Variational formulation of Poisson's equation with Boltzmann carrier statistics at quasi-equilibrium is used to find analytical solutions of the potential distribution in multidimensional semiconductor devices with resonably good accuracy. To demonstrate this approach, short-channel effects in the threshold voltage of fully-depleted SOI MOSFET's are analyzed, The usefulness of our idea is extended further to determine the quasi-Fermi potential of the floating gate of an EEPROM device, which has been determined to be very difficult to find by conventional methods.