An isolated atom of germanium has 32 electrons, arranged in subshells according to this scheme: This element has the same crystal structure as silicon and, like silicon, is a semiconductor. Which of these electrons form the valence band of crystalline germanium?
The electrons from subshells 4s and 4p will form the valence band isolated atom of germanium.
An isolated atom of germanium has 32 electrons arranged in subshells:
Materials whole resistivity values lie between that of conductors and insulators are called semiconductors. Examples are silicon, germanium, etc. At room temperatures the number of free charge carriers is very less and hence they act like insulators. As we increase the temperature, the number of charge carriers that are free to move increase rapidly and hence they start behaving as conductors.
From the concept, we can get that the outermost subshells of an atom that are far from the nucleus of the atom form the valence band of an atomic structure.
Hence, according to the atomic orbital concept, there are 4 electrons in the valence band due to the contribution from subshells and that is the highest filled band.
Doping changes the Fermi energy of a semiconductor. Consider silicon, with a gap of 1.11eV between the top of the valence band and the bottom of the conduction band. At 300K the Fermi level of the pure material is nearly at the mid-point of the gap. Suppose that silicon is doped with donor atoms, each of which has a state 0.15eV below the bottom of the silicon conduction band, and suppose further that doping raises the Fermi level to 0.11eV below the bottom of that band (Fig. 41-22). For (a) pure and (b) doped silicon, calculate the probability that a state at the bottom of the silicon conduction band is occupied. (c) Calculate the probability that a state in the doped material (at the donor level) is occupied.
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