The datasheet provides comprehensive information about the physical characteristics and maximum ratings of the device. For example, you can look up which terminals are the base, collector and emitter.
ECE Course Notes. Learn It! Figure 1. This property means that a BJT can be used as an amplifier or a switch. The action is fundamentally the same as with the PNP transistor, but current always travels opposite the direction of electron flow. In a PNP transistor, hole flow is in line with the current, because holes are modeled as positively charged particles.
Since electrons flow from the emitter to the collector in NPN transistors, this means that current flows in the opposite direction; that is, from the collector to the emitter. A negative potential is placed across the base and emitter. The excess free electrons in the emitter are pushed toward the base, where a small percentage recombines with a small number of holes.
The doping level of the emitter is much higher than that of the base however, so there are far more free electrons than holes. Instead, the majority of the electrons are pulled through the base into the collector. A small number of holes also makes its way from the base into the emitter, aiding in emitter-base current. Using the example of two diodes to model a bipolar junction transistor is useful not only for comprehension, but also for testing BJTs.
A multimeter can be used to test basic functionality as long as it has an integrated diode tester. First, the base is one single region, whereas the middle region of the two diode model is two distinct regions connected by a conductor. Second, the base is much narrower than the n-type region in a standard P-N junction diode. Third, the base is very lightly doped compared with the emitter, which is the most heavily doped region of a BJT. These factors reduce carrier recombination in the base. They allow the BJT to function primarily via minority carrier diffusion from the base to the collector.
For instance, a PNP transistor functions by injecting holes into the base, where they are the minority carrier whereas an NPN transistor injects electrons into the base. Holes are a minority carrier when they are in an n-type material, and electrons are a minority carrier when they are in a p-type material. There a four modes of operation for bipolar junction transistors: forward-active, saturation, reverse-active, and cut-off.
This is the standard mode of operation for most BJTs. The base-emitter junction is forward biased, and the base-collector junction is reverse biased. Moreover, impurities are commonly added to the silicon via doping to make the layers behave as desired. It's the p-type layers in a BJT's job to attract the electrons that enter the transistor through its input circuit.
The n-type layers, on the other hand, should encourage electrons to flow out of the transistor. This push-and-pull effect between the layers enables the electrical current to be amplified and controlled.
The configuration of the layers in a BJT determines the electrical current's flow and ensures that a charge can't return in the same direction as it came.
This setup protects the system from overheating or becoming damaged. Bipolar junction transistor BJP workings. Source: Electronics Tutorials. When electrical contacts are placed on all three silicon layers and the current is switched on, the electrons in the current will flow between the n-type and p-type silicon layers.
In an NPN sandwich, amplification occurs when a small positive current is attached to the p-type layer the base. Here, one of the n-type layers the emitter is negatively charged, while the other the collector is positively charged. In an NPN Sandwich, electrons are attracted to the base from the emitter. The electrons then move from the emitter to the collector; this movement creates a higher current flow between the two n-type layers.
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