Leakage current:  In an ideal forward biased diode the current will not flow in the reverse direction. However in a real PN junction diode, a very small amount of current flow in the reverse direction as a result of the minority carriers in the semiconductor. The level of leakage current is dependent upon three main factors.

• Voltage: As voltage increases beyond potential barrier value current flows in reverse direction
• Temperature: Leakage current increases with temperature rise.
• Material: Depended on semiconductor material and the amount of doping.

Diode Characteristics

Characteristics of diodes in response to applied voltage are useful for various applications in electronics. Most commonly it is used for making rectifiers where diodes are used in a circuit to convert Alternating current (AC) to Direct current (DC). The characteristics of a diode can be seen in the graph, in which various values of voltage are applied across a diode (from positive values to negative values) and the current output measured and plotted as shown in figure 14. 

Image source: technologyuk.net

The curves (graphs) represent the current output in relation to voltage applied in Silicon and Germanium diodes at 25⁰C. As per the graph the current do not increase significantly until the voltage exceed the potential barrier of 0.2V for Germanium and 0.6 for silicon in forward biased. Once the applied voltage exceeds the potential barrier value, the current increases tremendously.

In the reverse biased ideally the depletion layer will not allow any current to flow backward and act as an insulator. However, in practical, there will be a leakage current flowing in the reverse direction due to the movement of minority carriers. If the voltage across the diode is increased to ‘reverse biased breakdown voltage’, current flow in the reverse direction increases rapidly to a high value, causes the diode to overheat and fail. The phenomenon is known as AVALANCHE BREAKDOWN and the diode will be permanently damaged. The breakdown voltage for Silicon can be any value from a few volts up to 1000V and for Gemanium100V depending on the construction of the diode and the level of doping.