When a diode is first manufactured, the electrons in the n-doped region tend to flow to the p-doped region due to diffusion effect. In the process, a depletion region is formed until the electric field in the depletion region cancel out the diffusion effect. Hence an equilibrium is reached. (or so I thought) However, since this two effect cancel out each other exactly, why would we need a minimum voltage of about 0.3-0.5 V to forward bias it? Wouldn't any voltage cause current to flow, since the diffusion+external voltage source is now greater than the electric field in the depletion zone alone?