(a) Briefly describe the processes of charge carrier drift in semiconductors and the situation in which it occurs, and show how the drift of electrons and holes contributes to the current density. Explain what determines the electrical conductivity of a semiconductor. Give an explanation of the temperature dependence of conductivity in a doped semiconductor, including the relevant power-laws. Sketch a schematic plot of this dependence. (b) A voltage of 0.1 mV is applied between the two ends of a cylindrical wire of germanium (Ge), with a length of 10 cm and a radius of 0.2 mm. The rod is doped with 10^23 cm^-3 phosphorus atoms and kept at 300 K. The current along the rod is measured at 10 mA. Making a reasonable assumption about the extent of dopant ionisation, calculate n and p, where each symbol has its usual meaning. See below for some material-specific constants. (c) Continuing from the previous part, (b), calculate the Fermi level, Ef. Comment on its position relative to the donor dopant level Ed, making reference to the validity of the approximation to the Fermi-Dirac distribution that underpins the use of effective densities of states. (d) Continuing from part, (b), calculate the electron mobility Ī¼n. (e) Continuing from the previous part, (c), determine the fraction of phosphorus ions that are ionised and, thus, verify that the assumption that you made in part (b) is valid. The band-gap Eg of Ge at 300 K is 0.66 eV. The phosphorous ionisation energy in Ge is Ec - Ed = 0.012 eV. Effective densities of states for Ge at 300 K are Nc = 10^19 cm^-3 and Nv = 5.6 x 10^18 cm^-3.