The effect of charged additives on the carrier concentrations in lone-pair semiconductors

The Street—Mott model of gap states, interpreted in terms of valence alternation defects, is used to calculate the temperature dependence of the carrier concentrations and of the conductivity as a function of the concentration of charged additives in chalcogenide semiconductors. When the charged additives are allowed to equilibrate with the valence alternation defects at elevated temperatures we find that the conductivity activation energy remains essentially unchanged although the conductivity may be increased by a few orders of magnitude for large concentrations of additives. Very much larger increases in conductivity result when the charged additives are prevented from equilibrating with the valence alternation defects. At high additive concentrations the chalcogenide material is expected to behave like a partially compensated and nearly degenerate semiconductor in agreement with observations of Ovshinsky (1977). The gradual unpinning of the Fermi level with increasing additive concentration is explained.