Coupled numerical analysis of Suspended gate field effect transistor (SGFET)

Suspended gate MOSFETs exist since last few decades. Resonant gate transistors were first demonstrated as a means of getting high Q devices [1]. The advantages of transistor based transduction as compared to the capacitive detection have also been demonstrated [2,3]. The behaviors of SGFETs have been studied using equivalent lumped parameter modeling methodology [4-7]. Numerical simulations involving "hybrid" FEA coupling between two different tools, ANSYS Multiphysics and ISE-DESIS, with the help of an external Perl script have also been tried [8]. Here ANSYS is used for the coupled electrostatic and structural physics calculations and ISE-DESIS provides the correct boundary conditions for the electrostatic domain using semiconductor physics. Another related study [9] solved the beam equation coupled with the Poisson equation numerically using finite difference and Newton Raphson method. Our work presents an easier modeling and analysis of a suspended gate MOSFET in COMSOL, successfully demonstrating a solution of a moving gate type device. In our analysis, we consider the air gap as a deformable continuum and we report the standard Id-Vg characteristics of the transistor. Commercially available packages such as ISE-DESIS specialize in only fixed gap analysis wherein air is also modeled as a material with a known permittivity. In such an analysis the capacitor formed due to the air gap remains fixed. We demonstrate here that this analysis can be easily carried out in COMSOL using its multiphysics features. Using two dimensional analysis involving structural mechanics domain, moving mesh ALE, convection and diffusion, and the electrostatics domain, the effect of moving gate and hence the moving air gap can be modeled and analyzed.