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Capo4love
Chemical analysis of a germanium crystal reveals indium at a level of 0.0003091 atomic percent. Assuming that the concentration of thermally excited charge carriers from the Ge matrix is negligible, what is the density of free charge carriers (free carriers/cm3) in this Ge crystal?
Use density of Ge. Convert it into atoms of Ge/cm^3. Then you can use the atomic percent (fraction) to find how many charge carriers are in the crystal (with volume 1 cm^3).