Hot carrier induced degradation mode depending on the LDD structure in NMOSFET´s

Hot carrier induced degradation was studied for LDD NMOSFETs with various gate-drain(LDD n^-) overlap and sidewall spacer lengths. The dependence of impact ionization efficiency on the LDI structure was clarified by experimental and simulated results. Four LDD degradation modes were found through the stress tests. The authors´ model and simulated results explain these four modes, caused by the electron trap location in the gate oxide. In order to achieve high reliability and high drivability, the sidewall spacer should be short enough so that the n⁺region can reach under the gate edge, on condition that the gate-drain overlap is long enough to maintain a constant lateral electric field profile.