Sub-threshold and active-region characterization of ion-implanted buried-channel MOS transistors

The drain current versus gate voltage characteristics of ion-implanted buried-channel MOS transistors were measured and compared with a theory based on the in-depth impurity, mobile carrier, potential and mobility distribution. In more heavily implanted transistors, a characteristic sub-threshold current almost independent of gate voltage is obtained both experimentally and theoretically. For these cases, a new definition of threshold voltage is suggested. According to their potential distribution at threshold, implanted transistors are classified into three groups. The depths and widths of buried channels fall into the transition region from surface to bulk carrier mobility. Models for the mobility distribution including surface and impurity scattering as well as field dependence were used to calculate the active-region conductance.