Before revision: Generally, the material is sufficiently doped so that there is many orders-of-magnitude difference between the two concentrations, and so this equation can be simplified to where nm and μm are the doping level and mobility of the majority carrier respectively.
After revision: Generally, the material is sufficiently doped so that there is many orders-of-magnitude difference between the two concentrations, allowing this equation toa difference of be simplified to where nm and μm are the doping level and mobility of the majority carrier respectively.