When a $p-n$ junction diode is forward biased

Assertion (A): We cannot form a $p-n$ junction diode by taking a slab of a $p$-type semiconductor and physically joining it to another slab of a $n$-type semiconductor.

Reason (R): In a $p$-type semiconductor $\eta_e \gg \eta_h$ while in a $n$-type semiconductor $\eta_h \gg \eta_{e^*}$

A pure Si crystal having $5 \times 10^{28}$ atoms $\mathrm{m}^{-3}$ is dopped with 1 ppm concentration of antimony. If the concentration of holes in the doped crystal is found to be $4.5 \times 10^9 \mathrm{~m}^{-3}$, the concentration (in $\mathrm{m}^{-3}$ ) of intrinsic charge carriers in Si crystal is about

In an extrinsic semiconductor, the number density of holes is $$4 \times 10^{20} \mathrm{~m}^{-3}$$. If the number density of intrinsic carriers is $$1.2 \times 10^{15} \mathbf{m}^3$$, the number density of electrons in it is