The impurities would cause a change in conductivity, as conductivity is based on the number of holes or electrons in the valence or conduction bands of the semiconductor.There are an equal number of electrons and holes in an intrinsic semiconductor because for each electron promoted from the valence band to the conduction band, there is one hole created in the valence band.While the electronic structure of a semiconductor and insulator appear the same, the band gap energy between the conduction and valence bands is much smaller, which allows for electrons to be excited across the band gap, allowing for conductivity.This property makes semiconductors an option as a material for use in high temperature electrical applications. This rise in conductivity with increased temperatures is opposite of Metals, as metals decrease in conductivity as temperature increases. Figure 4 clearly shows the large rise in conductivity at high temperatures, just as there is a rise in carrier concentration in the same temperature region. Notice the similarity between Figure 3 & 4, as the electrons and holes are the source of conductivity in intrinsic semiconductors. The conductivity of silicon, based on the carrier density data from Figure 3, was plotted using Eq 3 and shown in Figure 4. The band gap energy E gap is related by \[E_\): and Eq 3. Band gap energy, which is dependent on the material, is also temperature dependent and decreases with temperature to a limited extent based on the material. In intrinsic semiconductors there is an equal number of electrons and holes in the material for every electron to be promoted across the gap, there is a hole left behind. These electron hole pairs are attracted to each other by their electric charge and are called an exciton. The number of electrons crossing the gap is temperature dependent and depends on the specific intrinsic material. As temperature increases, electrons in the valence band may gain enough energy to jump the band gap into the conduction band, and they leave behind a hole, which is an area of local positive charge the electron once occupied. Since the band gap, or forbidden region, has no probability of an electron occupying this region, the maximum energy an electron in a semiconductor can attain at 0 K is at the top edge of the valence band. This is directly related to the Fermi energy, which is the maximum energy of an electron at 0K.
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This temperature dependence is because at 0K, there are no electrons in the conduction band. The properties of semiconductors are strongly dependent on temperature. While the band structure of semiconductors may look very similar to that of an insulator, the band gap between the conduction and valence bands in a semiconductor is of much lower energy, typically less than 4eV. Figure 1 - Band Diagram of an Intrinsic Semiconductor, showing Fermi Energy, Conduction & Valence bands, and Band Gap.