Semiconductors only exhibit measurable electrical conductivity at high temperatures. The reason for this dependence on temperature is the band structure of the electron energy levels, which comprise a conduction band, a valence band and an intermediate zone, which in pure, undoped semiconductor materials cannot be occupied by electrons at all. As the temperature increases, more and more electrons are thermally excited from the valence band into the conduction band, leaving behind “holes” in the valence itself. These holes move under the influence of an electric field as if they were positive particles and contribute to the current much as electrons do. In order to determine the conductivity of pure, undoped germanium, this experiment involves sending a constant current through the crystal and measuring the voltage drop as a function of temperature. The data measured can be described by an exponential function to a good approximation, whereby the separation of bands appears as a key parameter.
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