When an impurity is doped into an intrinsic semiconductor,the conductivity of the semiconductor

The acceptor level of a $p$-type semiconductor is $6 \ eV$ above the valence band. The maximum wavelength of light which can create a hole would be: (Given $hc = 1242 \ eV \ nm$) (in $nm$)

Explain the recombination coefficient of an intrinsic semiconductor and derive the relation $n_i^2 = n_e n_h$.

The charge carriers in a $p$-type semiconductor are

The length of a germanium rod is $0.928 \ cm$ and its area of cross-section is $1 \ mm^2$. If for germanium $n_i = 2.5 \times 10^{19} \ m^{-3}$,$\mu_h = 0.15 \ m^2 V^{-1} s^{-1}$,and $\mu_e = 0.35 \ m^2 V^{-1} s^{-1}$,then the resistivity is:

In $n$-type semiconductor,electrons are majority charge carriers,but it does not show any negative charge. The reason is: