collector junction is a reverse-biased P-N junction. It means that the N-type material at the collector terminal is connected to the positive voltage (VBC) and the P-type material at the base terminal is connected to the negative voltage (VBB). This biasing arrangement blocks current flow from the collector to the base.
Working principle and operation:
The operation of a BJT is based on the flow of charge carriers (electrons and holes) across the base region. When a small forward current (known as the base current, IB) is applied to the base-emitter junction, electrons from the N-type emitter region are injected into the P-type base region, and holes are injected from the base region into the emitter region. The injected electrons and holes recombine within the base region, and a small fraction of them recombine at the base-collector junction.
The base region's thinness and the fact that it is lightly doped compared to the emitter and collector regions make it more likely for the electrons and holes to reach the base-collector junction. At the base-collector junction, the reverse-biased voltage causes a depletion region to form, acting as a barrier for most of the electrons and holes.
However, a small portion of electrons, known as minority carriers (holes in the NPN transistor), can diffuse through this depletion region and enter the collector region. This process of electron injection into the collector region is known as "minority carrier injection" and forms the collector current (IC).
The collector current (IC) is much larger than the base current (IB) due to the transistor's internal amplification effect. The ratio of collector current to base current is called the "current gain" or "beta" (β) of the transistor, denoted as IC/IB or β.
In summary, when a small current is applied to the base-emitter junction, a larger current flows between the collector and the emitter, allowing the BJT to amplify a small input signal. This property makes it a crucial component in various electronic applications, including amplifiers, switches, and digital logic circuits.