PCS of semimetals, semiconductors, and dielectrics
The distinctive feature of semimetals and semiconductors is, first of all, the low density of charge carriers compared with ordinary metals. Thus, in typical semimetals as arsenic, antimony, and bismuth, the carrier density n decreases from about 2×1020 (As) to 3×1017 (Bi) per cm3, whereas in metals, n is 1022−1023 cm3. These values can be still lower by several orders of magnitude in semiconductors depending on the doping level. Accordingly, the Fermi energy decreases from several eV in metals to tens of meV and lower in semimetals (Fig. 11.1). Because of the band structure peculiarities, an effective mass of charge carriers both in semimetals and semiconductors can be one order of magnitude lower as compared with the metals. The small effective mass leads to the decrease of the Larmor radius to a value comparable with the contact size already in an easily attainable fields of about a few Tesla. The effect of a magnetic field on the EPI spectra in As and Sb was described in Section 8.3. Additionally, the de Broglie wavelength at the Fermi energy λ B ∝ n −1/3 increases in semimetals up to tens of nanometers (Fig. 11.1), i. e., it may become comparable with the contact dimension. This leads to the influence of the quantum interference effects on the contact conductivity. Again, the low carrier density increases a screening length r s ∝ n −1/6 (Fig. 11.1) in the case of semiconductor-metal contact, resulting in Schottky barrier formation (see Fig. 2.5) with a low concentration of carriers at the surface. All mentioned features lead both to the new interesting phenomena in point contacts as well as to difficulties in their interpretation, in particular how to separate bulk properties from the surface influence.
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