Modeling and Simulation of Subthreshold Swing and Subthreshold Current in Short Channel Fully-Depleted UTB Recessed Source/Drain SOI MOSFETs

Abstract

Fully Depleted Silicon-On-Insulator is a promising technology but there were limitations in it due to the ultra thin Silicon body which increases the series resistance. So research was done to find new devices to overcome this limitation. One such device is Re S/D SOI MOSFETs where the source and drain regions penetrate in to the BOX region to reduce the contact and series resistance. This device was analyzed widely due to its high current driving capability and has good subthreshold behavior.
The analysis first started with the modeling of surface potential in the channel using evanescent model. To obtain the surface potential in the channel we need to solve the second order Poisson‟s equation. We use evanescent mode analysis to obtain the solution of this equation. In this style of modelling we split the equation into 1-D Poisson‟s equation for obtaining long channel potential and 2-D Laplace equation which when solved gives the short channel solution. The back and front channel surface potentials were modeled and compared with the simulation results. There was a clear dominance of back surface over the front due to the coupling effect from recessed source and drain in lightly doped channel region.
A detailed analysis of subthreshold characteristics was done to understand the device performance. The Subthreshold swing was mathematically modeled and the effect of different device parameters like Si film thickness, gate dielectric constant, gate oxide thickness and thickness of gate and source penetrations in BOX was observed.
The generalized subthreshold current has been modeled using MATLAB to find the numerical model results for variations in the device parameters and the corresponding results have been verified using device simulator ATLASTM from SILVACO. The model and simulation results have been plotted together and accuracy of model was verified.