Grain electronic conduction across

Each band can be filled with electrons in a similar fashion to filling a plastic bottle with sand. If that bottle of sand is completely full, it is possible to tilt it or even turn it upside down and the grains of sand will not move. If the plastic bottle is not completely full (i.e., a partially filled band), then the grains of sand can easily move when the bottle is tilted. They are not localized in one position but delocalized across the top surface. In a similar way, electrons of a partially filled band are delocalized across the crystal and can conduct electricity. The energy of the highest filled levels is called the Fermi energy. 

When a voltage V is applied across the double Schottky barrier, the barrier height (j) decreases with increase of the voltage. A small fraction of electrons can penetrate the depletion region. These electrons, which are accelerated in the depletion region, can excite the valence electrons to conduction band. Therefore, holes recombine with the trapped electrons at the grain boundary, and decrease the barrier height. 

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