Space-Charge Impedance in Photodiodes

High levels of injected space charge can strongly modify both the resistance and the reactance of photodiodes over their operating frequency range, but no substantive consideration of this effect has been published for p-i-n or uni-traveling-carrier (UTC) photodiodes. This letter describes the importance of the effect using a model that assumes a constant electron velocity, and relates published UTC photodiode results to the model. Inclusion of this effect in device design will benefit both linear and nonlinear (e.g., mixing) applications.