Differences between a diode and a Schottky barrier diode

There are various types of diodes (electronic components that need to be used properly according to their locations and applications). For example, Schottky barrier diodes are often used in power supply circuits, which are required to be highly efficient and compact.

In this article, we will explain topics like “What is a Schottky barrier diode?” “When do you use them?” and “How they differ from regular diodes.”

Table of Contents

• What is a Schottky barrier diode?
• Schottky barrier diodes are used in low voltage switching power circuits
• Is the Schottky barrier diode a universal diode?
• How are they different from Fast Recovery Diodes (FRD)?
• Summary of how to use diodes in power supply circuits

What is a Schottky barrier diode?

A Schottky barrier diode is a diode that uses a phenomenon called the Schottky barrier, wherein electricity flows in only one direction when a semiconductor and metal are joined. Since their structure is different from the PN junction made of the P-type / N-type semiconductors of general diodes, their electrical characteristics are different from those of general diodes.

Schottky barrier diodes are characterized by the fact that compared to diodes made by the PN junction, the time they take to go from the on-state to the off-state (reverse recovery time trr) is shorter, and their forward voltage V_F is lower.

This characteristic is very convenient for operating at high frequencies, such as when switching power supplies, and they are an indispensable electronic component for improving efficiency and miniaturizing current power supply circuits.

Schottky barrier diode structure (left) and electrical characteristics diagram (right). At the junction between the semiconductor and metal, there is an electrical wall called a Schottky barrier that allows electricity to flow in only one direction.

Reference: Schottky Barrier Diode | What is a diode? | Electronics Travel Tips | ROHM Co., Ltd.

Schottky barrier diodes are electronic components that are assigned unique circuit symbols so that they can be clearly identified on schematics. The line of the cathode part is bent into an S shape when the anode is seen on the left side, although some schematic editors have a square S shape instead of a curved line. There is a Zener diode (its cathode part is Z-shaped) in similar electronic parts, so be careful not to mistake the two.

Be careful not to confuse Zener diodes and Schottky barrier diodes, as they have similar schematic symbols.

Schottky barrier diodes are used in low voltage switching power circuits

I explained that the characteristics of Schottky barrier diodes are their “fast reverse recovery time” and “low forward voltage”, and that these characteristics can be utilized in switching power supply circuits.

Switching power supplies can be made smaller as the switching frequency is higher, but electronic components that can handle higher frequencies must be used. Schottky barrier diodes have a fast reverse recovery time and can handle high-speed switching.

Furthermore, since the forward voltage is low and the loss during rectification is small, switching power supply circuits that are compact, high efficiency, and generate low heat can be configured.

Due to these characteristics, Schottky barrier diodes are used on the secondary sides of ACDC converters and low-voltage DCDC converters.

Can Schottky barrier diodes be used anywhere?

So far, Schottky barrier diodes sound like high-performance universal diodes, but it should be noted that they cannot be used in all types of circuits.

First of all, Schottky barrier diodes have a limit of about 200V (and for many products, the limit is up to about 40V) even for products with high withstand voltages, so they cannot be used to control high voltages.

Even more troublesome is the tendency for thermal runaway to occur. Schottky barrier diodes have a large leakage current when they are off, while the leakage current can increase due to the influence of heat.

In a high voltage circuit, the effects of leakage current cannot be ignored. If the thermal design is inadequate, the increase in the leakage current due to heat generation will not stop the vicious cycle that further raises the temperature, resulting in thermal runaway. Please note that this thermal runaway will occur even below the guaranteed temperature.

Reference: Schottky Barrier Diode | What is a diode? | Electronics Travel Tips | ROHM Co., Ltd.

Although Schottky barrier diodes seem like they are high-performance, they are electronic components with a low withstand voltage that are easily affected by heat, so check the location of their use and the voltage of the circuit and try to use them in the right places.

How are they different from Fast Recovery Diodes (FRD)?

A diode similar to the Schottky barrier diode is a product called a Fast Recovery Diode (FRD). Fast recovery diodes are also diodes with a fast reverse recovery time. So, how are they different from Schottky barrier diodes?

Fast recovery diodes have a fast reverse recovery time and a high withstand voltage, but also have the disadvantage of a high forward voltage V_F, so they are used in circuits where high voltages that cannot be used with Schottky barrier diodes are applied.

Data sheet for 10A fast recovery diodes in the TO220FN package. The reverse voltage is high and the reverse current (leakage current) is low. However, the forward voltage V_F is high.

Reference: ROHM Super-Fast Recovery Diode RF1001T2DNZ

Data sheet for the 10A Schottky barrier diode, also in the TO220FN package. The forward voltage V_F is low, the reverse voltage is as low as 40V, and the reverse voltage is large. It is important to use them properly, because they have characteristics that are the opposite of those of FRD

Reference: ROHM Schottky Barrier Diode RB085T-40NZ

By the way, the drawback of the high forward voltage of fast recovery diodes is being solved by next-generation semiconductor silicon carbides (SiC). Since the SiC Schottky barrier diode (SiC-SBD) has both a withstand voltage of 1200V and a low forward voltage, the use of SiC-SBD may increase in place of the fast recovery diode in the future.

Reference: SiC SBD | What are SiC Power Devices? | Electronics trivia | ROHM Co., Ltd. – ROHM Semiconductor

Summary of how to use diodes in power supply circuits

Finally, we will introduce a summary of Schottky barrier diodes, an introduction of other diodes, and their proper use in power supply circuits.

Schottky Barrier Diode (SBD)

• Often used for low-voltage switching power supplies. It’s used for diodes that rectify on the secondary sides of DCDC converters and ACDC converters.
• Schottky diodes have a low forward voltage (V_F) and excellent reverse recovery time.
• Since the V_F is less than 1V even with a large current product, the forward loss is small, and the conversion efficiency is high.
• However, there is a large leak when reverse voltages are applied, and it is easily affected by heat, so thermal runaway may occur depending on the conditions.

Rectifier diode

• Diodes and bridge diodes used for the rectification of general commercial power supplies are also applicable.
• V_F is generally 1V or less, and the reverse recovery time is inferior to that of the above two diodes. However, switching performance is not required because 50Hz / 60Hz rectification is the main application.

Fast Recovery Diode (FRD)

• Used for the primary side of ACDC converters and inverter circuits because of its excellent switching characteristics and high withstand voltage.
• Excellent switching characteristics, but V_F is high. The V_F at high currents is about 2V.
• High withstand voltage in the reverse direction. Plus, it can handle a high withstand voltage of 800V.

Zener diode

• In power supply circuits, it is used for surge protection and overvoltage detection on the secondary side of the ACDC converter.
• The voltage is constant even if the current changes, and it is generally used in the opposite direction.
• Although other diodes are mainly used for rectification, Zener diodes are used as constant voltage circuits and protection devices.