Distribution of Step Heights of Electron and Hole Traps in SiON nMOS Transistors

The reliable operation of transistors is affected by electron and hole traps located inside the oxide and at the oxide/semiconductor interface. Each of the single defects can capture and emit a charge, alter the device electrostatics and thus affect the device behavior. As a consequence, a drift of the threshold voltage of the transistor can be measured. In nanoscale devices, the charge transition events of the defects can be observed as discrete steps in the device current. A significant benefit of investigating such scaled devices is, that we can study the impact of both electron and hole traps separately, which is not possible with their large-area counterparts. By probing scaled SiOX devices, the distribution of step heights caused by the single defects has been reported to follow a uni-modal exponential distribution. However, our results clearly reveal that the step heights of electron traps are bi-modal exponential distributed for positive BTI (PBTI) and negative BTI (NBTI) and for the hole trap/NBTI case, whereas a uni-modal distribution is observed for the hole trap/PBTI case. Of particular importance is the fact that if uni-modal distributions are considered for verification of the designs in circuit and device simulators the tail of the distributions, i.e. the large step heights, are fairly underestimated. This might give rise for an unexpected failure of the respective components. Furthermore, we demonstrate that for the underlying technology the charge sheet approximation (CSA) significantly underestimates the real impact of the defects too, which leads to pessimistic estimates for defect densities from experimental data.