MESH OPTIMIZATION FOR THE NUMERICAL-SIMULATION OF AMBIPOLAR TRANSPORT IN LOW LIFETIME SEMICONDUCTOR STRUCTURES

The current-voltage characteristics, carrier concentrations, electric field, electrostatic potential, space charge density and lifetime contours of a semiconductor device can be obtained by numerically solving the phenomenological transport equations. Most of the commercially available simulation programmes are not adapted to low lifetime compounds, such as semi-insulating GaAs, since for such semiconductors it is necessary to take into account the space charge variation of the localized centers (trapping or recombination centres). We will present, as an example, the case of a P+-SI-N+ GaAs diode. At thermal equilibrium and for low current levels we used an irregular mesh with a fine grid spacing in regions of high electric field or carrier concentrations, i.e. in the vicinity of the P+ and N+ contacts. At high current levels, in order to obtain convergence of the solutions, it was found necessary to adjust the mesh size in relation to the current dependent space charge distribution and to use an auto adaptive mesh whose structure followed the regions of high space charge density.