The diode forward voltage is the drop in electrical voltage that occurs when electrical current is conducted through a diode. Diodes are two-lead semiconductor devices that conduct an electrical signal in one direction but not the other. When a diode is conducting electricity, it is said to be forward biased and it consumes a small amount of the voltage passing through it in the process. The amount of voltage used by the diode itself when forward biased is called the diode forward voltage, diode voltage, or diode voltage drop.
Diodes are constructed of two pieces of the same type of material fused together with a lead attached to each end. One piece of the material, called the cathode, has an additive that makes it negatively charged. The other piece, called the anode, has an additive that makes it positively charged. When these two pieces are fused together, they exchange electrons at the point where they meet, which then becomes balanced, having neither a positive nor a negative charge. This area is called the depletion layer.
If a negative voltage is applied to the cathode of a standard silicon diode, the depletion layer widens, creating an electrical field that resists the voltage. A diode in this condition is said to be reverse biased. As a result, no electrical current can pass through the diode as the diode consumes all of the applied voltage. Hence, the voltage drop, or diode reverse voltage, is 100% of the voltage applied.
On the other hand, if a negative voltage is applied to the anode of a standard silicon diode, the negative voltage combines with the negative cathode joining forces. This force is strong enough to overcome the depletion layer and the positively charged anode of the diode. The diode then becomes forward biased and begins to conduct electrical current; however, the electrical force needed to overcome the depletion layer and travel across the positive anode of the diode requires the use of a small amount of the electrical voltage. This used voltage is the diode forward voltage and typically consumes about 0.7 volts in a standard silicon diode.
The diode forward voltage varies from one type of diode to another, depending on the base material used, the amount of charge added to the anode and cathode of the diode, and the diode’s intended application. In applications dealing with very low voltages, special diodes are used that have very thin depletion layers, weak anodes, and subsequently very small diode forward voltages. Likewise, there are special diodes that have diode reverse voltages that are less than 100%, allowing them to conduct electricity even when in a reverse biased condition.