CONDUCTION MECHANISM OF SELF-COMPENSATED-HIGHLY DISORDERED SEMICONDUCTORS (A POSSIBLE MODEL FOR SEMICONDUCTING GLASSES) .I. AMBIPOLAR CONDUCTIVITY

Ambipolar conduction in a self‐compensated highly disordered semiconductor with the Fermi‐level pinned close to the middle of the band gap is discussed and it is shown that predominant p‐type conduction occurs because of a higher effective level density in the narrower valence band, which usually is not compensated by a lower hole mobility due to a higher effective mass of holes. Because of a non‐negligible contribution of minority carriers, the Hall‐mobility is considerably lower than the actual carrier mobilities and can vanish and even invert its sign, if the major carrier transport takes place higher inside the bands where the effective masses increase markedly and/or if the Fermi‐level lies slightly above the middle of the band gap. The shift of the carrier transport into the bands is assumed to be caused by small potential barriers extending from the band edges into the band and being due to a sufficient density of charged centers. The results of this model are compared with experimental findings on a chalcogenide Ge0.33As0.2Te0.2S0.27‐glass yielding mn = 0.2 m0, mp = 1.6 m0, μn = 210 cm2/Vs, μp = 75 cm2/Vs, n = 5 × 107 cm−3, and p = 4 × 108 cm−3 for an observed semiconductivity of 5 × 10−9 Ω−1 cm−1 and Hall‐mobility of −0.5 cm2/Vs at room temperature. The activation energy for semiconductivity is 0.72 eV, the optical band gap ≈ 0.9 eV, yielding a barrier height within the bands of ≈ 0.25 eV. Copyright © 1969 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim