HIGH Electron Mobility Transistors (HEMTs) based on GaN are already established for high-power and high-frequency applications, such as mobile communications and radar. Discrete transistors and Monolithic Microwave Integrated Circuits (MMICs) are commercially available, however, emerging new technologies, such as ultra-broadband communication and hybrid vehicles, require further device optimization. On the other hand normally-off structures, while essential for some applications (both analog and digital), still suffer from several performance issues. In order to solve these problems and enable a cost and time effective optimization routine, device simulation tools are expedient.
This thesis discusses III-Nitride materials and material systems on which HEMTs are based. Own Monte Carlo simulations are supplemented by an extensive study of experimental and theoretical works available. Using the most recent findings for the band structure and accounting for all relevant scattering mechanisms the simulations show electron transport properties which are in agreement with those reported for GaN and AlN. For InN superior transport characteristics are predicted due to the lower band gap.
New transport models suitable for III-V materials are developed based on the extensive summary of available experimental and theoretical data and the own simulations results. They are subsequently implemented in the device simulator MINIMOS-NT. Established physical models for the lattice and thermal properties of the materials as well as models describing relevant effects are discussed with respect to HEMT specifics and material properties.
Several GaN-based device generations are simulated using the presented models and model parameters. A very good agreement with experimental data and excellent predictive results allow for extensive optimization studies of the gate geometry. Performance predictions for down-scaled devices in high-temperature operation are shown. Transconductance investigations are discussed with respect to the transport model used. InAlN/GaN structures are studied and their RF performance is analyzed accounting for improved material quality. At last two different approaches towards the realization of normally-off devices are examined: the recess gate technique and band structure engineering using an additional InGaN cap layer. Based on experimental DC data, the RF performance is predicted and compared.