3.1.2 Limitations of the PIF-Libraries for Three-Dimensional
Device Simulation

The libraries described above have been heavily used in the two-dimensional version of Minimos-NT. Despite of the implementation in Minimos-NT these libraries have some disadvantages which restrict their practical relevance for a multi-dimensional simulator.

• Although the libraries PAI, GRS, and GAS have been implemented for two- or three-dimensional applications they are not suitable for multi-dimensional applications. The libraries can either be compiled for two or for three dimensions. Thus, a two- and a three-dimensional version of Minimos-NT must be compiled which do not enable multi-dimensional mixed-mode simulations.
• The Geometry2DSupport library uses the slab method [95] to perform point location within the geometry. Due to this method the complete library has been designed to handle two-dimensional geometries only.

Other obstacles found during code analysis concerning the implementation and usage of the libraries of the two-dimensional version of Minimos-NT are:

• Calls to the PIF-Libraries are spread over the whole code. Thus, maintenance of the code is difficult and the probability that the developer introduces new errors is high.
• The library calls are very complex and have up to $20$ parameters with slightly or completely different meaning and may depend on each other. A typical call to read a value of a single quantity is gasPifGetReal which requires $17$ parameters.
• Each developer has to know exactly the internal description of each quantity. To identify a quantity four names and two integers are required. For the quantity handling several additional information is necessary like the segment and grid a quantity is defined on or quantity properties like if it stores vectors or is constant. Thus, these calls require a deep knowledge of the library internal representation of each quantity and, therefore, are error-prone which resulted in copy-paste-code to avoid failures.
• Before quantities are used, quantities have to be initialized. Because the handling of quantities is spread over several files and several functions it was hard to guarantee that the initialization was done before their first use. The developer had to find out if a quantity has been initialized before.
• Quantities can be created, deleted, and modified anywhere in the code. This resulted in a software product complex to understand, to maintain, and to modify.
• The Voronoi information is calculated by Minimos-NT. But to accomplish this, the handling of grids was implemented twice: For ortho-product and for triangular grids. The code mainly differed by names for variables and functions.

Thus, the PIF-Libraries which are used by the two-dimensional version of Minimos-NT are not well suited to handle the geometries, grids and attributes for multi-dimensional TCAD tools. Two solutions have been imaginable:

• Expanding the PIF-Libraries to three-dimensions. The advantage of this approach is, that the software architecture of Minimos-NT can be nearly left unchanged and most of the PIF-Library calls inside Minimos-NT can remain. The disadvantage is, that expanding of these libraries to three dimensions would lead in a complete redesign of the libraries. Nevertheless, Minimos-NT still would be restricted to grids, file-formats and functionality supported by these libraries.
• Development of a new interface which can use any kind of library to represent complete wafers like the Wafer-State-Server [108,109].

Within this thesis the second approach has been chosen. Therefore, a new quantity server as well as interfaces to read the device and grid descriptions have been developed. This approach allows to connect Minimos-NT to several existing libraries. An example of such a library is the so called Wafer-State-Server which has been developed at the Institute for Microelectronics within the past years. It is a library which can handle complex geometries and attributes distributed on the grid. The geometries may consist of several segments. Two- and three-dimensional structures are supported. Nevertheless, its two-dimensional capabilities are small. It provides several readers and writers for several formats such as HDF5 [110] or DFISE [111].