A Si sample is doped with arsenic impurities at a concentration of 10^17 cm^-3. Calculate the intrinsic electron concentration and compare it to the extrinsic electron cocncentration, assuming all the arsenic impurities are ionized. Use the effective mass of the electron and hole as 20% of the free mass of the electron.
We were given the answer as 6.3 x 10^8 cm^-3, but I have no idea where to begin on this as we weren't given a temperature so I can't calculate the intrinsic concentration.