Fast Recovery Diode: What is it And How Does it Work?

What is a Fast Recovery Diode?

A fast recovery diode (also known as a fast diode or fast switching diode) is defined as a semiconductor device with a short reverse recovery time.

Because fast recovery diodes have a lower reverse recovery time than conventional diodes, they are better suited to high-frequency rectification applications.

As fast recovery diodes can be turned off very quickly, they are often used in high-frequency applications. Fast recovery diodes are used in high-frequency switching up to 100 kHz.

When voltage is applied in reverse, the diode only withstands it once in reverse bias. During reverse bias, current flows for a short period, known as the reverse recovery time (T_RR).

Typically, the reverse recovery time (T_RR) for a fast recovery diode lies between tens of nano-seconds to 100 nano-seconds. While the reverse recovery time for a conventional diode lies between a few micro-second to tens of micro-second.

Hence the fast recovery diode has a reverse recovery time several orders of magnitude faster than a conventional diode.

How does a Fast Recovery Diode work?

Generally, fast recovery diodes are used for rectification purposes. If you have a low-frequency AC (sine) signal, it converts into a DC signal.

In a low-frequency signal, the period is longer, meaning the signal takes more time to complete a cycle. Thus, a conventional diode has enough time to switch states during the positive and negative half cycles.

As we know, the frequency is inversely proportional to the time period. Therefore, for high-frequency signals, the time period is minimal. And the signal will take a minimal time to complete the cycle.

In this case, the conventional diode cannot be used as it takes more time to change states. So, we need a quick semiconductor device and that has a low reverse recovery time.

The conventional diode is made from semiconductor material Gallium-Arsenide (GaAs). In a fast recovery diode, the gold (Au) is added with Gallium-Arsenide.

By adding gold as a semiconductor material, the reverse recovery time is reduced. Hence, it can switch fast from positive to negative half cycle. And it can be used in high-frequency applications.

Relationship Between Forward Current (I_F) and Reverse Recovery Time (T_RR)

When a diode is in forward bias, the current that passes through the diode is known as forward current (I_F).

The diode is designed to allow current only in one direction. It will block current in reverse bias and allow for flowing current in forward bias.

When a diode transfers from forward bias to reverse bias, the current will flow for some time. This time is known as reverse recovery time (T_RR).

In forward bias, electrons and holes pass through a junction, moving to the opposite side. In reverse bias, electrons and holes stay on their side, creating a depletion layer.

When the diode switches to reverse bias, electrons and holes take time to return to their sides. This time depends on the forward current.

The large forward current takes more time for electrons-holes to set at their side. The below figure shows the comparison recovery time for large and small forward current.

In a diode, the n-layer is thick, and holes take more time to come back. To reduce reverse recovery time, the traps are set in n-layer.

The holes are caught in traps, and it reduces reverse recovery time. An electron beam or diffused heavy metal is irradiated onto a PN junction to create a carrier trap. And it will catch holes that are going back.

Fast Recovery vs. Schottky Diodes

Schottky diode is a diode constructed by the junction of semiconductors with metal (such as gold, tungsten, platinum, molybdenum, chromium). It is also designed for fast switching applications as it has a characteristic of fast recovery time.

Let’s understand the difference between a fast recovery diode and a Schottky diode.

	Fast recovery diode	Schottky diode
Construction	Similar to a conventional diode, it has a PN junction.	In the Schottky diode, a barrier is formed by the contact between metal and semiconductor. This barrier is known as the Schottky barrier.
Material used	With semiconductor material, gold is added.	With N-type semiconductor material, metal is used to form a Schottky barrier.
Reverse recovery time (TRR)	Tens of nano-second to 100 nano-second. Compared to the Schottky diode, reverse recovery time is high. But it is very low compared to convention diode.	Less than 20 nano-second.
Forward voltage drops	0.5-2 V	0.4-1 V
Reverse withstand voltage	Less than 1200 V	Less than 150 V
Power consumption	High	Less
Application	Rectifier, Radio signal detector, Analog and digital communication circuit, industrial and commercial applications	SMPS, Voltage clamping and solar cell application, discharge and reverse current, protection, Radiofrequency (RF), and Detector diode.

Applications of a Fast Recovery Diode

The applications of a fast recovery diode are listed below:

• Mostly fast recovery diode is used in the application of a high-frequency rectifier.
• Power factor correction and power factor controller
• DC-DC converters and inverters
• Switching power supplies
• It is used in various industrial and commercial electronics circuits.
• Detect high-frequency RF waves
• For modulation purposes, it is used in various analog and digital commutation circuits.

Advantages of a Fast Recovery Diode

The advantages of a fast recovery diode are listed below:

• Low reverse recovery time
• High switching speed
• Good efficiency
• Low power loss

Disadvantages of a Fast Recovery Diode

Adding gold to a fast recovery diode increases recombination centers, resulting in a high reverse current.