Modeling and observation of photoconductivity in polycrystalline silicon

We have developed a phenomenological model for the photoconductivity in polycrystalline semiconductors which takes into account the contributions from the majority carriers due to mobility enhancement in addition to the contributions due to photogenerated excess (minority) carriers. The model is applicable for the cases of both partially and completely depleted grains at all intensities of illumination. The model is compared with the experimental data of photoconductivity as a function of intensity of illumination obtained in polycrystalline silicon at different temperatures. The data are in good agreement with the theoretical predictions. The results clearly demonstrate that in polycrystalline materials, photoconductivity is a majority carrier phenomenon at low optical illuminations irrespective of whether the suppression of grain boundary potential barriers is partial or complete. However, at high level conditions, the photoconductivity behavior becomes an excess carrier effect as observed in single crystals.