Upon some calculation I arrive to the expression: d2DPn(x)/dx2 = DPn(x)/Lh2 Where: DPn(x) = Pn(x) + Pno - excess minority carriers (holes) concentration in the n-type part of the pn junction. Now the roots to the characteristic equation are +/- 1/Lh where Lh is the length of the diffusion. Therefore the solution looks like: DPn(x) = Ae-1/Lh + Be1/Lh I know for a fact the solution is DPn(x) = DPn(0)e-x/Lh The initial conditions would be that: @ x = 0 we have hole concentration DPn(0) @ x = Lh we have DPn(Lh) = 0 But I have no idea how to arrive at the solution in bold. I'm missing something and I was thinking you could help. Thank you.