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

Mobilities of electrons and holes in a sample of intrinsic $Ge$ at room temperature are $0.35 \, m^2/V-s$ and $0.18 \, m^2/V-s$ respectively. If the electron and hole densities are each equal to $2.5 \times 10^{19} \, /m^3$,the $Ge$ conductivity will be.....$S/m$ (in $.12$)

In a sample of pure silicon,$10^{13} \text{ atoms/cm}^3$ of phosphorus is added. If all donor atoms are active,what will be the resistivity at $20 ^oC$ if the mobility of electrons is $1200 \text{ cm}^2/\text{V} \cdot \text{s}$? (in $\Omega \cdot \text{cm}$)

In a $p$-type semiconductor,the acceptor level is at $50 \text{ meV}$ above the valence band. To produce one hole,the maximum wavelength of the light photon required is (Planck's constant,$h = 6.6 \times 10^{-34} \text{ Js}$ and speed of light in vacuum,$c = 3 \times 10^8 \text{ m/s}$) (in $\mu \text{m}$)

Explain the concept of the hole in the semiconductor.

$A$ donor atom in a semiconductor has a loosely bound electron. The orbit of this electron is considerably affected by the semiconductor material but behaves in many ways like an electron orbiting a hydrogen nucleus. Given that the electron has an effective mass of $0.07 \, m_e$,where $m_e$ is the mass of the free electron,and the space in which it moves has a permittivity of $13 \, \varepsilon_0$,then the radius of the electron's lowermost energy orbit will be close to ................. $\mathring{A}$ (take the Bohr radius of the hydrogen atom as $0.53 \mathring{A}$).