Hot-carrier degradation for 90 nm gate length LDD-NMOSFET with ultra-thin gate oxide under low gate voltage stress

Abstract

The hot-carrier degradation for 90 nm gate length lightly-doped drain (LDD) NMOSFET with ultra-thin (1.4 nm) gate oxide under the low gate voltage (LGV) (at Vg = Vth, where Vth is the threshold voltage) stress has been investigated. It is found that the drain current decreases and the threshold voltage increases after the LGV (Vg = Vth) stress. The results are opposite to the degradation phenomena of conventional NMOSFET for the case of this stress. By analysing the gate-induced drain leakage (GIDL) current before and after stresses, it is confirmed that under the LGV stress in ultra-short gate LDD-NMOSFET with ultra-thin gate oxide, the hot holes are trapped at interface in the LDD region and cannot shorten the channel to mask the influence of interface states as those in conventional NMOSFET do, which leads to the different degradation phenomena from those of the conventional NMOS devices. This paper also discusses the degradation in the 90 nm gate length LDD-NMOSFET with 1.4 nm gate oxide under the LGV stress at Vg = Vth with various drain biases. Experimental results show that the degradation slopes (n) range from 0.21 to 0.41. The value of n is less than that of conventional MOSFET (0.5-0.6) and also that of the long gate length LDD MOSFET (~0.8).