February 1987

Document Type


Degree Name



Dept. of Electrical Engineering


Oregon Graduate Center


The electrical characteristics of N-channel accumulation-mode MOSFETs have been calculated using a model of the conduction process in polysilicon that is based on drift and diffusion of carriers as opposed to earlier models based primarily on thermionic emission. In this model the polysilicon grain boundary is treated as a highly resistive, but conducting medium. Segregation of dopant atoms to the grain boundary due to annealing is taken into account. There are two routes by which polysilicon MOSFETs are fabricated. Polysilicon films can be recrystallized to achieve a device quality single crystalline substrate using several techniques. The other method uses as-deposited films in conjunction with hydrogen passivation of the grain boundary. The model presented here is for these as-deposited films. This model takes into account the shielding of uncompensated ionized dopants in the channel, when a gate voltage is applied. Current-voltage characteristics are discussed in the three regimes of device operation: leakage, weak accumulation, and drive. Channel electron mobility, grain boundary barrier potential and surface potential are calculated as a function of applied gate voltage for different film properties. The properties considered are average grain size, doping density, grain boundary trap density and trapping level. Finally the current-voltage relations for a polysilicon MOSFET are calculated and compared to the reported experimental results.





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