What is Fast Recovery Diode? Symbol, Construction, Working & Applications

Fast Recovery Diode – Construction, Working, Characteristics, Advantages, Disadvantages and Applications

Diodes are semiconductor-based devices that allow current in only one direction. They are one of the most used electronic components used in modern circuits. They are used for switching, rectification, protection, etc. There are different types of diodes used in modern circuits for their specific function. A fast recovery diode is one of the many types of diode used for its fast reverse recovery time.

What is Fast Recovery Diode?

A fast recovery diode is a semiconductor-based PN junction diode that has a very fast reverse recovery time. It is also known as a fast diode or fast switching diode. Since it has a very fast reverse recovery time as compared to a conventional diode, it is best suited for high-frequency applications.

The diode conducts current in forward bias and blocks current in reverse bias. But due to the stored charge in its junction, it is unable to immediately block current when the applied voltage changes from forward to reverse bias. Due to stored charge carriers, it conducts in the reverse direction for a short duration of time called reverse recovery time.

A conventional diode has a large reverse recovery time and it is unable to block a high-frequency signal as the reverse half cycle conducts during the reverse recovery time. Therefore, the fast recovery diode is used for rectification of high-frequency signals.

The reverse recovery time T_rr of the fast recovery diode ranges from tens to hundreds of nanoseconds. Whereas the reverse recovery time of a conventional PN junction diode resides in tens of microseconds. Therefore fast diode is used for rectification of signals up to 100 kHz.

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Symbol of Fast Diode

The symbol of a fast diode (for example FR-106, FR-107 etc. diode) is the same as a conventional diode because both of them have similar operations except for their switching speed.

Construction

It is a PN junction diode having a similar structure as a conventional diode. P-type semiconductors and N-type semiconductors are joined together to form a PN junction.

The only difference between them is that the fast diode has recombination centers in its semiconductor material. The recombination centers help in recombining the stored charge carriers and reducing their lifetime. Thus the junction is quickly drained of the stored charge and the recovery reverse time is reduced.

Gold is added as recombination centers in the semiconductor material such as gallium arsenide (GaAs). It greatly reduces the reverse recovery time as compared to a conventional diode. However, there is a limitation because the reverse current increases with an increase in the number of recombination centers. Therefore the amount of gold being added is taken into consideration.

Working of FR Diode

A fast recovery diode has the same operation as a conventional diode as they both are used for rectification. When an AC signal passes through a diode, the positive half cycle passes through it while the negative half cycle is blocked. However, it doesn’t immediately block the negative half cycle. It requires an amount of time to recover from the conduction state to the blocking state.

As we know that the time period of a signal is inversely proportional to its frequency (T = 1/f). Time period means the time required to complete one cycle. Low-frequency signal has a large time period i.e. they are slow. Any conventional diode can rectify them as they do not require a small recovery time.

However, high-frequency signals have a very short time period and the duration of a negative half cycle is very small. A conventional diode having a large recovery time will conduct the fast negative half cycle. Therefore they require a diode that has a very small recovery time to block the negative half.

The fast recovery diode achieves this small recovery time by adding gold as recombination centers in its semiconductor. During the conduction state, the electrons and holes flow to the opposite side of the PN junction. When the applied voltage reverses, the electrons and holes start to move to their sides.

Electrons are fast and they recover pretty quickly but the holes take time as they are heavier than electrons. They recombine with each other and form the depletion region that blocks the current flow. The recombination centers placed at a short distance in the semiconductor help in the recombination of the holes thus providing a fast recovery time.

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Forward Current Vs Reverse Recovery Time

The reverse recovery time greatly depends on the forward current. Forward current is the current flowing through the diode in forward bias.

In forward bias, electrons and holes move to the opposite side of the PN junction to flow out of the opposite terminals. The number of these electrons and holes depends on the forward current. A large forward current requires a large number of electrons and holes.

The greater the number of electrons and holes on their opposite sides, the larger the time taken by them to move to their sides. Thus the reverse recovery time is increased.

The n-layer consists of the majority of charge carriers electrons while the p-layer consists of holes. In forward bias, the electrons move to the p-layer while the holes move to the N-layer. When the applied voltage is reversed, the majority charge carriers move to their original side. But the holes are heavier.

Therefore, during the construction, the n-layer of the semiconductor is designed larger as compared to the p-layer and filled with traps (recombination centers). Usually, Gold is used to catch the holes moving back and reduce the recovery time.

Hard Recovery and Soft Recovery

The reverse recovery of a diode can be classified into two types

During hard recovery, the reverse current decrease suddenly whose magnitude oscillates creating noise as well as causing power loss. Hard recovery is illustrated by the given graphs.

The reverse recovery time is fairly small but there is a large noise generated in the system.

During soft recovery, the reverse current softly and smoothly reduces to zero. There is no abrupt change in the reverse current, as a result, there is no noise generated in the system. The reverse recovery time is comparatively large with low power loss but the system has no noise.

Electrical Characteristics

Here are some electrical characteristics of a fast recovery diode.

• Forward Voltage: The forward voltage drop across the PN junction of the fast recovery diode is around 1.3 to 3.6 volts.
• Maximum peak reverse voltage: the maximum reverse voltage the fast diode can withstand ranges between 600V and 1000V.
• Reverse Recovery Time: The time it takes to recover from forward conduction to reverse Blocking state and ranges between 10ns and 200ns.
• Reverse Current: The maximum leakage current in reverse bias ranges from a few μA to tens of μA.

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Ultra High-Speed FRD

Such a type of FRD is specifically designed for its high speed even at the cost of high forward voltage. It has an extremely small reverse recovery time around 25 ns. But the forward voltage drop is fairly large in the range of 3 to 3.6 volts. It does have the disadvantage of high forward voltage but still, it is used for its high switching speed. Another advantage of ultra high-speed FRD is its soft recovery characteristics.

Reverse Recovery Characteristics

The given graph shows the comparison between the fast recovery diode and the conventional diode. It shows the change in the diode current with respect to time during the switching from forward bias to reverse bias.

As seen in the graph, the reverse current in the fast recovery diode quickly reduces to zero due to the recombination centers. Whereas in a conventional diode, the reverse current takes a long time to reduce to zero.

Advantages and Disadvantages of Fast Recovery Diodes

Advantages

Here are some advantages of a fast recovery diode

• The main advantage is its high switching speed.
• It has a very low reverse recovery time
• It has lower losses as compared to conventional diodes.
• Its efficiency is high as compared to a conventional diode.
• It has a lower reverse leakage current as compared to the Schottky diode.
• It can withstand higher reverse voltage as compared to the Schottky diode.

Disadvantages

Here are some advantages of a fast recovery diode

• With the addition of gold as recombination centers, the reverse leakage current in the diode increases.
• Compared to the Schottky diode, it has high power consumption.
• It has a higher forward voltage drop as compared to the Schottky diode.

Applications

Generally, a fast recovery diode is used in the high-speed application.

• They are usually used for the rectification of high-frequency signals.
• They are used in envelope detectors for RF signals.
• They are used in high-speed analog and digital communication.
• They are used in various automobile industries.
• They are used in high-speed DC to DC converters.

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