Degenerate semiconductor
A degenerate semiconductor is a semiconductor , the so strongly doped (or contaminated), is that it so well is electrically conductive as metals . Due to the foreign atoms, the Fermi level approaches the conduction band or lies within it, the band gap becomes very small or disappears ( English band-gap narrowing or gap-shift ). The effect can be explained by the shielding of the grid potential by the additionally introduced, free charge carriers. The occupation probabilities in the conduction and valence bands can no longer be described with the Boltzmann statistics .
In the case of degenerate semiconductors, the Fermi level is not within the forbidden band, but in the conduction band (n-semiconductor) or within the valence band (p-semiconductor). This occurs with extremely high doping (greater than 10 19 per cm 3 ). When a metal comes into contact with a degenerate n-semiconductor, the Schottky barrier is so small that it can be tunneled through in both directions. There is then an ohmic contact.
Silicon and germanium were not available in very pure form before 1940 (in the case of silicon before knowledge of the zone melting process ), which is why they were historically classified as metals due to this circumstance.