Analysis of new hot carrier degradation phenomena: 'W' or 'S' shape evolution of LDD NMOSFET

A new phenomenon has been observed for transistors with super steep channel retrograde well (SSRW) channel and lightly doped drain (LDD) structure. This is the 'W' or "S" shape evolution of drain current and transconductance (G/sub m/) during hot carrier stress. This "S" shape evolution does not follow the conventional power law degradation model A/spl middot/t/sup n/. Instead, it can be found that the degradation rate increases very quickly after stress for a given time, and the lifetime is much shorter than the prediction from the power law. A three-stage degradation model is proposed. First, series resistance increases in the LDD region and mobility degradation causes the current degradation. Secondly, a large threshold voltage (V/sub t/) shift results in the electric-field enhancement, which leads to hole trapping under the spacer. As a result, current and G/sub m/ re-increase. Finally, these trapped holes increase the electron-trapping rate and result in decrease of drain current and G/sub m/.