An analytic model for very thin SOI transistors

Summary form only given. Devices made in very thin films (1000 AA or less) have a number of characteristics which are different from those of devices made in bulk material or thick SOI. In order to design these devices properly, it is important to understand these differences. To do this, the authors have derived an approximation to Poisson's equation valid for thin silicon films. This approximation is equivalent to assuming that the silicon film is replaced by a sheet of charge and zero thickness. Although this seems to be a radical approximation, the range of validity is surprisingly wide, primarily because the total dopant charge in these thin films is so small. The results of this simple model indicate that long channel transistors fabricated in thin SOI do have a number of properties that are different from transistors fabricated in thicker SOI films or in bulk films. In particular, the threshold voltages of the transistors are independent of doping (if the transistor is thin enough or lightly doped enough), the threshold voltages are dependent only logarithmically on the thickness of the film (if the total dose remains constant), and the front threshold voltage is linearly dependent on the back gate voltage over a wide range of back gate voltages. The dependence of the subthreshold slope on both the front and back interface-state densities can also be obtained from this model.<>