Last revised: October 6, 2024
Reviewed by: Scott Orlosky, consulting engineer

Schottky diodes are also known as Schottky barrier diodes or hot-carrier diodes. They consist of a junction between a metal layer and a semiconductor element. The metal layer, the anode, is heavily occupied with conduction-band electrons. The semiconductor element, the cathode, is a lightly doped N-type semiconductor. When forward-biased, the higher-energy electrons in the N region are injected into the metal region, allowing the junction to operate in the ON state. Schottky diodes achieve high switching speeds as they give up their excess energy very rapidly as they fluctuate between ON and OFF states.

Image Credit: Aligra

*The primary difference between a Schottky diode and a traditional PN Junction diode is the use of a metal rather than a P-type semiconductor anode.

Doping

Semiconductors are either doped with N-type or P-type impurities. N-type semiconductors carry a negative charge as the dopant donates free electrons to the substrate. P-type semiconductors are positively charged as the covalent bond with P-type impurities is missing an electron. P-type semiconductors are not used in Schottky diodes and are replaced by a metal anode instead.

N-type and P-type semiconductors. Image Credit: ASDN

Carrier Signal

The carrier signal is transferred by an electrical current through the junction. In most diodes the carrier signal is comprised of both free electrons and shared electrons that migrate from loose molecular bonds. The majority carrier is the predominant carrier and the minority carrier is less prevalent, being either shared or free electrons. In Schottky diodes the majority carriers are the free electrons in the crystal lattice. Furthermore, these hot-carrier diodes operate only with majority carriers and because there are no minority carriers, there is no reverse recovery current as with other types of diodes.

Symbol for a Schottky diode

Performance                                           

Schottky diodes are used in high-frequency applications and fast-switching digital circuits. They have low forward voltages, no reverse recovery time, and high switching speeds but are limited by lower reverse voltage ratings and relatively high reverse leakage current.

Low Forward Voltage (V_f)

The forward voltage (V_f) of the diode is the rated potential difference between the anode and cathode when the anode is positively charged and electrons are migrating through the barrier. Schottky diodes operate at lower voltages and generate less heat while in use.

No Reverse Recovery Time (t_rr)

Reverse recovery time (t_rr) is the time taken for the reverse current (I_r) to reach a specified level when the reverse voltage is applied while the device is conducting in the forward direction. Schottky diodes exhibit a very rapid change in response to bias as they do not experience a reverse current since they have no recovery period. The free electrons mobilize only when a forward voltage is present and the time elapsed when the diode switches from conducting to non-conducting states is almost instantaneous.

High Switching Speeds

Schottky diodes are capable of achieving high switching speeds because of two basic principles. First there is no reverse recovery time and the diode readily switches from conducting to non-conducting state as the voltage potential alternates. Secondly, the voltage drop across the junction is much lower than that of a conventional PN junction diode.

This increases efficiency and switching speed as the potential differential between on and off states is less.

Low Reverse Voltage Ratings

One downfall of Schottky diodes is the low reverse voltage ratings. The thin barrier at the junction will allow reverse current to propagate when there is a less significant reverse bias than with other conventional diodes.

High Reverse Leakage Current

Associated with the low reverse voltage ratings Schottky diodes also possess high reverse leakage current. The reverse leakage current is a function of temperature and consequently Schottky diodes suffer from thermal instability.

Applications

Schottky diodes are mainly used in applications that require fast response to changes in operation. Some of the applications include

• Switching Power Supplies
• Switching Voltage Regulators
• Rectifiers
• Detectors
• Motor Drivers

Schottky Diodes FAQS

What are the advantages of using Schottky diodes?

Schottky diodes can switch ON and OFF much faster than PN junction diodes. They produce less unwanted noise compared to silicon or germanium diodes 

Due to their low forward voltage drop, they are more efficient in power applications.

What is thermal runaway in Schottky diodes and how can it be managed?

Thermal runaway occurs when the diode's reverse leakage current increases with temperature, leading to higher power dissipation and further temperature rise. This can be managed by proper thermal management techniques to dissipate heat effectively and prevent excessive temperature rise.

What are Trench Schottky diodes?

Trench Schottky diodes are an advancement over the original Schottky diodes, providing greater capabilities. They are designed to handle higher reverse blocking voltages and reduce leakage currents compared to planar Schottky diodes.

How does the reverse leakage current affect the performance of Schottky diodes?

The reverse leakage current in Schottky diodes significantly affects their performance in several ways:

Thermal Instability

Schottky diodes possess high reverse leakage current, which is a function of temperature. This can lead to thermal instability, as the leakage current increases with temperature, causing higher power dissipation and further temperature rise. This phenomenon can result in thermal runaway, where the diode's temperature continues to increase uncontrollably, potentially leading to device failure.

Efficiency

The high reverse leakage current can reduce the overall efficiency of the diode, especially in power applications. This is because the leakage current contributes to power losses when the diode is in the reverse-biased state. In applications like switching power supplies and DC-DC converters, where high efficiency is crucial, managing the reverse leakage current is essential.

Reverse Voltage Ratings

Schottky diodes typically have lower reverse voltage ratings compared to conventional PN junction diodes. The thin barrier at the junction allows reverse current to propagate with a less significant reverse bias. This characteristic limits the maximum reverse voltage that Schottky diodes can handle, making them less suitable for high-voltage applications.

Thermal Management

Due to the high reverse leakage current, effective thermal management becomes critical in applications involving Schottky diodes. Proper heat dissipation techniques must be employed to prevent excessive temperature rise and ensure reliable operation of the diode.

Trade-offs in Design

The electrical performance of a Schottky diode involves trade-offs between forward voltage drop, leakage current, and reverse blocking voltage. Designers must balance these factors to optimize the diode's performance for specific applications. For instance, a lower forward voltage drop generally comes at the price of increased leakage current, which must be managed to prevent thermal issues.

How can thermal management be effectively implemented in Schottky diode applications?

Effective thermal management in Schottky diode applications is crucial to ensure reliable operation and prevent thermal runaway. Here are several strategies to implement effective thermal management:

Heat Sinks

Using heat sinks can significantly improve thermal management by increasing the surface area for heat dissipation. Heat sinks are often attached to the diode package to draw heat away from the diode and dissipate it into the surrounding environment.

Thermal Interface Materials (TIMs)

Applying thermal interface materials between the diode and the heat sink can enhance thermal conductivity. TIMs fill microscopic air gaps and improve the thermal connection between surfaces, ensuring efficient heat transfer.

Proper PCB Design

Designing the printed circuit board (PCB) with thermal management in mind is essential. This includes using thermal vias, copper planes, and adequate spacing to facilitate heat dissipation. The PCB layout should ensure that heat is spread evenly and efficiently away from the diode.

Active Cooling

In high-power applications, active cooling methods such as fans or liquid cooling systems can be employed to maintain the diode's temperature within safe limits. Active cooling is particularly useful in environments where passive cooling methods are insufficient.

Thermal Management Software

Utilizing thermal management software can help in simulating and optimizing the thermal performance of the diode in its application. This software can predict hot spots and suggest design modifications to improve thermal performance.

Component Selection

Selecting Schottky diodes with lower reverse leakage currents can reduce the thermal load. Trench Schottky diodes, for example, are designed to handle higher reverse blocking voltages and reduce leakage currents compared to planar Schottky diodes 

Thermal Runaway Prevention

Implementing thermal runaway prevention techniques is crucial. This can include using temperature sensors and control circuits to monitor the diode's temperature and shut down the circuit if the temperature exceeds safe limits.

Environmental Considerations

Considering the operating environment is also important. Ensuring adequate ventilation and avoiding high ambient temperatures can help in maintaining the diode's temperature within safe limits.

Power Derating

Derating the power dissipation of the diode can also help in managing thermal issues. Operating the diode below its maximum rated power can reduce the thermal load and improve reliability.

By combining these strategies, effective thermal management can be achieved, ensuring the reliable operation of Schottky diodes in various applications.

Schottky Diodes Media Gallery

Resources:

GlobalSpec—What is the Ideal Rectifier for Fast Switching Applications? Comparing Trench and Planar Schottky

GlobalSpec—Electronics I Laboratory Manual

GlobalSpec—Types of Diodes, Their Characteristics and Applications

Electronics360—Thermal runaway in Schottky rectifier diodes and how to solve it

Image credits:

Aligra| ASDN | New World Encyclopedia | Skyworks Solutions, Inc.