Question
Write a short-note on p-type semiconductors.
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Answer
→As shown in Fig., to prepare this type of semiconductors, in pure $S i$ or $G e$, trivalent impurity like $A /, B , I n$ etc. are added. (In the outer most orbit, there are 3 electrons in such atoms, so they are called tri-valent.)
→The dopant has one valence electron less than the $S i$ and $G e$ atoms, and therefore, this atom can form covalent bonds with neighbouring three $S i$ atoms but does not have any electron to offer to the fourth $S i$ atom.
→So, a vacancy (empty space) or hole is created in the bond between the fourth neighbour and the trivalent atom, as shown in the Fig.
→Since the neighbouring Si atom in the lattice wants an electron in place of a hole, an electron in the outer orbit of an atom in the neighbourhood may jump to fill this vacancy, leaving a vacancy or hole at its own site.
→Thus the hole is available for conduction. Hole has the tendency to attract/accept an electron. Hence, such impurities are called acceptor impurities.
→Apart from this, at room temperature, some covalent bonds break and pair of electron and a hole is created.
→Thus, for such a material, the holes are majority carriers and electrons are minority carriers.
→Since, the holes behave as a positive charge due to deficiency of negatively charged electrons, from the first letter of the word positive, such extrinsic semiconductors doped with trivalent impurity are called $p$-type semiconductors.
→For $p$-type semiconductors. $n_h \gg n_e$.
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