TRANSISTORS AGE

Four ways time wins

BTI Bias-temperature instability: charge trapped near the gate dielectric shifts the threshold

HCI Hot-carrier injection: energetic carriers near the drain damage the interface

TDDB Time-dependent dielectric breakdown: a conducting path forms through the gate oxide

Electromigration Current pushes metal atoms along a wire until it opens: a wiring effect

A chip is sold with a promise of about ten years at high temperature.

BIAS-TEMPERATURE INSTABILITY

The threshold creeps

ΔV_th = A(V, T) · tⁿ

Under bias, charges tunnel into traps in and near the gate dielectric and slowly shift the threshold. The shift grows as a power law of time with a small exponent, and both higher voltage and higher temperature speed it up. A larger V_th weakens the drive current, so the circuit slows as it ages. Part of the shift recovers when the stress is removed.

Aging eats the timing margin: the chip has to be designed for the end of its life.

OXIDE BREAKDOWN

The weakest link

Defects pile up in the gate dielectric until a conducting path forms. Each transistor has a random lifetime, spread widely (a Weibull distribution), so the failure of the first one out of billions comes very much earlier than the average. That is why gate oxides are run at modest fields, and why a bigger chip has a shorter life at the same voltage.

A chip is a chain, and it breaks at its weakest oxide.

HOW CHIPS COPE

Margins, burn-in and monitors

Guard bands Design for the aged state: leave timing margin for the threshold shift a chip will have after ten years

Burn-in Run new chips hot and at high voltage so the weak ones fail in the factory, not in your hands

Aging monitors On-chip sensors track the shift and adapt the voltage or the clock

The sensor link The same slow charge trapping near an insulator is why ISFETs drift (Unit 13): aging and sensor drift are one physics

Understanding the trap is useful for both a processor and a biosensor.

CHECK THE MECHANISMS

Which aging mechanism?

Trapping, hot carriers, breakdown and migration: four ways time wins.

UNIT 26 STUDY COMPLETE

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Ready for the Fab Challenge?

You've seen how time changes a transistor, and that the slow charge trapping which ages chips is the same thing that makes sensors drift.