If a crystal of silicon or germanium is doped in such a way that its one part becomes n-type and the other p-type, then a p-n junction would be formed in between. On one side of this junction, there would be a region of n-type having free electrons as majority charge carriers. On the other side of the junction, there would be a region of p-type having holes as majority charge carriers as shown in Fig. 19.8 n this figure, the black dots show electrons which being free, are moving randomly. While small circles represent holes. The holes donor have a random motion. They move while remaining in their respective orbits.
Just after the formation of p-n junction, some of the electrons of n-type region, due to their free random motion cross the junction and enter into the p-type region where the holes are in abundance. When an electron reaches the site of a hole, it fills up the vacant site and becomes a part of the orbit of impurity atom, having that hole. The impurity atom, with its three valence electrons was a neutral atom, but when an electron fills the whole present in its orbit, then the number of electrons in its orbit becomes four. In this way this atom is converted into a negatively charged ion. Note that in a crystal, every atom has a fixed seat, so the negative ion does not move from its place. In this way, as more and more electrons enter into the p-region from the n-region, a layer of immobile negative ions is formed in the p-region.
Now let us observe the changes which take place in the n-region. This region contains pentavalent impurity atoms which have five valence electrons in their last orbit. Four of these electrons are bound by covalent bonds while the fifth one is free. As this electron leaves its parent atom and enters into the p-region, the number of electrons in this particular impurity atom gets short by one due to which it is converted into an immobile positive ion. In this way as more and more electrons enter from n-region into p-region, a layer of positive ions, adjacent to the junction, is formed in the n-region.
These layers of positive and negative ions formed just across the junction in the n and p-regions are shown in fig. 19. 9. These layers of positive and negative ions create a potential difference across the sides of the junction. Therefore, a positive potential appears at the n-type side of the junction. This potential difference tends to stop the motion of electrons from n-region to p-region. As the electrons continue to cross the junction from n to p-region, the layer of positive and negative ions across the junction becomes wider. This also increases the potential difference created by these layers till it reaches to such a value that it completely stops the entry of the elections from n to p-region. As this potential difference does not allow the elections cross the junction from n to p-region, so it is known as potential barrier (fig. 19.9b). In case of silicon its value is 0.7 volt and in case of germanium its value is 0.3 volt.
The region of the layers of positive and negative ions across the junction does not contain free electrons or holes, so it is known as depletion region (fig. 19.9a).
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