A software package has been developed for simulating complex silicon and heterostructure devices in 3D. Device geometries are input with a mouse-driven geometric modeler, thus simplifying the definition of complex 3D shapes. Single components of the device are assembled through boolean operations. Tetrahedra are used for grid generation, since any plane-faced geometry can be tessellated with tetrahedra, and point densities can be adapted locally. The use of a novel octree-like data structure leads to oriented grids where desirable. Bad angles that prevent the convergence of the control volume integration scheme are eliminated mostly through topological transformations, thus avoiding the insertion of many redundant grid points. The discretized drift-diffusion equations are solved with an iterative method, using either a decoupled (or Gummel) scheme, or a fully coupled Newton scheme. Alternatively, generated grids can be submitted to a Laplace solver in order to calculate wire capacitances and resistances. Several examples of results illustrate the flexibility and effectiveness of this approach.

Dependence of the conduction properties of Al-Al2O3-Au sandwich structure on ambient gas pressure was investigated. It was found that the diode current decreases with time nearly exponentially in oxygen atmosphere and that the relaxation time is a function of oxygen pressure and applied voltage.

Low-field mobilities and velocity versus electric field relations are among the key input parameters for drift-diffusion simulations of field-effect and bipolar transistors. For example, most device simulations that treat scattering from ionized impurities contain mobilities or velocity versus field relations based on the Born approximation (BA). The BA is insensitive to the sign of the charged impurity and is especially poor for ionized impurity scattering because of the relatively strong scattering of long-wavelength carriers, which have low energies, and therefore violate the validity condition for the BA. Such carriers occur at high symmetry points in the Brillouin zone and are critical for device behavior. There has been a tendency in the past to assume that majority and minority mobilities are equal. This assumption can lead to incorrect interpretations of device data and thereby misleading design strategies based on such simulations. We have calculated the majority electron and minority hole mobilities in GaAs at 300 K for donor densities between 51016 and 11019 cm-3 and the majority hole and minority electron mobilities for acceptor densities between 51016 and 11020 cm-3. We have included all the important scattering mechanisms for GaAs: acoustic phonon, polar optic phonon, nonpolar optic phonon (holes only), piezoelectric, ionized impurity, carrier-carrier, and plasmon scattering. The ionized impurity and carrier-carrier scattering processes have been calculated with a quantum mechanical phase-shift analysis to obtain more accurate matrix elements for these two scattering mechanisms. We compare the total scattering rate for majority electrons due to ionized impurities based on exact phase shifts and on the BA used by Brooks-Herring. We also present additional data that show the differences between the exact phase-shift analyses and the BA for majority electron scattering rates as functions of carrier energy and scattering angle. These results show that the calculated low-field mobilities are in good agreement with experiment, but they predict that at high dopant densities minority mobilities should increase with increasing dopant density for a short range of densities. This effect occurs because of the reduction of plasmon scattering and the removal of carriers from carrier-carrier scattering because of the Pauli exclusion principle. Some recent experiments support this finding. These results are important for device modeling because of the need to have reliable values for the minority mobilities and velocity-field relations.

We have developed an efficient general-purpose two-dimensional device simulation system which consists of a solver, and pre- and post-processors. This system can easily handle any complicated device having a non-rectangular shape. It can also be applied to compound semiconductor devices with heterojunctions, including optical devices such as laser diodes. In order to handle any device, a new program for construction of device geometry is developed as a preprocessor. It has an efficient graphic interface to reduce the time required to input data for simulations, which is a very time consuming task for complicated devices. A new efficient data structure representing device geometry is introduced in the program. During postprocessing, any physical quantity can be displayed on the multi-window screen. In addition, a general-purpose solver for basic semiconductor equations is implemented in the system. Using this system, any device can be successfully analyzed in a unified manner and the turn-around time for the simulation is significantly reduced.