Negative differential resistance in ultrashort bulk MOSFETs

In our contribution negative differential resistance and single electron switching events in the channel of bulk MOSFETs with channel lengths down to 30 nm are demonstrated. First, reproducible unexpected periodic transconductance oscillations in the I_DxV_G characteristics of nMOSFETs are presented. The oscillations, present from sub-threshold up to strong inversion, are reproducible from sample to sample and with temperature cycling. No dependency of the oscillation period on gate oxide thickness or channel length could be observed and the period of the oscillations does not change in magnetic fields up to 15 T. Various electric transport models for small size MOS systems are analyzed. For several reasons, Coulomb blockade seems to be a rather plausible explanation for the observed effects. In the second part, another single electron switching phenomenon is studied. Namely, oxide traps are used as a probe into the local channel surface potential. Locking at the bias point dependence of the random telegraph signal (RTS) it is possible to estimate the trap location along the channel. It is shown that the behavior of the RTS does depend upon the properties of the trap and channel electrons, making RTS analysis a valuable tool to study effects as coulomb scattering, electron gas heating and the mechanisms that influence electrical channel formation in very small area devices