Technology Frontier | Current Trends in Schottky Diodes

As early as 1938, the German physicist Walter Schottky developed a model for metal-semiconductor contacts. In contrast to semiconductor-semiconductor contacts, Schottky diodes have a potential barrier due to the way the material composition is chosen, forming a depletion region at the interface of the semiconductor. This prevents electrical power below a certain power threshold from flowing between metal and semiconductor. Schottky diodes therefore offer a larger barrier to transfer than other metal-semiconductor diodes (e.g. ohmic contacts that exhibit ohmic resistance behavior).

Due to their high switching speed, Schottky diodes are mainly used in high-frequency applications up to the microwave range. This is also due to their low saturation capability. Therefore, they are often used in switching power supplies as protection diodes in the form of freewheeling diodes or rectifier diodes to reduce induced voltages and as demodulators in detection circuits.

However, not all Schottky diodes are the same. For example, most silicon is used for voltages up to 250V, while gallium arsenide, silicon carbide or silicon germanium are used as semiconductor materials for blocking voltages between 200 and 1700V. Silicon Schottky diodes have a low threshold voltage of about 0.4V, even below 0.1V at low operating currents. This is much lower than the voltage of a semiconductor-semiconductor junction of about 1V. Schottky diodes can therefore be switched in parallel with the collector-base junction of silicon bipolar transistors to prevent saturation of the transistor and enable the transistor to switch to the blocking state significantly faster. However, they have higher leakage currents than silicon-based semiconductor-semiconductor diodes and quickly lead to high conduction losses at higher blocking voltages.

To reduce these drawbacks, Littelfuse recently launched two new product series: MBR and DST power semiconductors. The MBR series of rectifier diodes are based on Schottky diode technology on silicon, which not only has low leakage current, but also provides high temperature resistance and low forward voltage. They are ideal for high-frequency switching power supplies, freewheeling diodes, DC/DC converters, uninterruptible power supplies and polarity protection. They also meet all general requirements for commercial and industrial applications. These diodes suitable for high junction temperature conditions in harsh high-temperature environments have a guard ring for increased strength and durability. Developers often use MBR Schottky rectifier diodes because they have extremely fast switching characteristics, low forward voltage and low leakage current, and high resistance to high junction temperatures. Since they generate less heat than conventional diodes, they also reduce thermal and electrical losses.

Compared to the MBR series, the Schottky barrier rectifiers of the DST series have a very low forward voltage and lower leakage current, resulting in higher system efficiency. At the same time, they have a high saturation capability, which makes them unsuitable for ultra-high frequency applications. Otherwise, the two series have almost the same characteristics and are suitable for the same applications. Depending on the circuit design of the specific application, they are available in a variety of package types and single-chip or dual-chip configurations. Littelfuse can also customize diodes for specific applications. The company also consistently responds to individual customer needs in terms of functionality, packaging and delivery, allowing it to develop the perfect solution for specific application situations.

Typical applications for Schottky diodes include, for example, solar panels or switch-mode power supplies. The reason for this is that switch-mode power supplies now often operate at frequencies above 20kHz, sometimes even up to 200kHz in smaller applications. Therefore, although DSTs have gained popularity, MBR diodes are mainly used in this field. Today, switch-mode power supplies are increasingly being used to convert AC power to DC, including power adapters for laptops and tablets, USB chargers for smartphones, built-in power supplies for desktop computers, network equipment, set-top boxes, wireless routers and many other devices. If a piece of equipment has a processor or memory unit required for its operation and is powered by AC (for example, via a socket at home or an alternator in a car), then a switch-mode power supply is usually installed.

Power rectifiers are therefore mainly used in power supply applications, where the AC current from the alternator must be converted to a DC current of 5V, 9V, 16V or 24V for various subsequent devices. In automobiles, DC-to-DC power supplies convert the 12V or 24V current from the battery to a DC current of 5V, 9V, 16V or other voltages for various devices in the car. While reliable conversion of current and its voltage is usually the main goal of DC-to-AC or DC-to-DC power supplies and other electrical devices, preventing reverse flow is the goal of solar modules. Solar cells made of semiconductor materials generate free charge carriers due to the energy provided (for example, electromagnetic radiation from the sun). In order to generate an electric current from these charge carriers, the positive and negative charge carriers must be oriented in different directions. This is usually done by using the internal electric field, which can be generated by semiconductor-semiconductor or metal-semiconductor junctions. The latter can better prevent possible backflow due to their lower forward voltage drop and thus prevent power loss in the rectifier.

MBR Schottky diodes used to dominate the market in all applications because they are easier and cheaper to produce. Far Eastern manufacturers in particular can offer these devices at very low prices. However, quality requirements, durability, resistance to harsh environments and individual adjustments are playing an increasingly important role; therefore, opting for more expensive products is often more rewarding in the long run. In addition, DST diodes are being used more frequently because, despite the complexity of their production, they are not much more expensive than the MBR version and ensure a faster return on investment due to their higher system efficiency. Very high-frequency applications are also being gradually solved by using improved technology; thus, DST is conquering more and more areas of application.