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

The carrier transport mechanism in MOS tunnel diodes and solar cells fabricated on cast polycrystalline silicon was investigated. The direct current-voltage and 100 kHz small signal capacitance-voltage characteristics of the diodes were measured at various values of device temperature ranging between 300-420 K. Polysilicon diodes which exhibited exponential I-V characteristics were chosen for analysis, and diodes located on large angle grain boundaries and on very small grains were excluded. In addition, several diodes and cells were fabricated on single-crystal silicon by identical processing and were measured and analyzed. Results show that the density and nature of defects present in the surface barrier region of the polysilicon material seem to have a significant influence on the mechanism of carrier transport across the barrier. The dominant transport mechanism becomes multistep tunneling with increase in the number of defects such as dislocations, incoherent twin boundaries, and precipitates, while in MOS tunnel diodes on single-crystal silicon the carrier transport mechanism was an activated process such as thermionic emission or minority-carrier injection. A milder influence was produced by stacking faults and coherent twin boundaries.