Schottky layers, charge carrier density

Light sensors made from a-Si H are either p-i-n or Schottky barrier structures. Unlike crystalline silicon, a p-n jimction is ineffective without the undoped layer, because of the high defect density in doped a-Si H. Illumination creates photoexcited carriers which move to the junction by diffusion or drift in the built-in potential of the depletion layer and are collected by the junction. A photovoltaic sensor (solar cell) operates without an externally applied voltage and collection of the carriers results from the internal field of the junction. When the sensor is operated with a reverse bias, the charge collection generally increases and the main role of the doped layers is to suppress the dark current. A Schottky device replaces the p-type layer with a metal which provides the built-in potential. 

Because the spacing between pores is always less than the width of the depletion layer and PS has a very high resistivity, Beale et al. proposed that the material in the PS is depleted of carriers and the presence of a depletion layer is responsible for current localization at pore tips where the field is intensified. This intensification of field is attributed to the small radius of curvature at the pore tips. For lowly doped p-Si the charge transfer is by thermionic emission and the small radius of curvature reduces the height of the Schottky barrier and thus increases the current density at the pore tips. For heavily doped materials the current flow inside the semiconductor is by a tunneling process and depends on the width of the depletion layer. In this case the small radius of curvature results in a decrease of the width of the depletion layer and increases the current density at pore tips. The initiation was considered to be associated with the surface inhomogeneities, which provide the initial localized high current density at small surface depressions. 

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