A semiconductor is a material whose conductivity lies between conductors and insulators. Semiconductors that are chemically pure, i.e. free from impurities, are called Intrinsic Semiconductors or Undoped Semiconductors or i-type Semiconductors. The most common intrinsic semiconductors are silicon (Si) and germanium (Ge), which belong to group IV of the periodic table. The atomic numbers of Si and Ge are 14 and 32, giving their electronic configuration as 1s2 2s2 2p6 3s2 3p2 and 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p2, respectively.
This shows that both Si and Ge have four electrons each in the outermost valence shell (shown in red). These electrons are called valence electrons and are responsible for the conduction properties of semiconductors.
The crystal lattice of silicon (the same is true for germanium) is in two dimensions as shown in Figure 1. Here it is observed that each valence electron of a pair of Si atoms forms one with the valence electron of the adjacent Si atom. covalent bond.
After pairing, the intrinsic semiconductor loses free charge carriers which are nothing but valence electrons. Therefore, at 0K the valence band will be filled with electrons while the conduction band will be empty (Figure 2a). At this stage, no electron in the valence band will gain enough energy to cross the forbidden energy gap of the semiconductor material. Thus intrinsic semiconductors act as insulators at 0K.
Be that as it may, at room temperature, nuclear power can make a few covalent bonds break, in this way producing free electrons as displayed in Figure 3a. The electrons in this manner delivered become energized and move from the valence band to the conduction band, crossing the energy boundary (Figure 2b). During this cycle, every electron abandons an opening in the valence band. The electrons and openings shaped in this manner are called characteristic charge transporters and are liable for the conductive properties displayed by natural semiconductor materials.
Albeit inborn semiconductors are fit for directing at room temperature, here we can take note of that the conductivity in this manner displayed is low since there are a couple of charge transporters. However, as the temperature builds, an ever increasing number of covalent bonds are broken bringing about an ever increasing number of free electrons. The quantity of free electrons, thusly, is the consequence of the development of additional electrons from the valence band to the conduction band. As the electron populace in the conduction band expands, the conductivity of the natural semiconductor likewise increments. In any case, the quantity of electrons (ni) in a characteristic semiconductor is consistently equivalent to the quantity of openings (pi).
At the point when an electric field is applied to a particularly characteristic semiconductor, the electron-opening matches can be expanded under its impact. For this situation, electrons move toward the path inverse to the applied field while openings move toward the electric field as displayed in Figure 3b. This implies that the heading wherein electrons and openings move are inverse to one another. This is on the grounds that when an electron of a specific molecule moves to one side, leaving an opening in its place, the electron of an adjoining particle recombines with the opening and has its spot. Anyway in doing as such, it would have left one more opening in its place. This should be visible as the development (for this situation to one side) of openings in a semiconductor material. These two developments, albeit inverse in bearing, bring about a complete progression of current through the semiconductor.