Schottky Diodes: Principle, Functions, and Applications

Schottky diodes are named after their inventor, the German physicist Walter H. Schottky, who developed the underlying theory in the 1930s. SBD is the common abbreviation for Schottky Barrier Diode. Unlike conventional diodes, SBDs are not formed by creating a PN junction between a P-type and an N-type semiconductor. Instead, they are constructed from the junction formed at the contact point between a metal and a semiconductor. This structure is known as a metal-semiconductor junction, and therefore, SBDs are also referred to as metal-semiconductor diodes or surface barrier diodes. They belong to a class of devices known as hot carrier diodes.

Schottky diodes are metal-semiconductor devices typically made using a precious or highly conductive metal (such as gold, silver, aluminum, platinum, or silicide) as the anode (positive electrode) and an N-type semiconductor as the cathode (negative electrode). A key characteristic of Schottky diodes is their significantly lower forward voltage drop, which typically ranges from 0.15V to 0.45V, much lower than the 0.6V to 0.7V of a standard silicon PN junction diode. This low forward voltage, combined with their very fast switching speed, makes them highly efficient for a wide range of applications. They are used for rectification to convert AC to DC, as switching elements in logic circuits, for signal amplitude limiting, and for high-frequency signal detection. In recent years, the development of advanced materials like Silicon Carbide (SiC) and Gallium Nitride (GaN) has further expanded their use into high-power and high-frequency applications, such as in electric vehicles, solar power inverters, and 5G communications.

Catalog

I. Working Principle of Schottky Diodes

Schottky diodes are metal-semiconductor devices constructed from a metal such as gold, silver, aluminum, or platinum, which serves as the anode (positive electrode), and an N-type semiconductor, which acts as the cathode (negative electrode). The rectifying behavior arises from the potential barrier formed at the contact surface between these two materials. Because N-type semiconductors have a high concentration of electrons and the metal has a very low concentration, electrons diffuse from the high-concentration semiconductor (B) to the low-concentration metal (A). Since there are no holes in the metal, there is no corresponding diffusion of holes from A to B. As electrons continuously diffuse from B to A, the electron concentration at the surface of the semiconductor decreases, disrupting its electrical neutrality and forming a potential barrier. The electric field of this barrier is directed from B → A. Under the influence of this field, electrons in the metal will also drift from A → B, which counteracts the electric field formed by the diffusion. When a space charge region of a certain width is established, the electron drift caused by the electric field and the electron diffusion caused by the concentration difference reach an equilibrium, creating a stable Schottky barrier.

The internal structure of a typical Schottky rectifier is based on an N-type semiconductor substrate, on which an N-epitaxial layer is grown, often using arsenic as a dopant. The anode is made from a barrier metal like molybdenum or aluminum. Silicon dioxide (SiO2) is used as a passivation layer at the edges to eliminate the electric field in this area and improve the reverse voltage rating of the device. The N-type substrate has a low on-state resistance, and its doping concentration is significantly higher than that of the epitaxial layer. An N+ cathode layer is formed under the substrate to reduce the contact resistance of the cathode. By carefully adjusting these structural parameters, a Schottky barrier is formed between the N-type substrate and the anode metal, as shown in the figure. When a forward bias is applied across the Schottky barrier (anode connected to the positive terminal, N-type substrate to the negative), the barrier layer narrows, and its internal resistance decreases, allowing current to flow. Conversely, when a reverse bias is applied, the Schottky barrier layer widens, and its internal resistance increases, blocking current flow.

Figure 1. The internal circuit structure of the Schottky diode

II. Functions of Schottky Diodes

1. Rectification

Leveraging the unidirectional conductivity of Schottky diodes, alternating current (AC) can be converted into a single-direction, pulsed direct current (DC). In a circuit, current can only flow from the positive pole (anode) of the Schottky diode to the negative pole (cathode). In a conventional PN junction diode, when a forward voltage is applied, the potential barrier decreases, allowing a large current to pass with a typical voltage drop of around 0.7V. When a reverse voltage is applied, the barrier increases, blocking current flow except for a small leakage current.

Schottky diodes perform this same function but more efficiently. They are widely used in low-frequency half-wave and full-wave rectifier circuits. A common component is the rectifier bridge, which often packages four diodes into a single component. A full bridge contains four diodes connected in a bridge rectifier configuration. A half-bridge packages two diodes, and two half-bridges can be used to form a full bridge rectifier circuit. During each cycle of operation in a bridge rectifier, two diodes conduct simultaneously, converting the AC input into a unidirectional DC pulsating voltage. The lower forward voltage drop of Schottky diodes in these circuits leads to lower power loss and improved efficiency compared to standard PN junction diodes.

Figure 2. Rectifier Circuit

2. Switching

Under a forward voltage, a Schottky diode exhibits very low resistance and is considered to be in an "on" state, acting like a closed switch. Conversely, under a reverse voltage, its resistance is very high, placing it in an "off" state, like an open switch. This behavior allows Schottky diodes to be used in the construction of various logic circuits. A key advantage of Schottky diodes in switching applications is their extremely fast switching speed. Because they are unipolar devices and rely only on majority carriers for conduction, there is no reverse recovery time associated with the recombination of minority carriers, which is a limiting factor in conventional PN junction diodes. This allows them to turn on and off much more quickly, making them ideal for high-frequency applications.

Figure 3. Switch circuit in OFF state

The circuit above shows a basic switching configuration. The diode is reverse-biased, so it does not conduct. Any AC signal applied at C1 cannot pass through the diode, and therefore no AC component is detected at C2.

Figure 4. Switch circuit in ON state

In this second figure, the diode is forward-biased and conducts, allowing the AC signal from C1 to pass through to the output at C2. This demonstrates the "on" state of the switch. In practical RF circuits, this design often includes additional components to prevent RF signals from interfering with the DC bias lines, but the fundamental principle of controlling the signal path with the diode's bias state remains common.

3. Amplitude Limiting

Amplitude limiting, or clipping, is a function used to restrict a signal's amplitude to a specific range. Schottky diodes are well-suited for this role, especially in high-frequency pulse circuits, carrier circuits, and signal amplifiers, due to their sharp voltage-current (V-I) characteristics and excellent switching performance. When a Schottky diode is forward-biased, its forward voltage drop remains relatively constant (e.g., ~0.3V). This characteristic is exploited to limit, or "clamp," the signal voltage. For example, if a signal exceeds the diode's forward voltage, the diode will conduct and divert the excess signal, effectively clipping the output at the diode's forward voltage level. This can be implemented in series or parallel to achieve various levels of amplitude limiting.

4. Freewheeling

In circuits with inductive loads, such as relays or motors, a freewheeling diode (also known as a flyback diode) is essential for protection. When the current to an inductor is suddenly interrupted, the collapsing magnetic field induces a large voltage spike in the reverse direction. This spike can damage other components in the circuit. A Schottky diode connected in parallel with the inductive load provides a safe path for the current to circulate (or "freewheel") until the energy is dissipated. This clamps the voltage spike and protects the circuit. Schottky diodes are preferred for this application due to their fast response time and low forward voltage drop, which allows them to quickly clamp the voltage and dissipate energy more efficiently.

Figure 5. Freewheeling diode circuit

The freewheeling diode is connected in reverse-bias across the coil. The negative terminal of the diode is connected to the positive side of the coil's power supply, and the positive terminal of the diode is connected to the negative side. This configuration ensures that the diode does not conduct during normal operation but provides a path for the induced current when the power is disconnected.

5. Detection

Detection, also known as demodulation, is the process of extracting a low-frequency signal (like an audio signal) from a high-frequency modulated carrier wave. Schottky diodes are excellent for this purpose due to their unidirectional conductivity and fast switching speed, which allows them to efficiently rectify the high-frequency signal, leaving the lower-frequency information signal. They are widely used in radio receivers, televisions, and communication equipment. For small-signal applications where the operating frequency is high and the signal is weak, Schottky diodes are preferred over standard PN junction diodes because their low junction capacitance and fast recovery time result in better high-frequency performance.

Figure 6. Demodulation Circuit

6. Variable Capacitance

Varactor diodes, also known as variable reactance diodes, are specifically designed to exploit the voltage-dependent capacitance of a reverse-biased junction. While this is a characteristic of all diodes, varactors are optimized for this purpose. When the reverse-bias voltage across the diode increases, the depletion region widens, and the junction capacitance decreases. Conversely, as the reverse voltage decreases, the capacitance increases. Schottky varactors are used in high-frequency applications such as voltage-controlled oscillators (VCOs), RF filters, and tuning circuits in television receivers and mobile communication devices. Their capacitance values are typically small, ranging from a few picofarads to hundreds of picofarads.

III. Application of Schottky Diodes in Digital Circuits

1. Dual Power Supplies

In systems requiring redundant power sources, Schottky diodes are often used to combine two power supplies. This is a common requirement in high-availability systems where a backup power source must take over seamlessly if the primary source fails. The diodes are used to create a simple OR-ing circuit, where the output voltage is supplied by whichever input source has the higher voltage. Schottky diodes are ideal for this application because their low forward voltage drop minimizes power loss and heat generation, which is critical for maintaining system efficiency.

Figure 7. Schottky diode applied to dual power

2. AND Gates

As shown in the figure below, a simple AND gate can be constructed using Schottky diodes. In this configuration, the output will be logic high (1) only if all inputs (A1 to An) are high. If any input is logic low (0), it will pull the output low through the corresponding diode. Because the input stage of most digital logic chips is high-impedance, the current drawn by this diode-based AND gate is very low (in the microampere range). The low forward voltage drop of the Schottky diodes ensures that the logic levels remain within the required specifications for the connected digital circuits.

Figure 8. Schottky diodes used as AND gate

3. OR Gates

Similarly, an OR gate can be constructed using Schottky diodes as shown below. In this circuit, the output will be logic high (1) if any of the inputs (A1 to An) are high. The output will only be logic low (0) if all inputs are low. This provides a simple and effective way to implement OR logic for signals without using a dedicated logic gate IC.

Figure 9. Schottky diodes used as OR gate

4. Application Examples

In digital circuit design, it is often necessary to implement simple logic functions like AND, OR, or signal inversion. While dedicated logic chips from the 74 series are an option, using discrete Schottky diodes can offer more flexibility and save board space. The figure below shows a simple two-way reset circuit. In this scenario, a reset signal can be generated by either a JTAG interface or an external reset button. Connecting both reset sources directly to the master controller's reset pin could cause issues; for example, pressing the key could damage the JTAG emulator. By using a Schottky diode array like the BAT54A to form an OR gate, the reset signals are isolated from each other. The master controller will be reset if either the JTAG outputs a low logic level or the reset key is pressed, without the two signals interfering with each other.

Figure 10. Simple two-way reset circuit

IV. Modern Developments and Materials

While the fundamental principles of Schottky diodes have remained the same, the technology has seen significant advancements since 2020, particularly with the adoption of wide-bandgap semiconductor materials like Silicon Carbide (SiC) and Gallium Nitride (GaN). These materials offer superior performance characteristics compared to traditional silicon.

Silicon Carbide (SiC) Schottky Diodes: SiC diodes provide much higher breakdown voltages, lower leakage currents, and better thermal conductivity than their silicon counterparts. This allows them to operate at higher temperatures and higher voltages, making them ideal for demanding applications such as electric vehicle (EV) charging stations, solar power inverters, and industrial power supplies. Major manufacturers now offer SiC Schottky diodes with voltage ratings up to 1700V.

Gallium Nitride (GaN) Schottky Diodes: GaN technology offers even faster switching speeds than SiC, pushing the boundaries of high-frequency power electronics. GaN-based Schottky diodes are increasingly used in applications like 5G communication infrastructure, advanced radar systems, and compact, high-efficiency power adapters for consumer electronics.

V. Conclusion

The unique structure of the Schottky diode, based on a metal-semiconductor junction, gives it a distinct set of characteristics that make it invaluable in modern electronics. Its low forward voltage drop, extremely fast switching speed, and high efficiency make it a superior choice for a wide range of applications, from high-frequency rectification and signal detection to power management and digital logic. While they have been a cornerstone of electronics for decades, the recent integration of advanced materials like SiC and GaN is ensuring that Schottky diodes will continue to be critical components in next-generation technologies, driving innovation in renewable energy, electric vehicles, and high-speed communications.

Update Information

This article was updated in October 2025 to reflect the latest information and technological advancements since its original publication in 2020.