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A semiconductor is irradiated with light such that carriers are uniformly generated throughout its volume. The semiconductor is $n$-type with $N_{D}=10^{19}$ per $\mathrm{cm}^{3}$. If the excess electron concentration in the steady state is $\Delta n=10^{15}$ per $\mathrm{cm}^{3}$ and if $\tau_{p}=10 \mu \sec$ [minority carrier life time] the generation rate due to irradiation

1. is $10^{20} e-h$ pairs $/ \mathrm{cm}^{3} / \mathrm{s}$
2. is $10^{24} e \cdot h$ pairs $/ \mathrm{cm}^{3} / \mathrm{s}$
3. is $10^{10} \mathrm{c} \cdot h$ pairs $/ \mathrm{cm}^{-} / \mathrm{s}$
4. cannot be deterained as the given data is insufficient.