Time-dependent gate oxide breakdown (or time-dependent dielectric breakdown, TDDB) is a failure mechanism in MOSFETs, when the gate oxide

Gate oxide

The gate oxide is the dielectric layer that separates the gate terminal of a MOSFET from the underlying source and drain terminals as well as the conductive channel that connects source and drain when the transistor is turned on. Gate oxide is formed by oxidizing the silicon of the channel to form...

 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

Wafer (electronics)

A wafer is a thin slice of semiconductor material, such as a silicon crystal, used in the fabrication of integrated circuits and other microdevices...

 due to electron tunneling

Quantum tunnelling

Quantum tunnelling refers to the quantum mechanical phenomenon where a particle tunnels through a barrier that it classically could not surmount. This plays an essential role in several physical phenomena, such as the nuclear fusion that occurs in main sequence stars like the sun, and has important...

 current, when MOSFETs are operated close to or beyond their specified operating voltages.

Test method

A general test method used for measuring this failure mechanism is called "charge to breakdown" (QBD

QBD (electronics)

QBD is the term applied to the charge-to-breakdown measurement of a semiconductor device. It is a standard destructive test method used to determine the quality of gate oxides in MOS devices. It is equal to the total accumulated charge passing through the dielectric layer just before failure. Thus...

).