7.4
semiconductor.
In a similar manner, we can derive for a p-type semiconductor at a low injection rate
with the electron capture coefficient c n and the electron lifetime τ n,SRH .
We see that the lifetime is related to the capture coefficients via
The lifetime of the minority carriers due to Shockley–Read–Hall recombination therefore
is indirectly proportional to the trap density N T . Hence, for a good semiconductor device it
is crucial to keep N T low.
The values of the minority-carrier lifetimes can vary a lot. When the trap
concentration in c-Si is very low, τ n (τ p ) can achieve values around 1 ms. On the other
hand, the intentional introduction of gold atoms into Si, which introduce efficient traps
into Si, can decrease τ n (τ p ) to values around 1 ns. Typical minority-carrier lifetimes in
most c-Si devices are usually around 1 µs. For an efficient collection of photo–generated
carriers in c-Si solar cells the minority–carrier lifetimes should be in the range of tens of
milliseconds.
Auger recombination
We already mentioned that direct recombination is not possible or at least very limited for
indirect semiconductors, because both transfer in energy and momentum must occur for
an electron in the conduction band to recombine with a hole in the valence band. In
indirect semiconductors, Auger recombination becomes important. In comparison to direct
and SRH recombination, which involve two particles, i.e. an electron and a hole, Auger
recombination is a three particle process, as illustrated in Figure 7.5.
semiconductor.
In a similar manner, we can derive for a p-type semiconductor at a low injection rate
with the electron capture coefficient c n and the electron lifetime τ n,SRH .
We see that the lifetime is related to the capture coefficients via
The lifetime of the minority carriers due to Shockley–Read–Hall recombination therefore
is indirectly proportional to the trap density N T . Hence, for a good semiconductor device it
is crucial to keep N T low.
The values of the minority-carrier lifetimes can vary a lot. When the trap
concentration in c-Si is very low, τ n (τ p ) can achieve values around 1 ms. On the other
hand, the intentional introduction of gold atoms into Si, which introduce efficient traps
into Si, can decrease τ n (τ p ) to values around 1 ns. Typical minority-carrier lifetimes in
most c-Si devices are usually around 1 µs. For an efficient collection of photo–generated
carriers in c-Si solar cells the minority–carrier lifetimes should be in the range of tens of
milliseconds.
Auger recombination
We already mentioned that direct recombination is not possible or at least very limited for
indirect semiconductors, because both transfer in energy and momentum must occur for
an electron in the conduction band to recombine with a hole in the valence band. In
indirect semiconductors, Auger recombination becomes important. In comparison to direct
and SRH recombination, which involve two particles, i.e. an electron and a hole, Auger
recombination is a three particle process, as illustrated in Figure 7.5.