When a trivalent impurity atom (like boron or aluminium) is added to a pure crystal during the crystal growth, the resulting crystal is called a p-type extrinsic semiconductor. Let us consider the case when trivalent aluminium (or boron) is added to pure (germanium or) silicon crystal.
As shown in Fig. 3, the three electrons of aluminium atom form covalent bonds with the valence electrons of neighbouring silicon atom. There is a deficiency of one electron which is called a hole. The remaining fourth atom of the silicon also tries to form a covalent bond. By thermal energy, it gets an electron from the neighbouring silicon and a hole is created in the adjacent atom. This process continues and the hole moves about in random way due to thermal effects. The dopant (Aluminium) accepts an electron from the neighbouring silicon to form covalent bond and it is called an acceptor impurity. The hole in the valence band moves freely and hence the current flows through the material. This type of semiconductor is called p-type semiconductor. Unlike intrinsic semiconductors, the concentration of holes and electrons are unequal in extrinsic semiconductors.
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