Question

One major difference in the behavior of semiconductors and metals is that semiconductors increase their conductivity as you heat them (up to a point), but the conductivity of a metal decreases as you heat it. Suggest an explanation.

Short answer

In metals, increased temperature results in more pronounced lattice vibrations, causing electrons to scatter and reducing their mobility, leading to a decrease in conductivity. Conversely, in semiconductors, higher temperatures cause more electrons to gain enough energy to jump from the valence band to the conduction band, promoting conductivity. This difference in behavior is due to the distinct structures of metals and semiconductors and the influence of temperature on electron mobility.

Step-by-step solution

Understand the structure of metals and semiconductors

Metals have a crystalline structure, where their valence electrons are not tightly bound to a single atom and easily move as a "sea of electrons." These electrons are responsible for the conductivity of a metal. Semiconductors, on the other hand, have a specific band gap between the valence band (where valence electrons are in lower energy states) and the conduction band (where valence electrons can move freely because they have higher energy). When semiconductors are at low temperatures, their electrons will mostly be in the valence band, leading to low conductivity.

Explain the influence of temperature on conductivity in metals

As the temperature of a metal increases, the lattice vibrations caused by the thermal energy become more pronounced. Due to these vibrations, the electrons moving through the crystal can scatter off the vibrating ions, which in turn reduces the overall mobility of the electrons. As the electron mobility decreases, the resistance of the metal increases, resulting in a decrease in conductivity.

Explain the effect of temperature on conductivity in semiconductors

In semiconductors, as the temperature increases, more electrons gain enough energy to jump from the valence band to the conduction band. As more electrons are promoted to the conduction band, they can now contribute to the conductivity as they are free to move within the material. The increase in the number of electrons conducting electricity outweighs the impact of increased lattice vibrations, leading to an overall increase in conductivity as the semiconductor is heated.

Conclusion

The difference in the behavior of semiconductors and metals when their temperature increases can be attributed to their distinct structures and the influence of temperature on electron mobility. In metals, the increased thermal vibrations caused by heating the material lead to a decrease in conductivity, while in semiconductors, the increase in temperature results in more electrons jumping from the valence band to the conduction band, thus leading to an increase in conductivity.