Built-in longitudinal field effects in sub-100-nm graded Si/sub 1-x/Ge/sub x/ channel PMOSFETs

Simulations and experiments have been conducted to study the built-in longitudinal field effects in sub-100-nm graded Si/sub 1-x/Ge/sub x/ channel vertical PMOSFETs. Results are compared for two operation modes with the source/drain (S/D) contacts interchanged, i.e., the "normal" mode, in which the built-in field is favorable to the drift, and the "reverse" mode, in which the built-in field is a retarding one. The linear drain current at V/sub DS/=-0.1 V is found to he very similar between the two modes. Furthermore, if the body is grounded, the saturation drain current, as simulated, in the normal mode is 20% lower than that of the reverse mode. With floating body effects, however, experimentally it is observed that the saturation drain current in the normal mode is 14% higher than in the reverse mode. The simulation reveals that in the linear condition, the benefits of the accelerating built-in field in the channel is negated by a weaker source injection resulting from a larger source-side built-in potential at the Si source junction, and also by a reduced or even reversed diffusion current in the normal mode; in the saturation condition, the built-in field is overshadowed by the large external field due to high drain biases. The measured off-state leakage current at T/sub DS/=-1.6 V in the normal mode is 100 times higher than in the reverse mode due to the floating body effects.