ABSTRACT
Semiconductors are materials that have moderately good conductivity, which is higher than that of insulators and lower than that of metals. The conductivity of sufficiently pure semiconductors decays by orders of magnitude when they are cooled down from room temperature to liquid helium temperature (at absolute zero temperature, the conductivity almost vanishes). A semiconductor in a very pure state resembles an insulator, whereas in a highly polluted state, it acts like a metal. Furthermore, irradiation with light can transform the semiconductor from insulator-like behavior to metal-like. The optical absorption spectra of semiconductors normally exhibit a threshold. Below the threshold frequency, light can pass through with practically no losses, whereas above it, the light is strongly absorbed.
