Before defining positive bias, let us know that a pn junction diode is a basic semiconductor device. It is a semiconductor crystal having an excess of acceptor impurities in one region, and an excess of donors in other region. These regions are called the p region and n region. In the p region, holes are the majority carriers and in n region, electrons are the majority carriers. Here we discuss that how we connect it in the circuit.
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There are two ways to connect the pn junction diode in any circuit. One is called the positive bias and the negative bias. Here we discuss the positive bias. A junction diode is said to be positive biased when the positive terminal of the external battery is connected to the p region and the negative terminal to the n region of the diode. In this situation, an external electric field E directed from p region towards n region is set up in the diode. The field E is much stronger than the opposing internal field Ei. Hence holes in the p region and electrons in the n region both moves towards the junction. These holes and electrons mutually combine just near the junction and cease to exist. For each electron hole combination, a covalent bond breaks up in the p region near the positive terminal of the battery. Of the electron and hole so produced, the holes move towards the junction, while the electron enters the positive terminal of the battery through the connecting wire. Just at this moment, an electron is released from the negative terminal of the battery and enters in the n region to replace the electron lost by combining with the hole at the junction. Thus, the motion of the majority carriers constitutes a current across the junction. This is called the positive current or the forward current. The electrons carry the current in the external circuit only.
In the positive bias, the applied electric field E dominates the small barrier field Ei. As a result, the majority carriers are pulled towards the junction. Hence, the width of the depletion layer decreases. It is due to the reason the junction diode offers a very low resistance for the current to flow in the positive bias.