Schottky diode

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The Schottky diode (named after German physicist Walter H. Schottky), also known as the Schottky barrier diode or hot carrier diode , is a diode semiconductor formed by semiconductor junction with metal. It has low voltage drop forward and very fast switching action. The cat whiskers detector used in the early days of wireless rectifiers and metals used in early power applications can be considered primitive Schottky diodes.

When an adequate forward voltage is applied, the current flows forward. The silicon diode has a forward voltage of about 600-700 mV, whereas Schottky's forward voltage is 150-450 mV. These lower forward voltage requirements allow for higher switching speeds and better system efficiency.

Video Schottky diode

Construction

A metal-semiconductor junction formed between metal and semiconductor, creates a Schottky barrier (not a semiconductor-like semiconductor conventional diode). Commonly used metals are molybdenum, platinum, chromium or tungsten, and certain silicides (eg, palladium silicides and platinum silicides), whereas semiconductors are usually n-type silicon. The metal side acts as anode, and the n-type semiconductor acts as a diode cathode. This Schottky barrier produces a very fast transition and low forward voltage drop.

Selection of metal and semiconductor combination determines the forward voltage of the diode. Both n-and-p-type semiconductors can develop Schottky barriers. However, the p-type usually has a much lower forward voltage. When reverse leakage current increases dramatically by lowering forward voltage, it can not be too low, so the normally used range is about 0.5-0.7 V, and p-type semiconductors are rarely used. Titanium silicide and other refractory silicides, which can withstand the temperature required for source/drain annealing in CMOS processes, typically have too low a forward voltage to be useful, so the process using these silicides therefore usually does not offer Schottky diodes.

With increasing doping of semiconductors, the width of the thinning region decreases. Under a certain width, the payload operator can penetrate the depletion region. At very high doping levels, the intersection does not behave as a rectifier anymore and becomes an ohmic contact. This can be used for the simultaneous formation of ohmic contacts and diodes, since the diodes will form between the n-type silicide and light regions, and the ohmic contacts will form between silicides and p-d-type or p-type which are doped.. A lightly doped p-type area poses a problem, since the resulting contact has too high a resistance to good ohmic contact, but too low forward voltage and too high reverse leakage to make a good diode.

Since the sides of the Schottky contacts are quite sharp, a high electric field gradient takes place around them, which limits how much the threshold voltage of the return voltage can be. Various strategies are used, from guard rings to overlapping metallization to deploy field gradients. The guard rings consume valuable dead areas and are used primarily for larger high-voltage diodes, while overlapping metallization is used mainly with smaller lower-voltage diodes.

Schottky diodes are often used as antisaturation clamps in Schottky transistors. The Schottky diode made of palladium silicide (PdSi) is excellent because the forward forward voltage (which must be lower than the forward voltage of the base-collector junction). The Schottky temperature coefficient is lower than the B-C connection coefficient, which limits the use of PdSi at higher temperatures.

For Schottky power diodes, parasitic resistance of the buried n layers and epitaxial-type layers becomes important. The resistance of epitaxial layers is more important than for transistors, because currents must traverse the entire thickness. However, functioning as a ballast resistor is distributed throughout the junction area and, under ordinary conditions, prevents localized thermal runaway.

Compared with p-n power diodes, Schottky diodes are less rough. The junction of direct contact with heat-sensitive metallization, the Schottky diode can therefore eliminate less power than the equivalent p-n counterpart of a deep connection buried before failure (especially during a reversed reversal). The relative advantage of the lower forward voltage of the Schottky diode is reduced at a higher forward current, where the voltage drop is dominated by the series resistance.

Maps Schottky diode

Back up recovery time

The most important difference between a p-n diode and a Schottky diode is reversed recovery time (t _rr ), when the diode switches from the conductor to the nonconducting state. In p-n diode, reverse recovery time can be in the order of several microseconds up to less than 100 seconds for fast diodes. Schottky diodes do not have recovery time, because nothing needs to be recovered from (ie, there is no charge depletion operator area at the intersection). Redirection times are ~ 100 ps for small-signal diodes, and up to tens of nanoseconds for specialized capacity of high capacity power diodes. With p-n-junction switching, there is also reverse feedback, which in high power semiconductors brings increased EMI noise. With a Schottky diode, the transition is essentially "instantaneous" with little capacitive loading, which is less of a concern.

This "instant" switch is not always the case. In high voltage Schottky devices, in particular, the guard ring structure required to control the breakdown geometry field creates a parasitic p-n diode with the usual time-recovery attribute. As long as this guard ring is not forward biased, it just adds capacitance. However, if the Schottky junction is driven hard enough, the forward voltage will eventually bias both the forward diode and the actual t _rr will be very impactful.

It is often said that Schottky diodes are semiconductor devices "carriers of the majority". This means that if the semiconductor body is a n-type doped, only the n-type operator (cellular electron) plays an important role in the normal operation of the device. The majority carriers are rapidly injected into the conduction band from metal contacts on the other side of the diode into free-moving electrons. Therefore, there is no slow random recombination of the n and p-type carriers involved, so that the diode can stop conduction faster than the usual p-n rectifier diode. This property in turn allows a smaller area of ​​the device, which also makes the transition much faster. This is another reason why Schottky diodes are useful in switch-mode power converters: high-speed diodes mean that circuits can operate at frequencies in the range of 200 kHz up to 2 MHz, enabling the use of small inductors and capacitors with greater efficiency. than is possible with other types of diodes. The small area Schottky diode is the heart of the RF detector and mixer, which often operates at frequencies up to 50 GHz.

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Limitations

The most obvious limitation of the Schottky diode is its relatively low reverse voltage rating, and its relatively high reverse current. For a silicon-metal Schottky diode, the return voltage is usually 50 V or less. Some high voltage designs are available (200 V is considered a high turning voltage). The upturned leakage current, as it rises with temperature, leads to a problem of thermal instability. This often limits useful back voltages to well below the actual ratings.

While higher back voltages can be achieved, they will present a higher forward voltage, proportional to other types of standard diodes. Such Schottky diodes will have no gain unless a large switching speed is required.

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Silicon carbide Schottky diode

Schottky diodes made from silicon carbide have a reverse current which is much lower than that of Schottky silicon diodes, as well as higher forward and reverse voltages. In 2011 they are available from manufacturers in variants up to 1700 V reverse voltage.

Silicon carbide has high thermal conductivity, and temperature has little effect on switching and thermal characteristics. With special packaging, Schottky silicon carbide diodes can operate at a connection temperature of more than 500 Â ° K (about 200 Â ° C), which allows passive radiation cooling in aerospace applications.

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Apps

Voltage clamping

While the standard silicon diode has a forward voltage drop of about 0.6 V and a 0.2 V germanium diode, the Schottky diode voltage drop in forward bias of about 1 mA is in the range of 0.15 V to 0.46 V (see 1N5817 and 1N5711), which makes them useful in voltage clamping applications and prevention of transistor saturation. This is due to the higher current density in Schottky diodes.

Reverses current and disposal protection

Due to the low-voltage low-rise Schottky diode; less energy is wasted as heat, making it the most efficient choice for efficiency-sensitive applications. For example, they are used in stand-alone ("off-grid") photovoltaic (PV) systems to prevent batteries from using through solar panels at night, called "blocking diodes." They are also used in network-connected systems with multiple strings connected in parallel, to prevent backflows from flowing from adjacent strings via shaded strings if "bypass diodes" have failed.

Modified mode power supply

Schottky diodes are also used as rectifiers in switched-mode power supplies. Low forward voltage and fast recovery time leads to increased efficiency.

They can also be used in "OR" supply circuits in products that have internal batteries and power adapter inputs, or similar. However, a high reverse reverse current presents a problem in this case, since any high impedance voltage sensing circuit (eg, monitoring the battery voltage or detecting whether an power adapter exists) will see the voltage from other power sources through the leakage diode.

Sample-and-hold circuit

Schottky diodes can be used in series samples and bridge-resistant circuit diodes. When compared to a common p-n junction-based diode bridge, Schottky diodes can offer advantages. The advanced biased schottky diodes do not have the storage of minority carrier costs. This allows them to switch faster than ordinary diodes, resulting in a lower transition time from the sample to the hold step. The absence of minority carriers' storage costs also results in lower step-retardance or sampling errors, resulting in more accurate sampling of outputs.

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Setting

Commonly encountered Schottky diodes include 1N58xx series rectifiers, such as 1n581x (1 ampere) and 1N582x (3 ampere) passages through holes, and surface-mounted SS1x (1 ampere) and SS3x (3 amperes) sections. The Schottky rectifier is available in a wide range of surface-mounted style packages.

Small signal schottky diodes such as the 1N5711 series, 1N6263, 1SS106, 1SS108, and BAT41-43, 45-49 are widely used in high frequency applications as detectors, mixers and nonlinear elements, and have replaced germanium diodes. They are also suitable for protection of electrostatic discharges (ESD) from sensitive devices such as III-V-semiconductor devices, laser diodes and, to a lesser extent, the line of exposure to CMOS circuits.

Schottky metal-semiconductor junctions are shown in the successors to the 7400 TTL family of logic devices, 74S, 74LS and 74ALS series, where they are employed as a Baker brace in parallel with bipolar transistor-base collector intersections to prevent their saturation, thus greatly reducing the turn-off delay they.

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Alternative

When less power dissipation is desired, MOSFETs and control circuits can be used instead, in an operating mode known as active rectification.

A super diode consisting of a pn-diode or Schottky diode and operational amplifier provides almost perfect diode characteristics due to negative feedback effects, although its use is limited to the operational amplifier frequencies used to handle.

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Electrowetting

Electrowetting can be observed when a Schottky diode is formed using a liquid metal droplet, eg. mercury, contact with semiconductors, eg. silicon. Depending on the type of doping and density in the semiconductor, the droplet deployment depends on the magnitude and the voltage sign applied to the mercury droplets. This effect has been called 'Schottky electrowetting'.

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See also

• Schottky Barrier
• Schottky effect (Schottky emissions)
• Heterostructure barrier varactor diode
• Active rectification
• Baker brace and Schottky transistor
• 1N58xx Schottky diodes
• Electrowet

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References

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External links

• Video tutorial on schottky diode characteristics
• "Characteristics of Schottky Diodes" - PowerGuru
• "Introduction to Schottky Rectifiers"
• "Does the lowest forward voltage drop from a real schottky diode always be the best option?" Technical application, IXYS Corporation.
• "Schottky diode" in Electronic Notes

Source of the article : Wikipedia