“…In the expression of the current density J n the quantities D n , µ n represent the mobility and the diffusivity of the electrons. This problem has been studied by several authors [20,21,31,32], especially in a fairly general framework, [13,15,16,30] and for more realistic boundary conditions by the author [13,16]. Uniqueness is not possible in general, it is obtained under restrictive hypotheses [14,17,25].…”