ABSTRACT
Many models for the S-shaped negative differential conductivity (SNDC) mechanism were introduced in chapter 2. Among these models, it is worth noting that the two-level impact-ionization model by Scho¨ll [1] has prevailed [2-4] as a universal interpretation of the SNDC and chaos caused by the impact-ionization avalanche in semiconductors. However, the SNDC mechanism may depend on the nature of the material. In particular, the negative differential mobility observed during the impact-ionization avalanche may be an important factor in understanding the SNDC mechanism in compound semiconductors such as GaAs [5] and CdSe [6]. Then, as described in chapter 2, the screening effect for electron mobility may play an essential role in the occurrence of the SNDC in GaAs [5, 7-10] or CdSe [6]. This book adopts the screening model [5] for the SNDC mechanism in compound semiconductors. In section 3.2, the screening model is introduced in such a way as to be available for numerical studies of nonlinear dynamics and chaos. Thermodynamical consideration of the SNDC is given in section 3.3, and pattern formation of a current filament induced by the impact-ionization avalanche is argued based on an equalareas rule [1] in section 3.4. Finally, measurement techniques of the filamentary current are presented in section 3.5. Before describing the model, the chapter starts with the scattering processes which participate in the generation and recombination processes and the SNDC mechanism.
