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The Schottky diode named after the German physicist Walter H. Schottky , also known as Schottky barrier diode or hot-carrier diode , is a semiconductor diode formed by the junction of a semiconductor with a metal. It has a low forward voltage drop and a very fast switching action.
The cat's-whisker detectors used in the early days of wireless and metal rectifiers used in early power applications can be considered primitive Schottky diodes. When sufficient forward voltage is applied, a current flows in the forward direction.
A silicon p—n diode has a typical forward voltage of — mV, while the Schottky's forward voltage is — mV. This lower forward voltage requirement allows higher switching speeds and better system efficiency. A metal—semiconductor junction is formed between a metal and a semiconductor, creating a Schottky barrier instead of a semiconductor—semiconductor junction as in conventional diodes. Typical metals used are molybdenum, platinum, chromium or tungsten, and certain silicides e.
This Schottky barrier results in both very fast switching and low forward voltage drop. The choice of the combination of the metal and semiconductor determines the forward voltage of the diode. Both n- and p-type semiconductors can develop Schottky barriers.
However, the p-type typically has a much lower forward voltage. As the reverse leakage current increases dramatically with lowering the forward voltage, it cannot be too low, so the usually employed range is about 0. With increased doping of the semiconductor, the width of the depletion region drops. Below a certain width, the charge carriers can tunnel through the depletion region.
At very high doping levels, the junction does not behave as a rectifier any more and becomes an ohmic contact. This can be used for the simultaneous formation of ohmic contacts and diodes, as a diode will form between the silicide and lightly doped n-type region, and an ohmic contact will form between the silicide and the heavily doped n- or p-type region. Lightly doped p-type regions pose a problem, as the resulting contact has too high a resistance for a good ohmic contact, but too low a forward voltage and too high a reverse leakage to make a good diode.
As the edges of the Schottky contact are fairly sharp, a high electric field gradient occurs around them, which limits how large the reverse breakdown voltage threshold can be. Various strategies are used, from guard rings to overlaps of metallization to spread out the field gradient. The guard rings consume valuable die area and are used primarily for larger higher-voltage diodes, while overlapping metallization is employed primarily with smaller low-voltage diodes. Schottky diodes are often used as antisaturation clamps in Schottky transistors.
Schottky diodes made from palladium silicide PdSi [ clarification needed ] are excellent due to their lower forward voltage which has to be lower than the forward voltage of the base-collector junction. The Schottky temperature coefficient is lower than the coefficient of the B—C junction, which limits the use of PdSi at higher temperatures. The resistance of the epitaxial layer is more important than it is for a transistor, as the current must cross its entire thickness.
However, it serves as a distributed ballasting resistor over the entire area of the junction and, under usual conditions, prevents localized thermal runaway. In comparison with the power p—n diodes the Schottky diodes are less rugged.
The junction is direct contact with the thermally sensitive metallization, a Schottky diode can therefore dissipate less power than an equivalent-size p-n counterpart with a deep-buried junction before failing especially during reverse breakdown. The relative advantage of the lower forward voltage of Schottky diodes is diminished at higher forward currents, where the voltage drop is dominated by the series resistance.
The most important difference between the p-n diode and the Schottky diode is the reverse recovery time t rr when the diode switches from the conducting to the non-conducting state. Schottky diodes do not have a recovery time, as there is nothing to recover from i.
With p—n-junction switching, there is also a reverse recovery current, which in high-power semiconductors brings increased EMI noise. With Schottky diodes, switching is essentially "instantaneous" with only a slight capacitive loading, which is much less of a concern. This "instantaneous" switching is not always the case. In higher voltage Schottky devices, in particular, the guard ring structure needed to control breakdown field geometry creates a parasitic p-n diode with the usual recovery time attributes.
As long as this guard ring diode is not forward biased, it adds only capacitance. If the Schottky junction is driven hard enough however, the forward voltage eventually will bias both diodes forward and actual t rr will be greatly impacted. It is often said that the Schottky diode is a " majority carrier " semiconductor device. This means that if the semiconductor body is a doped n-type, only the n-type carriers mobile electrons play a significant role in the normal operation of the device.
The majority carriers are quickly injected into the conduction band of the metal contact on the other side of the diode to become free moving electrons. Therefore, no slow random recombination of n and p-type carriers is involved, so that this diode can cease conduction faster than an ordinary p—n rectifier diode.
This property, in turn, allows a smaller device area, which also makes for a faster transition. The most evident limitations of Schottky diodes are their relatively low reverse voltage ratings, and their relatively high reverse leakage current.
For silicon-metal Schottky diodes, the reverse voltage is typically 50 V or less. Some higher-voltage designs are available V is considered a high reverse voltage. Reverse leakage current, since it increases with temperature, leads to a thermal instability issue.
This often limits the useful reverse voltage to well below the actual rating. While higher reverse voltages are achievable, they would present a higher forward voltage, comparable to other types of standard diodes. Such Schottky diodes would have no advantage  unless great switching speed is required. Schottky diodes constructed from silicon carbide have a much lower reverse leakage current than silicon Schottky diodes, as well as higher forward voltage about 1.
As of [update] they were available from manufacturers in variants up to V of reverse voltage. Silicon carbide has a high thermal conductivity, and temperature has little influence on its switching and thermal characteristics.
While standard silicon diodes have a forward voltage drop of about 0. This is due to the higher current density in the Schottky diode.
Because of a Schottky diode's low forward voltage drop, less energy is wasted as heat, making them the most efficient choice for applications sensitive to efficiency. For instance, they are used in stand-alone "off-grid" photovoltaic PV systems to prevent batteries from discharging through the solar panels at night, called "blocking diodes". They are also used in grid-connected systems with multiple strings connected in parallel, in order to prevent reverse current flowing from adjacent strings through shaded strings if the "bypass diodes" have failed.
Schottky diodes are also used as rectifiers in switched-mode power supplies. The low forward voltage and fast recovery time leads to increased efficiency. They can also be used in power supply " OR "ing circuits in products that have both an internal battery and a mains adapter input, or similar.
However, the high reverse leakage current presents a problem in this case, as any high-impedance voltage sensing circuit e. Schottky diodes can be used in diode-bridge based sample and hold circuits. When compared to regular p-n junction based diode bridges, Schottky diodes can offer advantages. A forward-biased Schottky diode does not have any minority carrier charge storage.
This allows them to switch more quickly than regular diodes, resulting in lower transition time from the sample to the hold step. The absence of minority carrier charge storage also results in a lower hold step or sampling error, resulting in a more accurate sample at the output. Due to its efficient electric field control Schottky diodes can be used to accurately load or unload single electrons in semiconductor nanostructures such as quantum wells or quantum dots. Small-signal schottky diodes such as the 1N,  1N,  1SS,  1SS,  and the BAT41—43, 45—49 series  are widely used in high-frequency applications as detectors, mixers and nonlinear elements, and have superseded germanium diodes.
Schottky metal—semiconductor junctions are featured in the successors to the TTL family of logic devices , the 74S, 74LS and 74ALS series, where they are employed as Baker clamps in parallel with the collector-base junctions of the bipolar transistors to prevent their saturation, thereby greatly reducing their turn-off delays. When less power dissipation is desired, a MOSFET and a control circuit can be used instead, in an operation mode known as active rectification.
A super diode consisting of a pn-diode or Schottky diode and an operational amplifier provides an almost perfect diode characteristic due to the effect of negative feedback, although its use is restricted to frequencies the operational amplifier used can handle.
Electrowetting can be observed when a Schottky diode is formed using a droplet of liquid metal, e. Depending on the doping type and density in the semiconductor, the droplet spreading depends on the magnitude and sign of the voltage applied to the mercury droplet. From Wikipedia, the free encyclopedia. Not to be confused with Shockley diode. Various Schottky-barrier diodes: Small-signal RF devices left , medium- and high-power Schottky rectifying diodes middle and right.
This section needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. Power Semiconductor Devices". Electrical engineer's reference book. Retrieved The Art of Analog Layout 2nd ed. Prentice Hall. Schottky rectifiers seldom exceed volts in their working peak reverse voltage since devices moderately above this rating level will result in forward voltages equal to or greater than equivalent pn junction rectifiers.
Power Electronics. Analog Integrated Circuit Design , Wiley. Page Couto Jr. Puebla, E. Chekhovich, I. Luxmoore, C. Elliott, N. Babazadeh, M. Skolnick, and A. Arscott and M. Gaudet "Electrowetting at a liquid metal-semiconductor junction" Appl. Electronic components.
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