The effect of high energy electron tail, and negative ions, on the sheath are studied. For high energy electron tail, we assume two temperature Maxwellian distribution, and solved the nonlinear Vlasov-Poisson equations with boundary conditions appropriate to an absorbing wall. Both of hot and cold ion cases are presented, and a numerical criterion, similar to Bohm sheath criterion, is obtained for the cold ion case.
The effect of negative ions is also studied, concluding that when the density of negative ion is large enough, the criterion is of the order of ion thermal speed, much less than Bohm speed.
