Overview: The article explores the significance of Schottky diodes in fast-switching electronic applications, highlighting their unique characteristics, working principles, and advantages over traditional PN junction diodes.

Almost all electronic manufacturing industries, including consumer electronics, wireless communications, and industrial control, rely on power electronic devices as essential components. Schottky barrier diodes (SBD) are the most important type of such device in power conversion systems, including frequency converters, switching power supplies, drivers, and other circuits. It offers faster switching, improved thermal durability, and increased efficiency. They have been widely available from a variety of suppliers in the power device market.

What is a Schottky diode?

Schottky diodes are metal-semiconductor junction diodes with a lower forward voltage drop than PN junction diodes. They are more typically utilized in high-speed switching applications such as computers, radio frequency, and rectifying applications that respond to rapid changes.

Schottky diodes are also known as hot-carrier diodes, which means that the thermionic emission current is the primary contributor to the overall diode current. They are symbolically represented, as shown in Fig. 1.

Fig. 1: Symbol of Schottky diode. Source: Rakesh Kumar, Ph.D.

Construction and Working of Schokky diode

Instead of a P-N junction, a metal and an N-type semiconductor come together to form a Schottky diode, as shown in Fig. 2. It has an anode pin and a cathode pin. The P-type area usually has metal anodes like gold, silver, platinum, tungsten, molybdenum, or chromium.

Under normal conditions, this junction creates a Schottky barrier, which is a potential energy barrier for electrons at the metal-semiconductor interface. This barrier (the depletion region in the junction) is relatively thinner when compared to the PN junction diode. 

When a forward bias is applied, the electrons in the N-type semiconductor gain enough energy to cross the Schottky barrier and enter the metal. Now, with higher kinetic energy, these electrons are referred to as "hot carriers" because they have more energy than in a typical PN junction diode. The flow of current in the Schottky diodes is only due to these majority carriers; in other words, they are unipolar devices. The unique feature of the Schottky diode is due to the absence of a minority carrier storage or recombination process.

Fig. 2: Illustration of Schottky diode. Source: Rakesh Kumar, Ph.D.

Advantages

These diodes are more advantageous in low-loss and high-frequency rectification applications because of their low forward voltage drop and quick reverse recovery behavior. The current flows at a lower forward voltage drop than regular PN junction diodes. These devices can provide significant power generation, signal detection, and frequency conversion capabilities for various applications.

Minimal forward voltage drop: 

Schottky diodes are well known for their low forward voltage drop compared to conventional PN diodes. Their voltage drop is relatively low, from 0.15 V to 0.45 V. In contrast, the voltage drop across the PN diode is around 0.6 V to 0.7 V. With the same current rating, the Schottky diode dissipates comparatively minimal power and generates less heat. 

The reason for this low forward voltage drop is that the metal-semiconductor junction creates a thinner depletion region, and the energy required by the electron to cross this barrier is relatively smaller. This highly benefits numerous high-power applications, producing higher efficiency, lower power loss, and reduced heat generation. 

Minimal reverse recovery time: 

When compared to traditional PN junction diodes, Schottky diodes are well known for their fast switching speeds and minimal reverse recovery times. Their recovery time is typically in the range of nanoseconds; this is because only the majority charge carriers (electrons) are involved in conduction, and there is no minority carrier storage or recombination process.

Silicon Carbide Schottky Diode

Silicon carbide (SiC) is the most extensively used wide-bandgap semiconductor material because it can be found on wafers with diameters up to 150 mm and relatively low defect densities. SiC Schottky diode has excellent switching and sensing capabilities, which are highly attractive in industrial applications where controlling critical processes across large temperature ranges is required. 

Since SiC has a wide bandgap, it can be used to make Schottky diodes with a higher barrier height. This controls the diode's exponential current-voltage (I-V) dependence and makes it good at rectifying temperatures far above Si. 

Gallium Nitride Schottky Diode

Gallium nitride (GaN) has found widespread application in power and high-frequency electrical devices. Free-standing GaN vertical diodes, in particular, have started to get increasing interest in recent years. The cost and constrained size of GaN substrates continue to be limitations. As a result, GaN SBDs on silicon or sapphire substrates are becoming widely used.

Challenges

The main disadvantage is their comparatively large reverse leakage current when compared to typical PN junction diodes. High leakage current from electric field crowding at the Schottky junction edge may cause the vertical SBDs to prematurely breakdown. Many different edge termination topologies have been suggested as ways to improve the breakdown voltage of GaN. These include p-type junction termination (JBSD), anode field plate (AFP), and nitridation-based (negative ion implantation) termination.

In vertical Si and SiC SBDs, different kinds of junction termination extensions (JTEs) and guard rings (GRs) were used to make the electric field less crowded at the anode edge. Additionally, commercially available SiC diodes still typically have a limited safe temperature range below 175°C.

A Schottky Diode To Consider

ZLLS1000TA

Diodes Incorporated is a manufacturer of the ZLLS1000TA Schottky diode. They are known for their low forward voltage drop and fast switching capabilities, making them suitable for various applications in power electronics.

ZLLS1000, as shown in Fig. 3, is a 40V high-current, low-leakage Schottky diode. Its advantages include low equivalent-on resistance, high current capacity, minimal forward voltage drop, and fast switching. It is available in compact SOT23 packaging, making it ideal for space-constrained applications. 

Fig. 3: ZLLS1000TA. Source: oemsecrets 

They are commonly used in switching-mode power supply circuits where the switching frequency is in the range of 10 to hundreds of kHz. They are also used in communication applications such as detection and RF/microwave circuits. They are also used in reverse current and protection circuits. They are widely used in DC-DC converters, strobes, mobile phones, charging circuits, and motor control.

Summarizing the Key Points

  • Schottky diodes offer superior performance with low forward voltage drop and fast switching speeds, making them ideal for high-frequency applications in power electronics.
  • Advancements in materials like silicon carbide and gallium nitride have further enhanced the capabilities of Schottky diodes, enabling efficient operation in industrial environments.
  • Specific models like the ZLLS1000TA from Diodes Incorporated are known for their compact design and high current capacity in power supply circuits and communication systems.

Reference

Ru Xu et al., “1.4-kV Quasi-Vertical GaN Schottky Barrier Diode With Reverse p-n Junction Termination,” IEEE Journal of the Electron Devices Society 8 (January 1, 2020): 316–20, https://doi.org/10.1109/jeds.2020.2980759.

Arne Benjamin Renz et al., “The Optimization of 3.3 kV 4H-SiC JBS Diodes,” I.E.E.E. Transactions on Electron Devices/IEEE Transactions on Electron Devices 69, no. 1 (January 1, 2022): 298–303, https://doi.org/10.1109/ted.2021.3129705.

Gheorghe Pristavu et al., “Lagging Thermal Annealing for Barrier Height Uniformity Evolution of Ni/4H-SiC Schottky Contacts,” I.E.E.E. Transactions on Electron Devices/IEEE Transactions on Electron Devices, January 1, 2024, 1–5, https://doi.org/10.1109/ted.2024.3361397.