![]() Hence, no electrons can diffuse from the emitter to the collector. The thermal energy of the electrons is not sufficient to move electrons from the emitter to this point. From this point, the electron is pulled to the collector by the electric field of the voltage source and those of the right depletion layer. Energy has to be applied to the electron to move it from the emitter to the point shown at this drawing. The electrons have to pass two depletion layers to be able to move from the emitter to the collector. Let's have a closer look at the situation inside of the transistor while it is attached to a voltage source between emitter and collector: How to make the emitter collector line conductive Thus, the left depletion layer is growing, too, until no more electrons can reach the middle layer.Īs soon as the state of equilibrium is reached, the middle layer is more negatively charged than before. The number of immobile charges is decreasing. In here, the electrons recombine with holes, by what some of the movable charges vanish during recombination processes. The electric field of the left depletion layer is weakened by the field of the voltage source, hence electrons start diffusing into the P-doped layer. The electric field of the voltage source is pointing into the same direction as those of the right depletion layer, by what those zone is growing. Those charges move around freely inside of the accordant layer. This will result in the following situation.Įlectrons are injected into the crystal at the emitter and they are extracted at the collector, by what additional holes are created at this area of the transistor. Let's attach the emitter to the negative and the collector to the positive terminal of a DC voltage source. aluminum)Īttaching voltage to emitter and collector Figure 3: Light blue = trivalent impurity atoms (e.g. Yellow = pentavalent impurity atoms (e.g. The (immobile) charges are located near the junction and not equally distributed over the whole crystal (see chapter diodes). In summation, the two N-doped layers become positively charged, while the P-doped middle layer (base) becomes negatively charged. Like explained at the chapter about diodes, a depletion layer is formed around the contact area by recombination of extra electrons and holes. There are two P/N junctions inside of a bipolar junction transistor. Transistor without being attached to a voltage source Figure 2: The lower drawing shows a NPN Transistor. The pins are called Emitter (E), Collector(C) and Base (B). All three layers can be attached to a voltage source. The upper drawing shows a PNP bipolar junction transistor. There are electrical connectors at all three layers of the device: A NPN transistor is made of two layers of N-doped material with a middle layer of P-doped material, while the middle layer of an PNP transistor is made of N-doped material and it is embedded in a sandwich of P-doped material. You can find the source code of the Java program at the download section.īipolar junction transistors ( BJT ) consist of a sandwich of variably doped semiconductor materials.
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