A silicon sample is $2\mathrm{.}5$ cm long and has a cross$-$sectional area of $0\mathrm{.}1$ cm$2.$ The silicon is n

type with a donor impurity concentration of Nd $=2$ x $1015$ cm$3.$ The resistance of the sample is

measured and found to be $70.$ What is the electron mobility?

$($b$)$ Three volts is applied across a $1-$cm$-$long semiconductor bar. The average electron drift

velocity is $104$ cm$/$s$.$ Find the electron mobility.