1. Introduction

The increasing functionality and complexity of TCAD simulation tools raises high demands for new software technologies supporting their development by teams consisting of people specialized in many different fields. Current simulator development has to be done by at least coordinating the work of physicists, numerical analysts, and software developers to obtain consistent projects. Thus concepts for the integration of their work like the usage of convenient interface abstraction techniques are demanded. Another important issue is the amount of time, often referred to in terms of the legendary ``person-year'' which has to be spent for the development of new simulators or models.

Solution concepts for these demands are generally summarized under the term ``Rapid Application Development'' strategies and address the following points:

• Modular development approaches heavily support the collaboration of a number of people and reflect their organization in groups specialized on specific tasks such as the development of the simulator kernel, equation solvers, time step control algorithms, and physical models.
• The reuse of code once developed should be supported to minimize the efforts for writing and debugging code for similar tasks. This challenge is commonly met by defining style guides for the code and the usage of object-oriented programming languages.
• To decrease the time required for the implementation and testing of new algorithms and models within existing applications often scripting languages are employed.
• All the above mentioned concepts should be efficient in terms of memory and CPU time consumption during the development, testing, and application of new models or simulators.

The evolution of various TCAD environments and simulators during the last decade reflects the trend to concentrate more and more on these requirements. An example for a large hierarchical structure of libraries serving for different tasks within an TCAD framework is documented for the VISTA TCAD environment [18,14]. The functionality of TCAD frameworks has commonly been achieved by utilizing various interpreted languages. Commonly known examples in this field are TMA Suprem-4 [60] and FLOOPS/FLOODS [24,23] based on TCL [34] interpreters as well as VISTA [38] and SIESTA [57] wrapped into LISP interpreters.

Various attempts have been made to support the scripting of physical models for ECAD and TCAD applications. The result is a number of different applications offering various scripting abilities, from the simple parameterization of fixed models towards the support for complete user defined models. These attempts can be classified into three main streams:

• A simple possibility is to support the definition of arbitrary models via user defined subroutines which will be compiled into the executable of the application. Examples for this approach are PROMIS 1.6 [19] and ZOMBIE [21]. In case this process is sufficiently supported by an according framework, it offers a very effective solution in terms of CPU usage. The most appealing disadvantage is that model developers have to be aware of the programming language and partially of the internal data and algorithm structures of the affected application. Another issue is that the lacking portability of algorithms defined in such a manner.
• The second approach is to use a general purpose extension language, like TCL [34] or Perl [65]. These languages are widely available on almost all modern operating systems. The advantage of using one of these is the high portability, the stability, and the vast amount of available documentation. The show stopping disadvantages are insufficient computational speed and lacking support for native data type representations of abstract data types. The latter could be addressed by some recently upcoming interpreters for object-oriented extension languages like C++ interpreters [27,28] or Python [26], but various tests did not reveal sufficient speed improvements over the other interpreter languages. Thus this approach is used within TCAD environments mostly for the parameterization or flexible control of fixed models. A recent example for such an approach can be found in [64] describing the implementation of a three-dimensional diffusion simulator utilizing a newly designed ``Simulation Command Language (SCL)''.
• The third approach is to equip TCAD simulators with modules capable of the discretization of analytical mathematical expressions. Advanced examples for that strategy can be found in [42], [25] for the description of models in the field of device simulations. Other implementations in process simulators which should be mentioned are ALAMODE [68], PROPHET [43], and Amigos [41,42]. A closer inspection shows that they implement all kinds of modeling languages varying from simple dial-an-operator schemes up to languages capable of handling complex analytical expressions independent from pre-defined operator libraries. None of this scripting languages is portable to other simulators, mostly because their implementation depends heavily on the exact layout of the grid and data structures of the simulator program. Another disadvantage of these modules is the restriction to the mathematical part of the formulation of new models. Aspects like variations in the iteration schemes, the input, and output of simulation data, handling of the simulation grid and other algorithms commonly used within simulator programs are left out.

The concepts described within this thesis are mostly resulting from the demand for enhanced and portable possibilities for the development of scriptable and extendible TCAD applications. The attempt is made to avoid the inherent problems of previously developed solution techniques and to provide new answers to the above described problems with the ``Algorithm Library''. The design concept of the ``Algorithm Library'' is to offer an open framework for the object-oriented implementation, management and documentation of any kind of algorithms and physical models required within TCAD applications. Additionally it provides the ``Model Definition Language'' - an object-oriented interpreter and compiler language - which is designed to meet the performance requirements of TCAD simulators by simultaneously offering advanced abstraction possibilities and language features supporting the joint development of models by physicists and computer scientists. The Algorithm Library is designed to contain solution concepts and support for all groups of people concerned with the development, implementation and application of TCAD tools and frameworks, e.g. framework engineers, application engineers, model developers, and TCAD engineers. Thereby it offers on the one hand a single interface for the interaction and communication between these user groups and on the other hand a single tool set for the solution of interdisciplinary problems. Special attention has been spent to the portability of the Algorithm Library to a number of different operating systems and compilers. Another major goal was to assure the applicability of its concepts to existing TCAD applications.