Snapback Induced Gate Dielectric Breakdown in Graded Junction MOS Structures

A new type of failure mode of the gate dielectric breakdown in an MOS transistor induced by the snapback phenomena is reported. Unlike a typical gate oxide breakdown which is caused by a voltage stress on the gate, this failure mode is caused by a source-drain bipolar current resulting from the snapback action. This failure mode results in a gate-to-drain short, and was found to require a minimum critical current, Idcrit, after the transistor goes in the snapback. The failure node is exhibited in the input protection structures used in an MOS circuit. The incidence of the failure mode increased with increasing grading of the source-drain junction. (i.e., the Idcrit decreased as the junction grading increased). The ESD breakdown of the inputs are also shown to be a direct result of this failure mode