On the mechanism of carrier transport in metal-thin-oxide semiconductor diodes on Polycrystalline silicon

A systematic experimental investigation was carried out to determine the dominant mechanism of carrier transport in MOS tunnel diodes and solar cells fabricated on Wacker cast polycrystalline silicon. A large number of diodes were fabricated and direct current-voltage and 100-kHz small signal capacitance-voltage characteristics were measured at various values of device temperature ranging between circa 300 and 420 K. Only those polysilicon diodes were chosen for analysis which exhibited exponential I - V characteristics. This excluded diodes located on large angle grain boundaries and on very small grains. For the sake of comparison, a few diodes and cells were fabricated, on single-crystal silicon also, by identical processing, and were measured and analysed. The measurements and their analysis reveal the following. The density and nature of defects present in the surface barrier region of the Wacker polysilicon material seem to have a significant influence on the mechanism of carrier transport across the barrier. With increasing number of such defects as dislocations, incoherent twin boundaries and precipitates, the dominant transport mechanism became multistep tunneling, while in MOS tunnel diodes on single-crystal silicon it was an activated process such as thermionic emission or minority-carrier injection. Stacking faults and coherent twin boundaries seemed to have a milder influence.