Some device design issues of the impact-ionization MOS (I-MOS)de vice are discussed in terms of the junction depth of the source extension region and the substrate doping concentration. It is found that the source extension region is needed to be as shallow as possible in order to minimize the avalanche breakdown voltage. Furthermore, it is observed that the dependence of the threshold voltage of the I-MOS device on the substrate doping concentration is contrary to that of the MOSFET, which is an interesting phenomenon. It is related to the junction abruptness between the channel and the i-region, which is explained by using the concept of maximum lateral electric field.