In this dissertation a framework necessary for the optimization of GaAs based High Electron Mobility Transistors (HEMT) with numerical simulations is presented. The principles of HEMTs are explained including the limits given by the used material system. The investigations also include the influence of crystal strain and quantization effects on the electron energy levels in the channel.
The device simulator MINIMOSNT used for the simulations is described. The most important models such as drift diffusion and hydrodynamic transport models including the necessary physical parameters are presented. One important feature is that the simulation area is split into segments for which different models can be applied. The segments are linked together by interface models. The applied models account for velocity overshoot and tunneling through a potential barrier.
The verification of the simulation results is based on comparison with DC and RF measurements, quantities extracted from measured Sparameters as well as comparison with Monte Carlo simulation results. Basically three steps are necessary to obtain a realistic description of the device. First the geometry and material composition which is used in the simulation has to be developed. Then an appropriate combination of models for the different regions have to be found. Finally uncertainties in device and physical parameters have to be considered in the fitting procedure. In this context one has to distinguish between proper physical modeling and pure reproduction of a measurement results. Only characteristics which are based on accurate physical modeling can be subject to predictive simulations.
Based on the developed simulation setup HEMTs for low noise, power, and RF applications are investigated. The simulations are verified with measurements and extracted parameters of various fabricated devices for all applications. Simulations employing only one consistent set of parameters for a number of devices which use similar epitaxial layers but different geometries are shown. Although only parameters defined by the manufacturing process are used to fit the simulation results they agree extremely well with the measured data. The geometries of these devices cover the complete range of technically relevant gate length's of 120 nm to 500 nm and recess dimensions from 70 nm to 600 nm. The current gain cutoff frequencies of investigated manufactured devices cover a range of 23 GHz to more than 120 GHz.
Based on the excellent agreement between simulated and measured data
accurate predictive device simulation can be performed. This is used both
to optimize device performance for given requirements and to reduce technological
effort by optimization of performance, yield, and cost.
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