The leakage current mechanisms of n-channel MOSFETs in subthreshold region have been investigated in dependence of channel doping profile and substrate doping level for several channel lengths, which are diffusion current, punchthrough current, and substrate current.
The subthreshold slope ($dV_G d(logI_D)$) increases as the substrate doping level and channel doping profile are increased, but the punchthrough current results in the decrease. Increasing the substrate-bias is shown to increase the punchthrough voltage. In the short channel MOSFET, the maximum drain voltage is determined by bulk punchthrough voltage. As some hole current generated by impact ionization flows to the source, a significant rise in drain current is observed.
