A new look at impact ionization - Part II: Gain and noise in short avalanche photodiodes

In Part I, a new theory fur impact ionization that utilizes history-depende nt ionization coefficients to account fur the nonlocal nature of the ioniza tion process has been described. In this paper, we will review this theory and extend it with the assumptions that are implicitly used in both the loc al-field theory in which the ionization coefficients are functions only of the local electric field and the new one. A systematic study of the noise c haracteristics of GaAs homojunction avalanche photodiodes with different mu ltiplication layer thicknesses is also presented. It is demonstrated that t here is a definite "size effect" for thin multiplication regions that is no t well characterized by the local-field model, The new theory, on the other hand, provides very good fits to the measured gain and noise. The new ioni zation coefficient model has also been validated by Monte Carlo simulations .