Theory of slow traps and random telegraph signals in ultra-small planar MOSFETs

Abstract

It is shown that ultra-small MOSFETs with heavily doped substrates contain a significant concentration of slow traps in their space-charge regions. Such a trap arises due to random doping fluctuations and is created if a few shallow impurities form a small-scale cluster, resulting in a high binding energy of the ground state of the particle on this multiply charged “nucleus.” Inside the space-charge region, this “nucleus” is surrounded by a high potential barrier, which turns it into a slow trap with capture and emission times not too different from each other. Since a multiply charged trap scatters free carriers much more than a singly charged trap, it generates an easily observable random telegraph signal (RTS) when it captures or emits a free carrier. Such traps, located near the silicon–dielectric interface, generate high-amplitude RTSs that interfere with the normal operation of ultra-small MOSFETs. The probability of their formation depends very sharply on the doping profile near the interface. Therefore, if it can be proven that the slow traps generating the observed RTSs are in silicon, this will provide a clue to reducing this probability. This paper develops a theory describing why a multiply charged silicon slow trap can generate a high-amplitude random telegraph signal, how to reliably distinguish whether the observed RTS is generated by a silicon or oxide trap, and what experimental data are required for this. An analysis of the relevant published data for the RTSs observed in such a MOSFET shows that they are generated by slow traps located in silicon rather than in oxide. The developed theory is in qualitative and reasonable quantitative agreement with the analyzed data.