III. The Hall effect in semiconducting powders

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Abstract
Two aspects of the electrical conduction through powder semiconductors are studied : the effect of band-bending at the surface of the grains and the percolative nature of the current flow. Conductivity and Hall effect measurements were made on ZnO powders and on mixed powders of conducting ZnO and insulating ZnS. The effects of surface band-bending on the pure ZnO samples were interpreted by a ‘composite-sphere’ model which indicates that, for a powder of mean grain diameter 0.6 μm, the Hall effect measurements can only be used to measure the carrier concentration in the grains for values of N D-N A > 1017. For the ZnO used here, N D-NA < 1016 cm−3 The mixed ZnO/ZnS system showed classical site percolation behaviour in conductivity, σ = σ0(x - x c)1.7; the critical volume fraction of ZnO was x c = 0.14. The Hall effect results are less conclusive and are discussed in conjunction with a survey of appropriate percolation theories.

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