The purpose of this paper is to shed further light on the operating characteristics and limitations of IMPATT diodes, particularly those with Read or modified Read structures. This has been achieved by developing efficient and economical computer Programs which incorporate all of the important material parameters and doping profiles in an exact manner. These computer programs are then employed to study the properties of high-efficiency Si and GaAs structures. Some very interesting properties of these devices and the effects of material parameters and doping profiles on their performance are presented and discussed. This leads to a better understanding of these devices and their limitations. Preliminary calculations have also been carried out on an InP IMPATT diode and the results are presented.