This book is presenting a new way of looking into the well-known impact ionization phenomena in reverse biased p-n junctions. It is intended as a text for postgraduate students in applied physics, electrical and electronics engineering and material science. It can exclusively serve as a reference for the scientists who are working in the area of semiconductor device simulation and modeling. The analytical approach of representing impact ionization phenomena based on multistage scattering model will be extremely helpful for the researchers working in the above-mentioned areas of research. The procedure of estimating the ionization rate of charge carriers in different semiconductors and monolayer grapheme nanoribbons via multistage scattering model has been presented in different chapters of this book. The influences of acoustic phonon scattering as well as externally applied steady magnetic field on the ionization rates has also been described in detail in subsequent chapters.