- Fast Recovery Diode Definition: 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.
- Reverse Recovery Time: Fast recovery diodes have a reverse recovery time (TRR) ranging from tens to 100 nanoseconds, making them suitable for high-frequency applications.
- Material and Construction: These diodes are made from Gallium-Arsenide (GaAs) with added gold, which reduces the reverse recovery time and allows fast switching from positive to negative half cycles.
- Forward Current Impact: The forward current (IF) affects the reverse recovery time (TRR), with larger currents requiring more time for electrons and holes to return to their sides in reverse bias.
- Applications: Fast recovery diodes are used in high-frequency rectifiers, power factor controllers, DC-DC converters, inverters, switching power supplies, and various industrial and commercial electronics circuits.
What is a Fast Recovery Diode?
A fast recovery diode (also called a fast diode or fast switching diode) is a PN-junction rectifier designed to stop conducting soon after its voltage reverses.
Its short reverse recovery limits stored charge and switching loss, which makes the device useful in high-frequency rectifiers, converters and freewheeling circuits.
A designer selects the diode for the circuit’s switching frequency and checks its recovery charge, reverse voltage, forward current and thermal limits under the stated test conditions.
When the applied voltage changes from forward bias to reverse bias, stored charge keeps a brief reverse current flowing. Reverse recovery time (TRR) measures the turn-off interval until that current falls to the datasheet threshold.
Reverse recovery time (TRR) is device-specific. Manufacturers state it at defined forward current, current fall rate, reverse voltage and junction temperature, so a single nanosecond range does not describe every fast recovery diode.
Compare parts at the same test conditions. A shorter recovery time and lower recovery charge usually reduce turn-off loss, while forward voltage and recovery softness also affect the result.
How does a Fast Recovery Diode work?
In a rectifier, the diode conducts during the permitted part of an AC waveform and blocks the opposite polarity. The circuit then filters the rectified waveform when a steady DC output is required.
A low-frequency waveform has a long period. A standard rectifier can finish turning off before the next switching event when that interval is long enough for its stored charge to clear.
Frequency is the inverse of period. As frequency rises, the time available for each conduction and blocking interval becomes shorter.
If a diode turns off too slowly, its reverse current increases switching loss and can add voltage overshoot, ringing or noise. A fast recovery device limits that interval when its ratings suit the circuit.
Most fast recovery power rectifiers are silicon PN-junction devices. Gallium arsenide and added gold do not define this diode class.
Manufacturers shorten carrier lifetime through device design and processes such as platinum diffusion, other lifetime-control dopants or electron irradiation. Faster recovery trades against properties such as forward voltage, leakage current and recovery softness.
Relationship Between Forward Current (IF) and Reverse Recovery Time (TRR)
During forward bias, the conducting current is the forward current (IF). This current injects minority carriers that store charge in a silicon PN junction.
The diode conducts in forward bias and blocks reverse voltage within its rating after turn-off. It cannot regain full blocking action until the stored charge has been removed.
When the circuit drives the diode from forward bias into reverse bias, a reverse current removes that charge. Reverse recovery time (TRR) covers this measured turn-off interval.
Forward conduction places minority carriers in the junction and drift region. The reverse transition sweeps out or recombines those carriers while the depletion region forms for voltage blocking.
More stored charge generally produces a larger or longer recovery event. The result depends on forward current, current fall rate, reverse voltage and junction temperature.
The figure compares recovery after a large forward current with recovery after a smaller current. Read it as a qualitative comparison because a datasheet test also fixes the other switching conditions.
Fast recovery designs reduce the lifetime of minority carriers in the drift region. Recombination centres help stored carriers disappear sooner during turn-off.
Manufacturing methods include heavy-metal diffusion and electron irradiation. These processes create recombination centres, but the exact method and its electrical trade-offs depend on the diode design.
Fast Recovery vs. Schottky Diodes
A Schottky diode forms its barrier at a metal-semiconductor junction. It is a majority-carrier device, so it has no stored minority-carrier recovery charge, although its junction capacitance still draws current during switching.
The table compares the device principles. Exact voltage, current, forward-drop and switching values must come from the selected part’s datasheet.
| Fast recovery diode | Schottky diode | |
| Construction | A bipolar PN junction stores minority carriers during forward conduction. | A metal-semiconductor barrier conducts mainly through majority carriers. |
| Material used | Usually silicon, with carrier lifetime controlled by the structure or a process such as platinum diffusion. | A metal contact on silicon or silicon carbide forms the Schottky barrier. |
| Reverse recovery time (TRR) | Specified under stated test conditions; stored charge produces a measurable recovery interval. | No stored minority-carrier recovery charge, but junction capacitance still produces switching current. |
| Forward voltage drops | Device-specific, with a trade-off between forward drop and recovery speed. | Often low in silicon parts; high-voltage silicon-carbide parts have different values. |
| Reverse withstand voltage | Device-specific; current silicon ultrafast rectifiers are available with ratings up to at least 1,200 V. | Device-specific; silicon parts reach 200 V and silicon-carbide Schottky parts reach at least 1,700 V. |
| Power consumption | Depends on conduction loss, recovery charge, switching conditions and temperature. | Depends on forward drop, capacitive charge, switching conditions and temperature. |
| Application | Rectifiers, radio signal detectors, analogue and digital communication circuits, industrial and commercial applications | SMPS, voltage clamping and solar cell applications, discharge and reverse-current protection, radio-frequency (RF) circuits, and detector diodes. |
Applications of a Fast Recovery Diode
Fast recovery diodes are used in circuits that need a PN rectifier to turn off quickly:
- High-frequency rectifier stages
- Power factor correction and power factor controllers
- DC-DC converters and inverters
- Switching power supplies
- Industrial and commercial electronic circuits
- High-frequency RF detection
- Modulation and switching in analogue and digital communication circuits
Advantages of a Fast Recovery Diode
A correctly selected fast recovery diode offers these operating benefits:
- Short reverse recovery time under its specified test conditions
- High switching speed in rectifier and freewheeling service
- Good system efficiency when forward and switching losses are balanced
- Lower turn-off loss than a slower PN rectifier in the same circuit
Disadvantages of a Fast Recovery Diode
Carrier lifetime control can increase reverse leakage or forward voltage while it reduces stored charge. Designers must compare these trade-offs at the circuit’s current, voltage, switching rate and temperature.
