Principle of Schottky Diodes

The Schottky diode is named after the German physicist Walter Schottky, whose work explained the rectifying behavior of metal-semiconductor contacts. It is also called a Schottky barrier diode (SBD). Unlike a conventional PN junction diode, a Schottky diode is formed by joining a metal to a lightly doped semiconductor, usually N-type silicon. This structure gives it a low forward voltage and very fast switching, making it one of the most widely used diodes in power supplies, protection circuits and high-frequency applications.

The Metal-Semiconductor Junction

When a metal such as platinum, molybdenum or a metal silicide contacts N-type silicon, electrons flow from the semiconductor into the metal until their energy levels align. This leaves a depletion region in the semiconductor and creates an energy barrier at the interface, known as the Schottky barrier. The height of this barrier depends mainly on the metal chosen and the semiconductor properties.

Under forward bias, the barrier seen by electrons in the semiconductor is reduced and they pass over it into the metal through a process called thermionic emission. Under reverse bias, the barrier increases and only a small leakage current flows. The device therefore rectifies, much like a PN junction, but the physics of conduction is different.

Majority Carrier Conduction

A key difference from PN diodes is that a Schottky diode conducts with majority carriers only. In an N-type device, electrons carry the current, and there is essentially no injection of minority carriers. In a PN junction diode, forward conduction stores minority charge in the junction region, and this charge must be removed before the diode can block reverse voltage. That removal causes reverse recovery current and switching losses.

Because a Schottky diode stores almost no minority charge, it switches off very quickly. The main remaining effect is the charging and discharging of its junction capacitance. This makes Schottky diodes well suited to high-frequency switching converters and RF detectors.

Key Electrical Characteristics

Forward Voltage

The lower barrier height results in a lower forward voltage than a silicon PN diode at the same current. This directly reduces conduction losses, which is valuable in low-voltage, high-current power supplies. Forward voltage decreases as temperature rises, and it increases with current due to the series resistance of the semiconductor body.

Reverse Leakage Current

The same low barrier that reduces forward voltage also allows more reverse leakage. Leakage in a Schottky diode is significantly higher than in a comparable PN diode and rises sharply with temperature. At high junction temperatures and high reverse voltage, leakage losses can become a significant part of total dissipation.

Reverse Voltage Rating

Silicon Schottky diodes are generally used at relatively low to medium voltages, because higher breakdown voltage requires thicker, more lightly doped drift regions that increase forward voltage. For high-voltage applications such as power factor correction, silicon carbide (SiC) Schottky diodes are used; they offer high blocking voltage with negligible reverse recovery.

The Fundamental Trade-Off

Schottky diode design involves balancing forward voltage against leakage. A lower barrier height gives lower forward voltage but higher leakage, while a higher barrier gives lower leakage but higher forward voltage. Manufacturers therefore offer different product lines optimized for low forward voltage, low leakage or high temperature operation. Trench Schottky structures improve this trade-off by using the trench geometry to reduce the electric field at the surface, allowing lower forward voltage for a given leakage level.

The leakage-temperature relationship also introduces a risk of thermal runaway. If heat from leakage raises the junction temperature, leakage increases further, generating more heat. Designers must ensure the heatsinking and operating conditions keep the device stable at maximum ambient temperature and reverse voltage.

Common Applications

• Switching power supply rectification: output rectifiers and freewheeling diodes in buck, boost and flyback converters.
• Reverse polarity protection: a series Schottky diode has lower voltage drop than a standard diode, though MOSFET-based solutions are often used when losses must be lower still.
• Power OR-ing: combining multiple supplies or a battery backup so that the higher source powers the load.
• Clamping and protection: small-signal Schottky diodes clamp signals to supply rails to protect inputs.
• RF and signal detection: fast response makes them suitable for mixers and detectors.

Selection Tips

• Reverse voltage margin: choose a rating with sufficient margin above the maximum working voltage, including ringing and transients.
• Forward voltage at real conditions: compare the forward voltage at the actual operating current and temperature, not just the headline value.
• Leakage at maximum temperature: check the datasheet curves at the highest expected junction temperature and reverse voltage.
• Thermal resistance and package: power packages with exposed pads need adequate PCB copper for heat dissipation.
• Consider alternatives: in low-voltage, high-current outputs, synchronous rectification with MOSFETs may be more efficient; for high voltage, SiC Schottky diodes may be the better choice.

Summary

The Schottky diode uses a metal-semiconductor barrier and majority carrier conduction to achieve low forward voltage and almost no reverse recovery. These advantages come with higher, temperature-sensitive leakage and limited voltage capability in silicon. Understanding this trade-off helps engineers select the right device for rectification, protection and high-frequency circuits. ST's STPS family of silicon Schottky rectifiers and its STPSC silicon carbide diodes are examples covering both ends of this range.

If you need Schottky rectifiers or related power components, XINJIEDA can help source them or review your BOM for suitable alternates.