A small concentration of minority carries is injected into a homogeneous semiconductor crystal at one point. An electric field of $10 \mathrm{~V} . \mathrm{cm}$ is applied across the crystal and this moves the minority carriers a distance of $1 \mathrm{~cm}$ is $20 \; \mu \; \mathrm{sec}$. The mobility (in $\mathrm{cm}^{2} /$ volt. sec) will be
- $1,000$
- $2,000$
- $5,000$
- $500,000$