A measurement method for the increase of digital switching time due to hot-electron stress

Channel hot-electron (CHE) injection poses a reliability problem for n-channel field-effect transistors with small design rules. One often assesses the reliability of a particular fabrication process and design by subjecting individual transistors, instead of an entire circuit, to continuous or pulsed voltage stress. Simple inspection of the linear region transconductance degradation and threshold voltage shift of the individual devices, however, does not yield direct information on the circuit performance impact of channel hot-electron stress. In this letter we describe a simple measurement method for the extraction of the increase of digital switching time due to channel hot-electron stress. One performs this measurement on discrete transistors so that reliability tests still employ these individual devices. We obtain good agreement between this method and a direct measurement of the increased switching time of a CMOS inverter. We also find that the switching time changes negligibly even when the linear transistor characteristics are severely degraded if this swiching delay is measured with the same source and drain terminals as were employed for the hot-electron stress.