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The electron and hole concentrations in an intrinsic semiconductor are $n_i$ per $\mathrm{cm}^3$ at $300 \mathrm{~K}$. Now, if acceptor impurities are introduced with a concentration of $N_A$ per $\mathrm{cm}^3$ (where $N_A \gg n_i$ ), the electron concentration per $\mathrm{cm}^3$ at $300 \mathrm{~K}$ will be

1. $n_{i}$
2. $n_i+N_A$
3. $N_A-n_{i}$
4. $\frac{n_i^2}{N_A}$