Mobility of electrons in a semiconductor is defined as the ratio of their drift velocity to the applied electric field. If, for an n -type semiconductor electrons is $10^{19} \mathrm{~m}^{-3}$ and their mobility is $1.6 \mathrm{~m}^2 /(\mathrm{V} . \mathrm{s})$ then the resistivity of the semiconductor (since it is an $n$-type semiconductor contribution of holes is ignored) is close