Investigation of the impact ionization in the hydrodynamic model

The effects of impact ionization are studied in the framework of a standard hydrodynamic model. Three prescriptions of impact ionization are implemented in the device simulator HFIELDS. They are: (1) a model of Scholl and Quade developed from the Boltzmann transport equation; (2) an empirical model of Baccarani and Stork; and (3) a postprocessor method. Three thin-base Si bipolar devices are simulated. The numerical results show that over a certain range of electric field, the multiplication factors simulated from the Scholl-Quade model, and the Baccarani-Stork model agree very well with the experiment. At very high fields, these models tend to underestimate the net generation rate. Invoking the postprocessor technique, good agreement is found between simulation and experiment. However, at high fields the postprocessor method can lead to erroneous base and collector currents. The limitations of the Scholl-Quade model and how it can be extended for high-field applications are considered