Diodes come in several different shapes and flavors, from tiny glass encapsulated Zener diodes to incredibly large rectifier diode PUK modules. Schottky diodes fall into this spectrum somewhere in the middle of things. What makes a Schottky special?
In general, diodes can carry many common parameters, so, as a point of reference, we can compare a Schottky to a “standard” diode, the extremely common Silicon P-N junction rectifier diode that almost everyone knows; think 1N400x.
The Schottky diode get its name from its unique construction. It is also this construction that gives it the characteristics that set it apart from the other diodes.
The Schottky diode is constructed differently than that of a Silicon P-N junction diode. Instead of two doped layers of semiconductor material, the Schottky diode uses a thin layer of metal bonded to N-type doped material. This combination of metal layered with N-type semiconductor is also known as a M-S junction (Metal-Semiconductor junction), and the convergence where these two materials meet is referred to as a Schottky Barrier, aptly named after Walter H. Schottky and thus giving the diode its name.
Depending on the manufacturer’s secret sauce, this metal seems to be any one of the precious metals, like platinum, tungsten, gold, and so on.
The Schottky barrier lends itself to some unique characteristics that make the diode ideal for some applications. The M-S junction creates a narrower electron depletion region than that of the P-N junction.
The narrower depletion region gives the Schottky diode these advantages:
- Low forward voltage
- When forward biased, the Schottky will only require 0.3-0.4V to start conducting vs 0.6-0.7V for a P-N junction. This can be beneficial in applications where power savings is an absolute must, such as battery driven and solar cell applications.
- The low forward voltage makes Schottky diodes ideal for guarding sensitive devices from overvoltage.
- High speed switching
- This narrow depletion zone lends itself well to high speed switching applications where the diode needs to turn on and off quickly between forward and reverse biased. This characteristic lends well to switched power supplies such as buck boost converters.
- Low noise
- A narrow depletion zone creates a less capacitive diode. This means that compared to a P-N junction diode, ringing and other capacitive noise can be avoided, making Schottky diodes the go-to choice for RF circuits.
- Performance advantages
- Compared to a P-N junction diode of similar specifications, a Schottky diode will consume less power and will tend to be more thermally efficient at dissipating heat in high power applications.
On the other side of the narrow depletion coin there are disadvantages:
- Leakage current
- Since not every diode is perfectly ideal, they will suffer from a leakage current when reverse biased. Schottky diodes are prone to high leakage currents due to the narrow depletion zone compared to their P-N junction counterparts.
- Lower reverse voltage
- Due to the narrower depletion zone, Schottky diodes cannot withstand high reverse voltages in comparison to a P-N junction diode; for Schottky diodes, typically around the 50V range, and for P-N junctions, typical starting at 500V going well into the Kilovolt range.
The unique characteristics of the Schottky barrier lend themselves well to a plethora of applications in electrical and electronic circuits. Some more obvious than others, here are a few:
- Voltage clamping – due to their low forward voltage, Schottky diodes are often used to protect devices from overvoltage or reverse voltage situations by clamping the voltage to a concise voltage rail, sometimes called a rail-to-rail steering circuit.
- Efficient blocking diode – again due to the low forward voltage, Schottky diodes can be used in Photovoltaic systems to prevent batteries from discharging through the solar cells when production is lower than the battery voltage. When the solar cells are fully producing, the Schottky only requires a small overhead while charging the batteries.
- Switch-mode power supplies – because of their efficiency and fast recovery time, Schottky diodes are often used inside high efficiency power supplies and DC-DC voltage converter circuits.
- Battery operated devices – when low voltage overhead and efficiency are key, Schottky diodes are the de facto standard. In battery powered devices, voltage overhead is at a premium, so when a blocking diode is required the low forward voltage of a Schottky is the answer. There is a time in battery operated devices where a Schottky diode is unlikely to be of use; when it is being reverse biased much of the time. The leakage current will cause the battery to drain faster than if a conventional P-N junction diode is used.
There are thousands more applications where Schottky diodes fit the bill. When an application turns up that requires a diode, consider a Schottky instead of grabbing the 1N400x out of habit. Available Schottky diodes: https://www.digikey.com/short/zhh8rq
During the next browsing session on the Digi-Key website, check out the Reference Design Library (https://www.digikey.com/reference-designs/en) to see how many schematics contain a Schottky.