Doping of a semiconductor (with small impurity atoms) generally changes the resistivity as follows.

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

If $N_h$ and $N_e$ are the numbers of holes and conduction electrons in an extrinsic semiconductor,respectively,then:

Suppose a pure $Si$ crystal has $5 \times 10^{28}$ atoms $m^{-3}$. It is doped by $1$ ppm concentration of pentavalent $As$. Calculate the number of holes. Given that $n_i = 1.5 \times 10^{16} m^{-3}$.

What is the relation between the number density of holes $(n_h)$ and free electrons $(n_e)$ in an intrinsic semiconductor at room temperature?

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