This paper presents experimental results and explanations on the doping profile dependence of the electrical behavior of the vertical impact ionization MOSFET (I-MOS). The device is fabricated as a gated n(+)ip(+)in(+) structure, where the p(+) region is a (as grown) 3 nm thin highly doped delta layer. The final shape of the doping profiles strongly depends on the thermal budget during processing and influences the electrical characteristics. Especially the subthreshold slope S strongly depends on the shape of the doping profiles. Values of S as low as 1.06 mV/dec were measured using this device concept. We will explain the effects influencing the electrical behavior by measurements and simulations. (C) 2007 Elsevier Ltd. All rights reserved.