Schottky diode

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Schottky diode
Schottky.jpg
Various Schottky-barrier diodes: Small-signal RF devices (left), medium- and high-power Schottky rectifying diodes (middle and right)
Type Passive
Invented Walter H. Schottky
Pin configuration anode and cathode
Electronic symbol
Schottky diode symbol.svg

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.

Walter H. Schottky German physicist

Walter Hans Schottky was a German physicist who played a major early role in developing the theory of electron and ion emission phenomena, invented the screen-grid vacuum tube in 1915 while working at Siemens, co-invented the ribbon microphone and ribbon loudspeaker along with Dr. Erwin Gerlach in 1924 and later made many significant contributions in the areas of semiconductor devices, technical physics and technology.

Diode abstract electronic component with two terminals that allows current to flow in one direction

A diode is a two-terminal electronic component that conducts current primarily in one direction ; it has low resistance in one direction, and high resistance in the other. A diode vacuum tube or thermionic diode is a vacuum tube with two electrodes, a heated cathode and a plate, in which electrons can flow in only one direction, from cathode to plate. A semiconductor diode, the most commonly used type today, is a crystalline piece of semiconductor material with a p–n junction connected to two electrical terminals. Semiconductor diodes were the first semiconductor electronic devices. The discovery of asymmetric electrical conduction across the contact between a crystalline mineral and a metal was made by German physicist Ferdinand Braun in 1874. Today, most diodes are made of silicon, but other materials such as gallium arsenide and germanium are used.

A semiconductor material has an electrical conductivity value falling between that of a conductor, such as metallic copper, and an insulator, such as glass. Its resistance decreases as its temperature increases, which is behaviour opposite to that of a metal. Its conducting properties may be altered in useful ways by the deliberate, controlled introduction of impurities ("doping") into the crystal structure. Where two differently-doped regions exist in the same crystal, a semiconductor junction is created. The behavior of charge carriers which include electrons, ions and electron holes at these junctions is the basis of diodes, transistors and all modern electronics. Some examples of semiconductors are silicon, germanium, gallium arsenide, and elements near the so-called "metalloid staircase" on the periodic table. After silicon, gallium arsenide is the second most common semiconductor and is used in laser diodes, solar cells, microwave-frequency integrated circuits and others. Silicon is a critical element for fabricating most electronic circuits.

Contents

When sufficient forward voltage is applied, a current flows in the forward direction. A silicon diode has a typical forward voltage of 600–700 mV, while the Schottky's forward voltage is 150–450 mV. This lower forward voltage requirement allows higher switching speeds and better system efficiency.

Construction

1N5822 Schottky diode with cut-open packaging. The semiconductor in the center makes a Schottky barrier against one metal electrode (providing rectifying action) and an ohmic contact with the other electrode. Schottky Diode Section.JPG
1N5822 Schottky diode with cut-open packaging. The semiconductor in the center makes a Schottky barrier against one metal electrode (providing rectifying action) and an ohmic contact with the other electrode.

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.g., palladium silicide and platinum silicide), whereas the semiconductor would typically be n-type silicon. [1] The metal side acts as the anode, and n-type semiconductor acts as the cathode of the diode; meaning conventional current can flow from the metal side to the semiconductor side, but not in the opposite direction. This Schottky barrier results in both very fast switching and low forward voltage drop.

In solid-state physics, a metal–semiconductor (M–S) junction is a type of electrical junction in which a metal comes in close contact with a semiconductor material. It is the oldest practical semiconductor device. M–S junctions can either be rectifying or non-rectifying. The rectifying metal–semiconductor junction forms a Schottky barrier, making a device known as a Schottky diode, while the non-rectifying junction is called an ohmic contact.

Schottky barrier potential energy barrier in metal-semiconductor junctions

A Schottky barrier, named after Walter H. Schottky, is a potential energy barrier for electrons formed at a metal–semiconductor junction. Schottky barriers have rectifying characteristics, suitable for use as a diode. One of the primary characteristics of a Schottky barrier is the Schottky barrier height, denoted by ΦB. The value of ΦB depends on the combination of metal and semiconductor.

p–n junction semiconductor–semiconductor junction, formed at the boundary between a p-type and n-type semiconductor

A p–n junction is a boundary or interface between two types of semiconductor materials, p-type and n-type, inside a single crystal of semiconductor. The "p" (positive) side contains an excess of holes, while the "n" (negative) side contains an excess of electrons in the outer shells of the electrically neutral atoms there. This allows electrical current to pass through the junction only in one direction. The p-n junction is created by doping, for example by ion implantation, diffusion of dopants, or by epitaxy. If two separate pieces of material were used, this would introduce a grain boundary between the semiconductors that would severely inhibit its utility by scattering the electrons and holes.

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 0.5–0.7 V, and p-type semiconductors are employed only rarely. Titanium silicide and other refractory silicides, which are able to withstand the temperatures needed for source/drain annealing in CMOS processes, usually have too low a forward voltage to be useful, so processes using these silicides therefore usually do not offer Schottky diodes.[ clarification needed ]

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.

Schottky transistor combination of a transistor and a Schottky diode that prevents the transistor from saturating by diverting the excessive input current

A Schottky transistor is a combination of a transistor and a Schottky diode that prevents the transistor from saturating by diverting the excessive input current. It is also called a Schottky-clamped transistor.

For power Schottky diodes, the parasitic resistances of the buried n+ layer and the epitaxial n-type layer become important. 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. [2]

Reverse recovery time

The most important difference between the p-n diode and the Schottky diode is the reverse recovery time (trr), when the diode switches from the conducting to the non-conducting state. In a p–n diode, the reverse recovery time can be in the order of several microseconds to less than 100 ns for fast diodes. Schottky diodes do not have a recovery time, as there is nothing to recover from (i.e., there is no charge carrier depletion region at the junction). The switching time is ~100 ps for the small-signal diodes, and up to tens of nanoseconds for special high-capacity power diodes. 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 trr 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 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. This is another reason why Schottky diodes are useful in switch-mode power converters: the high speed of the diode means that the circuit can operate at frequencies in the range 200 kHz to 2 MHz, allowing the use of small inductors and capacitors with greater efficiency than would be possible with other diode types. Small-area Schottky diodes are the heart of RF detectors and mixers, which often operate at frequencies up to 50 GHz.

Limitations

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 (200 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 [3] unless great switching speed is required.

Silicon carbide Schottky diode

Schottky diodes constructed from silicon carbide have a much lower reverse leakage current than silicon Schottky diodes, as well as higher forward voltage (about 1.4-1.8V at 25 °C) and reverse voltage. As of 2011 they were available from manufacturers in variants up to 1700 V of reverse voltage. [4]

Silicon carbide has a high thermal conductivity, and temperature has little influence on its switching and thermal characteristics. With special packaging, silicon carbide Schottky diodes can operate at junction temperatures of over 500  K (about 200 °C), which allows passive radiative cooling in aerospace applications. [4]

Applications

Voltage clamping

While standard silicon diodes have a forward voltage drop of about 0.6 V and germanium diodes 0.2 V, Schottky diodes' voltage drop at forward biases of around 1 mA is in the range of 0.15 V to 0.46 V (see the 1N5817 [5] and 1N5711 [6] ), which makes them useful in voltage clamping applications and prevention of transistor saturation. This is due to the higher current density in the Schottky diode.

Reverse current and discharge protection

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.

Switched-mode power supplies

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.g., monitoring the battery voltage or detecting whether a mains adapter is present) will see the voltage from the other power source through the diode leakage.

Sample-and-hold circuits

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. [7]

Charge control

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. [8]

Designation

SS14 schottky diode in a
DO-214AC (SMA) package
(surface mount version of 1N5819) SS14 1A DO-214 Schottky diode.jpg
SS14 schottky diode in a
DO-214AC (SMA) package
(surface mount version of 1N5819)

Commonly encountered schottky diodes include the 1N58xx series rectifiers, such as the 1N581x (1 ampere) and 1N582x (3 ampere) through-hole parts, [5] [10] and the SS1x (1 ampere) and SS3x (3 ampere) surface-mount parts. [9] [11] Schottky rectifiers are available in numerous surface-mount package styles. [12] [13]

Small-signal schottky diodes such as the 1N5711, [6] 1N6263, [14] 1SS106, [15] 1SS108, [16] and the BAT41–43, 45–49 series [17] are widely used in high-frequency applications as detectors, mixers and nonlinear elements, and have superseded germanium diodes. [18] They are also suitable for electrostatic discharge (ESD) protection of sensitive devices such as III-V-semiconductor devices, laser diodes and, to a lesser extent, exposed lines of CMOS circuitry.

Schottky metal–semiconductor junctions are featured in the successors to the 7400 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.

Alternatives

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

Electrowetting can be observed when a Schottky diode is formed using a droplet of liquid metal, e.g. mercury, in contact with a semiconductor, e.g. silicon. 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. [19] This effect has been termed ‘Schottky electrowetting’. [20]

See also

Related Research Articles

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MOSFET transistor used for amplifying or switching electronic signals

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A bipolar junction transistor is a type of transistor that uses both electrons and holes as charge carriers.

Thyristor semiconductor device with three or more p-n junctions, having two steady states: off (non-conducting) and on (conducting)

A thyristor is a solid-state semiconductor device with four layers of alternating P- and N-type materials. It acts exclusively as a bistable switch, conducting when the gate receives a current trigger, and continuing to conduct until the voltage across the device is reversed biased, or until the voltage is removed. A three-lead thyristor is designed to control the larger current of the Anode to Cathode path by controlling that current with the smaller current of its other lead, known as its Gate. In contrast, a two-lead thyristor is designed to switch on if the potential difference between its leads is sufficiently large.

Varicap semiconductor diode used as voltage-controlled capacitor

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Silicon controlled rectifier semiconductor electronic device with three p-n junctions, mainly used in devices where the control of high power is demanded

A silicon controlled rectifier or semiconductor controlled rectifier is a four-layer solid-state current-controlling device. The principle of four-layer p–n–p–n switching was developed by Moll, Tanenbaum, Goldey and Holonyak of Bell Laboratories in 1956. The practical demonstration of silicon controlled switching and detailed theoretical behavior of a device in agreement with the experimental results was presented by Dr Ian M. Mackintosh of Bell Laboratories in January 1958. The name "silicon controlled rectifier" is General Electric's trade name for a type of thyristor. The SCR was developed by a team of power engineers led by Gordon Hall and commercialized by Frank W. "Bill" Gutzwiller in 1957.

Tunnel diode type of semiconductor diode

A tunnel diode or Esaki diode is a type of semiconductor diode that has negative resistance due to the quantum mechanical effect called tunneling. It was invented in August 1957 by Leo Esaki, Yuriko Kurose, and Takashi Suzuki when they were working at Tokyo Tsushin Kogyo, now known as Sony. In 1973, Esaki received the Nobel Prize in Physics, jointly with Brian Josephson, for discovering the electron tunneling effect used in these diodes. Robert Noyce independently devised the idea of a tunnel diode while working for William Shockley, but was discouraged from pursuing it. Tunnel diodes were first manufactured by Sony in 1957, followed by General Electric and other companies from about 1960, and are still made in low volume today.

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Metal rectifier

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Power MOSFET power MOS field-effect transistor

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1N4148 signal diode standard silicon switching diode

The 1N4148 is a standard silicon switching signal diode. It is one of the most popular and long-lived switching diodes because of its dependable specifications and low cost. Its name follows the JEDEC nomenclature. The 1N4148 is useful in switching applications up to about 100 MHz with a reverse-recovery time of no more than 4 ns.

Active rectification

Active rectification, or synchronous rectification, is a technique for improving the efficiency of rectification by replacing diodes with actively controlled switches such as transistors, usually power MOSFETs or power BJTs. Whereas normal semiconductor diodes have a roughly fixed voltage drop of around 0.5-1 volts, active rectifiers behave as resistances, and can have arbitrarily low voltage drop.

In electronics, leakage is the gradual transfer of electrical energy across a boundary normally viewed as insulating, such as the spontaneous discharge of a charged capacitor, magnetic coupling of a transformer with other components, or flow of current across a transistor in the "off" state or a reverse-polarized diode.

1N58xx Schottky diodes

The 1N58xx is a series of medium power, fast, low voltage Schottky diodes, which consists of part number numbers 1N5817 through 1N5825.

References

  1. ‘’Laughton, M. A. (2003). "17. Power Semiconductor Devices". Electrical engineer's reference book. Newnes. pp. 25–27. ISBN   978-0-7506-4637-6 . Retrieved 2011-05-16.
  2. Hastings, Alan (2005). The Art of Analog Layout (2nd ed.). Prentice Hall. ISBN   0-13-146410-8.
  3. "Introduction to Schottky Rectifiers" (PDF). MicroNotes. 401. Schottky rectifiers seldom exceed 100 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.
  4. 1 2 "Schottky Diodes: the Old Ones Are Good, the New Ones Are Better". Power Electronics.
  5. 1 2 "1N5817 Datasheets (PDF)". Datasheetcatalog.com. Retrieved 2013-01-14.
  6. 1 2 "1N5711 Datasheets (PDF)". Datasheetcatalog.com. Retrieved 2013-01-14.
  7. Johns, David A. and Martin, Ken. Analog Integrated Circuit Design (1997), Wiley. Page 351. ISBN   0-471-14448-7
  8. O. D. D. Couto Jr., J. Puebla, E. A. Chekhovich, I. J. Luxmoore, C. J. Elliott, N. Babazadeh, M. S. Skolnick, and A. I. Tartakovskii Charge control in InP/(Ga,In)P single quantum dots embedded in Schottky diodes (2011), Physical Review B 84, 125301. doi : 10.1103/PhysRevB.84.125301
  9. 1 2 "SS14 Datasheets (PDF)". Datasheetcatalog.com. Retrieved 2013-11-23.
  10. "1N5820 Datasheets (PDF)". Datasheetcatalog.com. Retrieved 2013-11-23.
  11. "SS34 Datasheets (PDF)". Datasheetcatalog.com. Retrieved 2013-11-23.
  12. Bourns Schottky Rectifiers.
  13. Vishay Schottky Rectifiers.
  14. "1N6263 Datasheets (PDF)". Datasheetcatalog.com. Retrieved 2013-01-14.
  15. "1SS106 Datasheets (PDF)". Datasheetcatalog.com. Retrieved 2013-01-14.
  16. "1SS108 Datasheets (PDF)". Datasheetcatalog.com. Retrieved 2013-01-14.
  17. "BAT4 Datasheets (PDF)". Datasheetcatalog.com. Retrieved 2013-01-14.
  18. Vishay Small-Signal Schottky Diodes.
  19. S. Arscott and M. Gaudet "Electrowetting at a liquid metal-semiconductor junction" Appl. Phys. Lett. 103, 074104 (2013). doi : 10.1063/1.4818715
  20. S. Arscott "Electrowetting and semiconductors" RSC Advances 4, 29223 (2014). doi : 10.1039/C4RA04187A