The subthreshold behavior of SOS MOST´s

MOST subthreshold behavior is of importance in many modern dynamic and very-low-power circuits. SOS MOST´s exhibit quite generally a lower transconductance than bulk Si MOST´s. Comparison between SOS and bulk Si MOST´s is made on the basis of a simple model in the weak inversion region. Experiments with n-and p-channel SOS MOST´s fabricated with epi Si layer thicknesses ranging from 0.1 to 3 µm confirm the predicted decrease of transconductance in weak inversion with decreasing thickness. Quantitative agreement between model and experience is obtained if a ∼350-Å thick nonconductive Si layer near the Si-sapphire interface is assumed. A transconductance jump observed for epi Si thickness equal to the surface maximum depletion width has not yet been explained. Further experiments including fabrication process, back-gate voltage measurements, and device dimensions were performed in order to investigate the low-transconductance origin. It is concluded that the only relevant parameters are the epi Si layer thickness and the high density of fast states at the Si-sapphire interface.