The effect of polysilicon grain boundaries on MOS based devices

Abstract

Analysis is provided of the flow of current through undoped polysilicon. Charge is assumed to be trapped in grain boundaries giving potential barriers across which the free electrons and holes pass. Barriers height depends on gate and drain voltage, as does charge in the grain. At low current the electrostatic effect of the free charge is much less than trapped. There is, therefore, no gradual channel and flux of charge is controlled by the differences in concentration in adjacent grains(quasi-diffusion) as in the subtreshold region of MOSFETs. When the number of free carriers becomes comparable with that on the gate, there is an ohmic change in potential, and the channel current is drift limited (quasi-drift) by analogy with the gradual channel condition in MOSFETs. Variation field effect current with gate and drain voltages is compared with measurements at various temperatures