Two-Dimensional Modeling of Gallium-Arsenide Submicron Field Effect Transistor Structures.

The operation of submicron GaAs MESFETs has been studied using numerical simulation. Drift-diffusion and energy-momentum models were developed. The models were formulated using finite difference approximations on a novel two-dimensional staggered mesh. The energy-momentum model is based on a modified form of the energy conservation moment of the Boltzmann transport equation. A static Monte Carlo model was used to solve for relevant relaxation times. In a third modeling approach, the transport equations are formulated in terms of a geometry factor. The geometry factor defines the shape of the conducting channel in MESFET. An analytical solution is used to find the geometry factor and the transport equations are solved in one dimension. This "quasi-two-dimensional" modeling approach was found to be considerably faster than the full two-dimensional simulations. The accuracy of the quasi-two-dimensional model was determined by comparison to the two-dimensional solutions. The numerical stability of all the simulations was evaluated using a von Neumann stability analysis technique extended for use in the staggered mesh systems employed by the study. The validity of the computer simulations was established by comparing model results with previously published simulations and measured device characteristics. The applicability of drift-diffusion transport theory to small GaAs MESFETs was explored by comparing the results obtained from drift-diffusion and energy-momentum models when device doping and gate length were varied. The three different modeling approaches were used to analyze conventional MESFET structures and a novel MESFET structure, the electron diffusion FET. The electron diffusion FET contains a thin low-doped region in the conducting channel of the FET between more highly doped contact regions. This structure was found to have a higher cutoff frequency than conventional MESFETs. The small-signal performance of submicron MESFETs was also evaluated by analyzing the transient response of the simulated devices. Small-signal equivalent circuits were derived and the results compared with small-signal elements obtained from incremental change partitioning.Ph.D.Electrical engineeringUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/161454/1/8712201.pd