Reverse biasing
Usually, the reverse resistance of a diode is some megohms under the room temperature and decreases by tens times as the temperature rises. The reverse current is a leakage current at the source rated voltage. Typically, silicon diodes have 1 to 10 uA and germanium 200 to 700 uA of leakage current. This value includes thermally produced current and surface-leakage current. When a diode is reverse biased, only these currents take place. The diode current is very small for all reverse voltages lower than the breakdown voltage. Nevertheless, it is much more dependent on temperature.
At breakdown, the diode goes into avalanche where many carriers appear suddenly in the depletion layer. With a rectifier diode, breakdown is usually destructive. To avoid the destructive level under all operating conditions, a designer includes a derating (safety factor), usually of two.
