A definition in semiconductor physics, carrier lifetime is defined as the average time it takes for a minority carrier to recombine. The process through which this is done is typically known as minority carrier recombination.

The energy released due to recombination can be either thermal, thereby heating up the semiconductor (thermal recombination or non-radiative recombination, one of the sources of waste heat in semiconductors), or released as photons (optical recombination, used in LEDs and semiconductor lasers).

Carrier lifetime plays an important role in bipolar transistors and solar cells.

In indirect band gap semiconductors, the carrier lifetime strongly depends on the concentration of recombination centers. Gold atoms act as highly efficient recombination centers, silicon for some high switching speed diodes and transistors is therefore alloyed with a small amount of gold. Many other atoms, e.g. iron or nickel, have similar effect.[1]

## Carrier lifetime in semiconductor lasers

In semiconductor lasers, the carrier lifetime is the time it takes an electron before recombining via non-radiative processes in the laser cavity. In the frame of rate equations model, carrier lifetime is used in the charge conservation equation as the time constant of the exponential decay of carriers.

The dependence of carrier lifetime on the carrier density is expressed as:[2]

$\frac{1}{\tau_n(N)}= A + BN + CN^2$

where A, B and C are the non-radiative, radiative and Auger recombination coefficients and $\tau_n(N)$ is the carrier lifetime.

## References

1. Alan Hastings - The Art of Analog Layout, 2nd ed (2005, ISBN:0131464108)
2. L.A. Coldren and S.W. Corzine, "Diode Lasers and Photonic Integrated Circuits", Wiley Interscience, 1995