A diode forward voltage refers to the voltage drop that occurs when an electrical current passes through a diode in an electrical circuit. It is generally independent of the amount of current passing through the diode, and will be the same for a given family of diodes. For silicon diodes, which are generally the most common, the diode forward voltage drop is about 0.7 volts (V).
Diodes are devices that pass electric current in one direction only, and therefore may be thought of as a kind of one-way valve for electrons. When the electrons are moving in the correct direction to pass through the diode, the diode is said to be forward biased. Thus, the diode forward voltage drop is also known as the forward bias voltage drop.
Diodes work by having a small bit of doped semiconductor material sandwiched between their electrodes, which pass electrons in one direction but not the other. This material has two zones, the n zone, which is rich in negatively-charged electrons, and the p-zone, which is poor in electrons. The p zone may be thought of having positive holes that can accept the electrons from the n zone. When the two zones are brought together to form a p-n junction, electrons transfer from the n-zone to the p-zone until all the n-zone electrons available for use as current carriers are used up, and the p-n junction becomes an insulator.
If fresh electrons are injected into the depleted n-zone, while they are removed from the saturated p-zone, electric current will flow across the junction. This is what happens when the diode is forward biased. The electrons must be pushed by a voltage with enough force to cross the p-n junction, and this push is the source of the diode forward voltage, or forward bias voltage drop.
If the polarity of the circuit is reversed, and the attempt is made to inject electrons into the p-zone while they are removed from the n-zone, the diode is said to be reverse biased and no current will flow. If this reverse-bias voltage exceeds a certain value — the maximum reverse voltage — the diode may fail and pass large currents, in a process known as reverse voltage breakdown. The diode will then be permanently damaged.
As stated, different types of diodes have different forward voltage drops. The forward voltage drop of a simple light-emitting diode (LED) may range from 1.4 to 4 V, depending on the type. For the Schottkey diode, the forward voltage drop is usually only 0.2 V.