A diode is an electronic device that controls the direction of current flow in a circuit. The standard diode allows electric current to flow in a forward direction, but not in the reverse direction. One type of diode can conduct current in the reverse direction under certain conditions, however. This special type of diode is the reverse diode.
Diode construction involves two segments of a semiconductor material, such as silicon. One segment has a positive charge, called the anode. The other segment has a negative charge, called the cathode. In manufacture, these two segments are fused together, forming a PN junction, which identifies one portion as positive and the other as negative. Metal leads are then usually attached to the ends, opposite the junction, to form a diode.
The PN junction is the focal point of a diode’s operation. When the two segments of material fuse together, they cancel out each other’s electrical charge in a narrow band across the PN junction called the depletion region. This area of the diode favors neither a positive nor a negative electrical charge, and acts as an insulator between the two segments of the diode.
Under normal operation, a diode functions much like an electronic check valve. If a negative voltage is applied to the diode’s cathode, the charge combines with the diode’s internal electrical charge. When this happens, the insulation of the depletion region at the PN junction holds, preventing electrical current from passing through the diode. A diode operating in this state is in reverse diode operation, or reverse bias.
If, however, a negative voltage is applied to the diode’s anode, the voltage moves into the positively charged section of the diode. When it reaches the junction, the charge will have enough electrical energy to bridge the depletion region. At this point, the diode will conduct electrical current and allow it to continue to flow until the voltage is removed. Diodes operating in this state are in forward diode operation, or forward bias.
The insulation of the depletion region, however, can only withstand a certain level of voltage. Should the voltage become too high while the device is operating in a reverse diode state, the depletion region will fail and allow a surge of electrical current to pass. This phenomenon is called an avalanche and typically destroys a standard diode when it occurs.
While the avalanche phenomenon is something to be generally avoided, engineers found that blocking voltage until it reached a predetermined level, then allowing it to pass, could be a useful tool in the development of electronic technology. They then began to design diodes with very specific depletion regions that could withstand the horrendous effects of an avalanche. Since their inception, these types of diodes have found their way into virtually every area of electronics.
In operation, a reverse diode functions like a standard diode. A negative voltage is applied to its cathode, and the diode blocks it. Should that voltage, however, continue to increase to a predetermined level, called the breakdown voltage, the diode will undergo a controlled avalanche and begin to conduct electrical current in the reverse direction safely. These diodes go by many names, including avalanche diodes, breakdown diodes, or reverse diodes.