Physical mechanisms responsible for short channel effects in MOS devices

The physical mechanisms responsible for short-channel effects in MOS devices are described. It is shown that short-channel effects are caused by the penetration of junction fields into the channel region. These fields effectively decrease the substrate doping, thus leading to a lower threshold voltage and a larger depletion region depth. Computer simulations explicitly verify the lower effective substrate doping and larger depletion depth in short-channel devices. The junction fields are found to decay exponentially with distance from the junction edges and to vary sinusoidally in the vertical direction. Both the decay length and the wave length are found to be proportional to (3tox+ Wc). The usefulness of the new physical understanding is demonstrated in a comparison of the short-channel effects of conventional and buried channel MOSFETs using both computer simulation and experimental results. Some of the limitations of charge sharing models are also discussed.