3-7. Leakage current

When forward bias is applied to the barrier, the diffusion potential drops. As a result, electrons in the semiconductor’s conduction band diffuse across the depletion region to the metal, resulting in a forward current from the metal to the semiconductor. Carrier diffusion occurs where the density of majority carriers in the semiconductor becomes higher than that of majority carriers in the metal. Therefore, electrons diffuse from the semiconductor to the metal, causing current to flow.
When reverse bias is in the range beyond the diffusion potential, the carrier density of the metal becomes higher than that of the semiconductor, causing current to flow in the reverse direction. This current is called leakage current. Since an SBD has a low carrier density at an energy level higher than the diffusion potential, the magnitude of leakage current is much lower than that of the forward current.
Since the diffusion potential of the SBD is lower than that of the pn junction diode, current begins flowing across the SBD at a lower voltage than in the case of the pn junction diode. SBDs tend to exhibit higher leakage current since their crystal structure tends to be disturbed at the heterogeneous junction of a metal and a semiconductor.