In a semiconductor,the ratio of the number of electrons to the number of holes is $7/5$ and the ratio of their currents is $7/4$. What is the ratio of their drift velocities?

The Bohr radius of the fifth electron of phosphorus (atomic number $Z = 15$) acting as a dopant in silicon (relative dielectric constant $\varepsilon_r = 12$) is ....... $\mathring{A}$.

In an $n$-type semiconductor:

Which one of the following statements is false?

In a $P$-type semiconductor, the acceptor level is $57 \, meV$ above the valence band. What is the wavelength of light in $\mathring{A}$ required to create a hole? (Given: $h = 6.6 \times 10^{-34} \, J \cdot s$, $c = 3 \times 10^8 \, m/s$, $1 \, eV = 1.6 \times 10^{-19} \, J$)

$A$ $Ge$ specimen is doped with $Al$. The concentration of acceptor atoms is $10^{21} \text{ atoms}/m^3$. Given that the intrinsic concentration of electron-hole pairs is $10^{19} /m^3$, the concentration of electrons in the specimen is: