Electrical characteristics of small size ion-implanted MOSFET``s are characterized based on analytical model that includes short channel, narrow width, and velocity saturation effects. Theoretical results on threshold voltage shift in short channel or narrow width devices are in good agreement with the experimental results. A comparison of the electrical characteristics of MOSFET``s with and without ion-implantation leads to the conclusion that the field implant dose significantly affects the threshold voltage behaviour in narrow width devices. Also considered is the external resistance that is important for the characteristics of short channel devices. Under these considerations, the Level 2 Model in SPICE 2.G is extended to the ion-implanted MOSFET``s in Part I and simple, but accurate MOSFET model in Part II.