Sub-Threshold Surface Potential Analysis Of Fully Depleted SOI MOSFET Based On Semi-Analytical Method

With the development of integrated circuits,MOS devices have entered the era of deep sub-microns,the structure and size of the traditional MOS devices will reach the physical limits,which will have some great impacts on the MOS device,such as short channel effect(SCE),drain induced barrier lowering effect(DIBL)and so on.In order to solve these problems,the researchers sought to find new materials,develop new device structures and commit to produce ideal semiconductor devices.Compared with traditional bulk silicon devices,SOI devices not only have high ability of suppressing short channel effect,but also have more advantages,such as low power consumption,no latch effect,small parasitic capacitance,high integration,strong anti-radiation ability,high speed,simple fabrication process and so on.In order to study SOI MOSFET devices more deeply,we need to establish a precise and simple potential model.The specific work is as follows.In the first chapter,the development and existing problems of traditional MOS are introduced in the thesis,to solve the problems we introduce SOI MOSFET,and analyze the significance,research status and application of SOI MOSFET devices.In the second chapter,the research methods of SOI MOSFET model are reviewed,we discussed the solution,the advantages and disadvantages of the existing parabolic approximation model and quasi-two-dimensional model.Based on this,the sub-threshold surface potential model of SOI MOSFET based on semi-analytical method is analyzed emphatically.In this model,the device structure is divided into three regions by means of rectangular equivalent source,to establish the potential equation and boundary condition of each region.In the following,according to the potential of each region and the potential phase to establish the convergence conditions.The third chapter introduces the solution of sub-threshold surface potential model based on semi-analytical SOI MOSFET.The main process of the solution is as follows:firstly,according to the separation variable method and the characteristic function expansion method to obtain the potential expression with the undetermined coefficient for each region.Secondly,as the potential is same when they at the interface.Supposing the upper surface potential at the interface between the oxide layer and the silicon film interface is ?1(x)and the lower surface potential at the interface between the silicon film and the buried oxide layer is ?2(x),and then replace the undetermined coefficient in the potential equation with the upper and lower surface potentials,in this way we can avoid the presence of sinh and cosh in the equation leads to equation does not converge.Thirdly,according to the electric displacement is same when they at the interface,we can get two equations which containing the upper and lower surface potentials.According to the characteristic of the boundary condition and the characteristic of the trigonometric function,the upper and lower surface potentials should be calculated as generalized Fourier series,in this way we can avoid the large number of calculations caused by the existence of sine and cosine functions in the Fourier series.Finally,the equations are solved by orthogonal function,and we can get the matrix equations which obtained calculate undetermined coefficients.We can get the required coefficient through the mathematical calculation tool MATLAB,and then take the coefficient into the upper and lower surface potential and potential equation,you can get very intuitive potential and surface potential of the analytical formula.Semi-analytical model is different from the numerical model and analytic model,which is based on the two proposed,not only has a good accuracy,but also has a clear analytical expression,with a strong practicality and value.The four chapter introduces the verification and analysis of fully depleted SOI MOSFET model.Firstly,the model is validated by PDE tool,and then comparing the contour maps of each region.Secondly,comparing the surface potential calculated by the model with the surface potential obtained by the SILVACO simulation which is on the same size,we know that the accuracy of the model is verified.From the comparing results,it can be seen that the calculated results and the SILVACO simulation results can be well matched with different channel length,oxide layer thickness,doping concentration and silicon film thickness.Therefore,the model can well reflect the surface potential with the change of the device parameters.