Simple and Robust Simulation of MOS Transistors

P. Hopper, L. Evans, D. Lauderbeck, and Dr. P. Blakey

 

The MINIMOS series of simulators developed by Professor Siegfried Selberherr and his group at the Technical University of Vienna predict the electrical characteristics of MOS transistors. These simulators incorporate well developed physical models, numerical efficiency, good software engineering, reliability, robustness, and ease of use. S-MINIMOS is a SILVACO development of MINIMOS 4.

Several features make S-MINIMOS robust yet easy to use. Automatic grid construction relieves the user from the task of defining an appropriate mesh. Non-convergence problems are rare because of the incorporated modern numerical algorithms and good initial guess strategies. The input syntax, focusing on specification of the problem rather than on solution techniques, is convenient and easy to use. Industry users familiar with MINIMOS routinely use MINIMOS whenever it is applicable. Special simulations requiring more general purpose simulators are handled by advanced users. MINIMOS users often outnumber PISCES users by about five to one in companies that use both these simulators.

 

S-MINIMOS Enhancements

The first version of S-MINIMOS added graphics capabilities. Many new features have now been added to make it useful for additional applications and even easier to apply to industrial problems. The new features of S-MINIMOS include additional physical models, improved numerical techniques, a sophisticated GUI, state-of-the-art post-processing visualization capabilities, automated scheduling of repetitive design tasks, and additional parameter extraction capabilities.

S-MINIMOS now uses capabilities provided by the MASTER Framework. MASTER uses existing TCAD framework concepts and extensions that automate design, manage data, and manage simulation over a network (network farming) for multi-run numerical experiments (refer to The Bug Exterminator, September/October 1991).

S-MINIMOS is supplied with the DeckBuild and TonyPlot modules of the MASTER Framework so that it can run standalone. DeckBuild provides a convenient, hierarchical graphical interface for problem specification. It also provides multi-run and multi-tool specification capabilities. TonyPlot is a sophisticated interactive visualization tool designed for use with process and device simulators.

Several tasks that require many runs have been automated and it is simple to automate new tasks. S-MINIMOS generates all required input decks, runs them, extracts the specified MOS parameters, and lists the results. Figure 1 shows threshold voltage versus gate length for two different thicknesses of gate oxide. The x-axis is gate length in microns, and the y- axis is threshold voltage in millivolts. The results demonstrate the anticipated short channel effects. The entire task took only 32 minutes, running on a Sun workstation!

 

Figure 1. Threshold voltage versus gate length
ran in about 30 minutes on a SparcStation.

 

 

New Physical Models

In addition to the original avalanche and energy balance models (Figure 2), new physical models in S-MINIMOS are from collaboration with Dr. Armstrong and his group at the Queen's University of Belfast, Northern Ireland. The additional physical models are: the ability to model SOI devices; a recessed source-drain capability; band gap narrowing; Joule heating effects; and two improved impact ionization models.

 

 

Figure 2.Drain and substrate currents calculated
using the S-MINIMOS Energy Balance Module.

 

One of these models describes the reduction in impact ionization close to the semiconductor-insulator interfaces. The other is a non-local ballistic model. These added capabilities significantly extend the range of situations for which S-MINIMOS can be used.

 

Summary

S-MINIMOS is a modern powerful simulator of MOS devices. It is robust and easy to use. The combination of S-MINIMOS and a general purpose simulator such as HFIELDS-2D or S-Pisces 2B, is very effective for companies that develop, manufacture, or use MOS technologies.