Ordering, covalent-bond vacancy

Prompted by the structure of the periodic table of the elements, electrons were assumed to occur in concentric shells around the nucleus with a positive charge of Z units, equal to the number of extranuclear electrons. In any period of 8 elements, arranged in order of increasing Z, electrons are postulated to occupy an increasing number of sites (from 1 to 8) at the corners of a cube centred at the nucleus. Any vacancy in the shell of eight enables the relevant atom to share an electron with a neighbouring atom to form a covalent bond and to complete the octet of electrons for that shell. This view has now endured for almost hundred years and still forms the basis for teaching elementary chemistry. The simple planetary model, proposed by Bohr, allows for only one electron per orbit and has little in common with the Lewis model. 

If an impurity or dopant atom with a valency of three, such as boron, is substituted for silicon, three of the four valence electrons of the dopant atom will be held in covalent bonds. One of the covalent bonds will be missing an electron. An electron from a neighboring silicon-to-silicon covalent bond, however, can easily jump into this electron vacancy, thereby creating a vacancy, or hole, in the silicon-to-siHcon covalent bond. Thus, these trivalent dopants accept free electrons, thereby generating holes, and are called acceptors. These additional holes upset the balance between the electron and hole populations, and so there are now more holes than electrons. This is called a P-type semiconductor, in which the holes are the majority carriers, and the electrons are the minority carriers. In a P-type semiconductor the hole concentration is generally many orders of magnitude larger than the electron concentration. 

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