Direct tunneling and gate current fluctuations

A comprehensive study of correlated gate leakage and drain current fluctuations in nMOS devices using non-equilibrium Green´s function calculations has been carried out. A simulation model combining 3D self-consistent electrostatic potentials accounting for random discrete dopants and charged oxide traps with a 1D and 2D transport description of direct-tunneling gate leakage has been developed. The influence of the charge state of the trap on the direct-tunneling current has been investigated. A considerable local change in current density around the trap has been observed. By varying the position of the trap it has been found that oxide defects close to the drain and source regions have a higher impact on the gate leakage. A statistical analysis of nMOSFETs by varying the configuration of the random discrete dopants has been performed. The trap has been positioned close to the drain to achieve a worst-case scenario. The reduction in direct-tunneling current due to charging of a single trap has been calculated for each device. Gate current reductions below one percent have been found. The experimentally measured large gate leakage fluctuations can thus not be accounted for with direct tunneling.