Time-dependent gate oxide breakdown
Time-dependent gate oxide breakdown (or time-dependent dielectric breakdown, TDDB) is a failure mechanism in MOSFETs, when the gate oxide breaks down as a result of long-time application of relatively low electric field (as opposite to immediate breakdown, which is caused by strong electric field). The breakdown is caused by formation of a conducting path through the gate oxide to substrate due to electron tunneling current, when MOSFETs are operated close to or beyond their specified operating voltages.
The most commonly used test for the investigation of TDDB behavior is "constant stress". Constant stress tests can be applied in form of constant voltage stress (CVS) or constant current stress. In the former, a voltage (that is often lower than the breakdown voltage of the oxide) is applied to the gate, while its leakage current is being monitored. The time it will take for the oxide to break under this constant applied voltage is called the time-to-failure. The test is then repeated several times to obtain a distribution of time-to-failure. These distributions are used to create reliability plots and to predict the TDDB behavior of oxide at other voltages.
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