Gigaohm-range polycrystalline silicon resistors for microelectronic applications

The objective of this work was to investigate the conduction properties of very high resistance devices formed from undoped chemical-vapor-deposited polycrystalline silicon. Test structures having resistances as high as 600 GΩ at 5 V were fabricated, of a size suitable for microelectronic device applications. Detailed measurements of current-voltage characteristics in the dark and with photoexcitation, the effect of resistor length, and the temperature dependence of resistance, were made. The data is interpreted in terms of a model based on avalanche breakdown of the reverse-biased n⁺-i junction, with the current limited by the remaining quasi-neutral i-region. Theoretical current-voltage curves and the dependence of effective resistance on device length are calculated with the model, showing all the qualitative aspects of the data. Incorporation of gigaohm-range load resistors into a 16K CMOS static RAM cell is described. The work shows the dominant effects of grain boundaries in controlling current transport in undoped polysilicon, providing high-diffusivity paths for impurity diffusion, and apparently determining the reverse breakdown behavior of the junctions present.