Schottky diodes are also known as Schottky barrier diodes or hot-carrier diodes. They consist of a junction between a metal layer and a semiconductor element. The metal layer, a cathode, is heavily occupied with conduction-band electrons. The semiconductor element, an anode, is a lightly doped N-type semiconductor. When forward-biased, the higher-energy electrons in the N region are injected into the metal region, allowing the junction to operate in the ON state. Schottky diodes achieve high switching speeds as they give up their excess energy very rapidly as they fluctuate between ON and OFF states.

Image Credit: Aligra

*The primary difference between a Schottky diode and a traditional PN Junction diode is the use of a metal rather than a P-type semiconductor anode.

Doping

Semiconductors are either doped with N-type or P-type impurities. N-type semiconductors carry a negative charge as the dopant donates free electrons to the substrate. P-type semiconductors are positively charged as the covalent bond with P-type impurities is missing an electron. P-type semiconductors are not used in Schottky diodes as they are replaced by a metal anode.

N-type and P-type semiconductors. Image Credit: ASDN

Carrier Signal

The carrier signal is transferred by an electrical current through the junction. In most diodes the carrier signal is comprised of both free electrons and shared electrons that migrate from loose molecular bonds. The majority carrier is the predominant carrier and the minority carrier is less prevalent, being either shared or free electrons. In Schottky diodes the majority carriers are the free electrons in the crystal lattice. Furthermore, these hot-carrier diodes operate only with majority carriers and because there are no minority carriers, there is no reverse recovery current as with other types of diodes.

Performance

Schottky diodes are used in high-frequency applications and fast-switching digital circuits. They have low forward voltages, no reverse recovery time, and high switching speeds but are limited by lower reverse voltage ratings and relatively high reverse leakage current.

Low Forward Voltage (V_f)

The forward voltage (V_f) of the diode is the rated potential difference between the anode and cathode when the anode is positively charged and electrons are migrating through the barrier. Schottky diodes operate at lower voltages and generate less heat while in use.

No Reverse Recovery Time (t_rr)

Reverse recovery time (t_rr) is the time taken for the reverse current (I_r) to reach a specified level when the reverse voltage is applied while the device is conducting in the forward direction. Schottky diodes exhibit a very rapid change in response to bias as they do not experience a reverse current since they have no recovery period. The free electrons mobilize only when a forward voltage is present and the time elapsed when the diode switches from conducting to non-conducting states is almost instantaneous.

High Switching Speeds

Schottky diodes are capable of achieving high switching speeds because of two basic principles. First there is no reverse recovery time and the diode readily switches from conducting to non-conducting state as the voltage potential alternates. Secondly, the voltage drop across the junction is much lower than that of convention PN junction diodes. This increases efficiency and switching speed as the potential differential between on and off states is less.

Low Reverse Voltage Ratings

One downfall of Schottky diodes is the low reverse voltage ratings. The thin barrier at the junction will allow reverse current to propagate when there is a less significant reverse bias than with other conventional diodes.

High Reverse Leakage Current

Associated with the low reverse voltage ratings Schottky diodes also possess high reverse leakage current. The reverse leakage current is a function of temperature and consequently Schottky diodes suffer from thermal instability.

Applications

Schottky diodes are mainly used in applications that require fast response to changes in operation. Some of the applications include

• Switching Power Supplies
• Switching Voltage Regulators
• Rectifiers
• Detectors
• Motor Drivers

Resources:

Frank Hughes - The Schottky Diode

Image credits:

Aligra| ASDN | New World Encyclopedia | Skyworks Solutions, Inc.