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A diode is a common electronic device with two leads; it allows an electrical signal to pass in one direction, but will block a signal that tries to pass in the other direction. In operation, a diode continuously switches back and forth between conducting in the desired direction and blocking in the undesired direction. When a diode switches, it takes a brief moment, called the recovery time, to recover and change from conducting to blocking. During the recovery time, a small amount of signal can pass in the wrong direction. A fast recovery diode is a diode designed to have as small of a recovery time as possible, so that the undesired signal does not disrupt high-power or high-frequency equipment.
Standard semiconductor diodes are made from two pieces of material, such as silicon. One piece is positively charge, called the anode, and the other is negatively charged, called the cathode. Such diodes are called PN junction diodes, after the two charged sections and the switching effect that takes place at the junction where the two pieces meet.
When an electrical current enters the cathode, it cannot pass through the negatively charged cathode of the diode, which shares the same electrical charge, and is blocked. A current entering through the anode, however, can pass through the positively charged anode and continue through the cathode and out of the other side of the diode and on to the rest of the circuit. In most applications, such as when converting an AC signal to DC, a diode switched between conducting and blocking regularly.
During the time that a diode is conducting, the current passing through the diode builds up a negative charge in the normally positive anode of the diode. When it then switches to its blocking mode, that built-up charge allows electrical current to flow through the diode in the reverse direction until the charge dissipates. The time it takes this charge to dissipate, and the diode to begin to block the signal fully, is called the diode’s recovery time.
For most applications, the recovery time of a standard diode, which is usually less than 100 milliseconds long, is not an issue. Likewise, the signal that passes through the diode during the recovery time is often too weak to be of concern. In certain high-speed, high-frequency or high-power applications, however, the recovery time of a diode can be of critical importance and require the use of a fast recovery diode.
Operationally, a fast recovery diode usually overcomes the long recovery time of a standard diode by using a metal segment in place of one of the semiconductor segments, such as in a Schottky diode. Another type of fast recovery diode, called a gold-doped diode, uses gold or platinum additives to increase the conductivity of one of the diode's segments. In practice, the use of metal instead of semiconductor provides for a more highly conductive diode. This higher conductivity allows the charge built up in the diode to dissipate at a much faster rate, usually in the tens of nanoseconds range, which greatly shortens the diode’s recovery time.