Semiconductor glossary · Test and reliability

What Is an Acceleration Factor?

An acceleration factor converts time in an accelerated test into the equivalent time under use conditions.

An acceleration factor is the ratio of lifetime under use conditions to lifetime under accelerated test conditions. It tells you how many hours of use one test hour represents.

Waiting 10 years to verify a 10-year lifetime is impractical. A shorter test at a higher temperature can be used because the logarithm of lifetime forms a straight line against reciprocal absolute temperature. The line can then be extended from the test condition toward the use condition.

The slope is Ea/k; the ratio is the acceleration factor

Arrhenius straight line

Lifetime L on a logarithmic scale is linear with 1/T in K⁻¹. The slope of ln L is Ea/k; the ratio of lifetime at the use and test conditions is the acceleration factor.

The Renesas public handbook gives the Arrhenius equation for reaction rate K as K＝Λexp(−Ea/kT), and writes lifetime L as L＝A exp(Ea/kT).

Taking logarithms gives ln L＝ln A＋Ea/kT. The handbook explains that this makes the logarithm of lifetime linear with reciprocal temperature. When the horizontal axis is 1/T in K⁻¹, the slope is Ea/k; multiplying the slope by the Boltzmann constant k gives the activation energy Ea.

On this line, the ratio of lifetimes at two temperatures is the acceleration factor. Raising the test temperature increases the ratio.

Calculate the ratio from two temperatures

Calculation

Calculate the ratio from two temperatures

ln(L1 / L2) ＝ 11606 × (1/T1 − 1/T2) × Ea

T1: absolute use temperature
T2: absolute test temperature
Ea: activation energy (eV)
L1 / L2: acceleration factor

Boltzmann constant: 8.6157×10⁻⁵ eV/K

Multiplying the difference between the reciprocal temperatures by the activation energy gives the logarithm of the lifetime ratio. That ratio is the acceleration factor used to convert test time to use time.

ln(L1/L2)＝11606(1/T1 − 1/T2)Ea

The handbook defines L1 and L2 as the lifetimes at temperatures T1 and T2 and states the equation above. The three required inputs are the two absolute temperatures and the activation energy. It gives the Boltzmann constant as 8.6157×10⁻⁵ eV/K.

Activation energy differs by failure mechanism. The handbook says that a representative value must be determined for each mechanism.

Use it directly to convert failure rate

The same handbook gives an example of estimating failure rate from test results under JIS-C5003. The calculation uses the 0.92 factor for a 60% confidence level and divides it by total test time.

That result is the failure rate under accelerated test conditions. The handbook says that the failure rate under use conditions is obtained by dividing it by the acceleration factor between the test and use conditions.

Because the use-condition failure rate is the accelerated-test failure rate divided by the acceleration factor, estimating the factor at c times its true value makes the use-condition failure-rate estimate 1/c times its true value. The choice of activation energy directly affects the conclusion.

For a new device, derive it from a purpose-built test

The ROHM public white paper describes two tests for GaN HEMTs, DHTOL and SALT, and shows how an acceleration factor is derived from SALT.

It also says that JEDEC JEP182 provides guidance for dynamic on-resistance test methods for GaN HEMTs. A dedicated test fills the gap outside standard testing.

When the material or structure changes, the dominant failure mechanism can change too. The acceleration factor equation may stay the same, but the activation energy used in it must be measured again.

Trade-off

Applying stronger acceleration makes failures observable sooner, reducing schedule and cost. It also raises the risk that a mechanism unlike the one under use conditions becomes dominant, making the resulting acceleration factor invalid. Assuming a larger activation energy produces a larger acceleration factor and a more optimistic lifetime estimate. Speeding up the test reduces confidence in the estimate unless the mechanism remains valid.

Frequently asked questions

What is an acceleration factor?

It is the ratio of lifetime under use conditions to lifetime under accelerated test conditions. It tells you how many hours of use one test hour represents.

How is it calculated?

The Renesas handbook defines lifetimes L1 and L2 at temperatures T1 and T2 and gives ln(L1/L2)＝11606(1/T1 − 1/T2)Ea.

What is Ea?

Ea is the activation energy. Its value depends on the failure mechanism and determines how strongly temperature accelerates that mechanism.

What value is used for the Boltzmann constant?

The same handbook gives 8.6157×10⁻⁵ eV/K.

Can stresses other than temperature produce acceleration?

Yes. The handbook describes the Eyring model as a generalization that includes non-temperature stresses such as humidity and voltage.

How is the acceleration factor used in a failure-rate calculation?

The handbook says to divide the failure rate measured under accelerated test conditions by the acceleration factor between the test and use conditions.

Is it used for newer devices such as GaN?

Yes. The ROHM white paper derives an acceleration factor from a SALT test and uses it to predict lifetime.

Compare related concepts

Activation energy (English) — the parameter that controls the acceleration factor
Lifetime extrapolation (English) — extending the fitted line toward use conditions
Voltage acceleration (Japanese) — acceleration by stress other than temperature
Weibull distribution (Japanese) — describing variation in test data
Accelerated testing (Japanese) — designing a test to reproduce failures in less time
Failures in time, FIT (Japanese) — expressing failure rate on a time basis

Sources

Renesas Electronics, Semiconductor Reliability Handbook, Rev.2.50 (January 30, 2017) (accessed September 21, 2026)
ROHM, Approach to the Lifetime of EcoGaN in Soft-Switching Operation, Rev.001 (June 2024) (accessed September 21, 2026)