Diode

A diode is an electronic component with two terminals that allows electric current to flow through it in only one direction, blocking its passage if the current flows in the opposite direction, not only It is used for the circulation of electric current but rather it controls and resists it. This causes the diode to have two possible positions: one with the current (forward biased) and one against the current (reverse biased).

Structure of the diode.

This term is generally used to refer to the semiconductor diode, the most common nowadays; It consists of a piece of semiconductor crystal connected to two electrical terminals. The vacuum diode (now no longer used, except for high power technologies) is a vacuum tube with two electrodes: a foil as anode, and a cathode.

In a simplified way, the characteristic curve of a diode (I-V) consists of two regions: below a certain potential difference, it behaves as an open circuit (does not conduct), and above it as a closed circuit with a very small electrical resistance. Due to this behavior, they are often called rectifiers, since they are devices capable of suppressing the negative part of any signal, as an initial step to convert an alternating current into a direct current. Their operating principle is based on the experiments of Lee De Forest.

The first diodes were vacuum tubes or valves, also called thermionic valves, consisting of two electrodes surrounded by vacuum in a glass tube, with an appearance similar to that of incandescent lamps. The invention was developed in 1904 by John Ambrose Fleming, an employee of the Marconi company, based on observations made by Thomas Alva Edison.

Like incandescent lamps, vacuum tubes have a filament (the cathode) through which current flows, heating it by the Joule effect. The filament is treated with barium oxide, so that when it is heated it emits electrons into the surrounding vacuum, which are electrostatically conducted towards a plate, curved by a double spring, positively charged (the anode), thus producing conduction. Obviously, if the cathode does not heat up, it will not be able to give up electrons. For that reason, circuits using vacuum tubes required time for the tubes to warm up before they could work, and the tubes burned very easily.

History

Vacuum diode, commonly used until the invention of the semiconductor diode, the latter also called solid state diode.

Although the solid-state semiconductor diode became popular before the thermionic diode, both were developed at the same time.

In 1873 Frederick Guthrie discovered the principle of operation of thermal diodes. Guthrie discovered that a positively charged electroscope could be discharged by approaching a hot piece of metal, without the need for it to touch it. The same was not the case with a negatively charged electroscope, reflecting this that current flow was only possible in one direction.

Independently, on February 13, 1880, Thomas Edison rediscovered the principle. In turn, Edison was investigating why the carbon filaments in light bulbs burned out at the positive end. He had built a bulb with an extra filament and one with a metal foil inside the lamp, electrically insulated from the filament. When he used this device, he confirmed that a current did flow from the glowing filament through a vacuum to the metal foil, but this only happened when the foil was positively connected.

Edison designed a circuit that replaced the bulb with a resistor with a DC voltmeter, and obtained a patent for this invention in 1884. It apparently had no practical use at the time. So the patent was probably a precaution, in case someone found a use for the so-called Edison effect.

About 20 years later, John Ambrose Fleming (consulting scientist at the Marconi Company and former Edison employee) realized that the Edison effect could be used as a precision radio detector. Fleming patented the first thermionic diode in Great Britain on November 16, 1904.

In 1874 German scientist Karl Ferdinand Braun discovered the unidirectional conducting nature of semiconductor crystals. Braun patented the crystal rectifier in 1899. Selenium copper oxide rectifiers were developed for high power applications in the 1930s.

Indian scientist Jagdish Chandra Bose was the first to use a semiconductor crystal to detect radio waves in 1894. The semiconductor crystal detector was developed into a practical device for receiving wireless signals by Greenleaf Whittier Pickard, who invented a silicon crystal detector in 1903 and received a patent for it on November 20, 1906. Other experiments tested a wide variety of substances, of which the mineral galena was widely used. Other substances offered slightly better performance, but galena was the most widely used because it had the advantage of being cheap and easy to obtain. At the beginning of the radio era, the semiconductor crystal detector consisted of an adjustable wire (the so-called cat's whisker) which could be manually moved across the crystal to obtain an optimal signal. This problematic device was quickly superseded by thermionic diodes, although the semiconductor crystal detector returned to frequent use with the advent of inexpensive germanium diodes in the 1950s.

At the time of their invention, these devices were known as rectifiers. In 1919, William Henry Eccles coined the term diode from the Greek dia, meaning separate, and ode (from ὅδος), meaning path.

Thermionic and gas-state diodes

Symbol of a vacuum or gaseous diode. From top to bottom, its components are, the anode, the cathode, and the filament.

Thermionic diodes are thermionic valve (also known as vacuum tube) devices, consisting of an array of electrodes packed in a glass under vacuum. The first models were very similar to the incandescent lamp.

In thermoionic valve diodes, a current through the filament to be heated indirectly heats the cathode, another internal electrode treated with a mixture of Barium and strontium oxide, which are alkaline earth oxides; these substances are chosen because they have a small work function (some tubes use direct heating, where a tungsten filament acts as both heater and cathode). Heating causes thermionic emission of electrons in a vacuum. In forward bias, the anode was positively charged, thus attracting electrons. However, electrons were not easily transported from the unhot anode surface when the thermionic valve was reverse biased. Also, any current in this case is negligible.

For most of the xx century, thermoionic valve diodes were used in analog signal, rectifier, and power applications. Currently, valve diodes are only used in exclusive applications such as rectifiers in electric guitars, audio amplifiers, as well as specialized high-voltage equipment.

Semiconductor diode

Training of the region of exhaustion, on the chart z.c.e.

A modern semiconductor diode is made of a semiconductor crystal like silicon with impurities in it to create a region containing negative charge carriers (electrons), called the n-type semiconductor, and a region on the other side containing negative charge carriers (electrons). positive charge (holes), called a p-type semiconductor. The diode leads are attached to each region. The in-crystal boundary of these two regions, called a PN junction, is where the importance of the diode takes its place. The crystal conducts a stream of electrons from the n side (called the cathode), but not in the opposite direction; that is, when a conventional current flows from the anode to the cathode (opposite to the flow of electrons).

By joining both crystals, a diffusion of electrons from crystal n to crystal p (J_e) is manifested. When a diffusion current is established, fixed charges appear in an area on both sides of the junction, an area that is called the depletion region.

As the diffusion process progresses, the depletion region increases in width, deepening the crystals on both sides of the junction. However, the accumulation of positive ions in zone n and negative ions in zone p creates an electric field (E) that will act on the free electrons in zone n with a certain displacement force, which will oppose the current of electrons and will end up stopping them.

This electric field is equivalent to saying that a voltage difference appears between zones p and n. This potential difference (V_D) is 0.7 V for silicon and 0.3 V for germanium crystals.

The width of the depletion region, once equilibrium has been reached, is usually of the order of 0.5 microns, but when one of the crystals is much more doped than the other, the area of space charge is much larger.

When the diode is subjected to an external voltage difference, it is said that the diode is polarized, and it can be direct or reverse polarization.

Forward bias of a diode

Direct polarization of the pn diode.

In this case, the battery lowers the potential barrier of the space charge zone, allowing the passage of electron current through the junction; that is, the forward biased diode conducts electricity.

For a diode to be forward biased, the positive pole of the battery must be connected to the anode of the diode and the negative pole to the cathode. Under these conditions we can see that:

• The negative pole of the battery repels the crystal-free electrons, so these electrons are directed towards the p-n union.
• The positive pole of the battery attracts the electrons of valence of the crystal p, this is equivalent to saying that it pushes the holes towards the p-n union.
• When the potential difference between the terminals of the battery is greater than the potential difference in the space load zone, the crystal-free electrons n, acquire the energy sufficient to jump to the holes of the crystal p, which have previously moved to the p-n binding.
• Once a free electron from the n zone jumps to the p zone through the space load zone, it falls into one of the multiple holes of the p zone becoming a valence electron. Once this occurred the electron is attracted by the positive pole of the battery and moves from atom to atom until reaching the end of the crystal p, from which it enters into the conductor thread and reaches the battery.

In this way, with the battery giving up free electrons to zone n and attracting valence electrons from zone p, a constant electric current appears through the diode until the end.

Reverse bias of a diode

Reverse polarization of the pn diode.

In this case, the negative pole of the battery is connected to zone p and the positive pole to zone n, which increases the space charge zone, and the voltage in said zone until the value of the battery voltage, as explained below:

• The positive pole of the battery attracts the free electrons of the zone n, which come out of the crystal n and are introduced into the driver within which they move to the battery. As the free electrons leave the zone n, the pentavalent atoms that were previously neutral, being removed from their electron in the orbital conduction, acquire stability (8 electrons in the valence layer, see semiconductor and atom) and a net electric charge of +1, making them positive ions.
• The negative pole of the battery yields free electrons to the trivalent atoms of the zone p. Let us remember that these atoms only have 3 electrons of valence, so once they have formed the covalent bonds with the silicon atoms, they have only 7 electrons of valence, being the electron that is missing the so-called hollow. The case is that when the free electrons ceded by the battery enter the p zone, they fall into these holes with what the trivalent atoms acquire stability (8 electrons in their valence orbital) and a net electrical charge of -1, thus becoming negative ions.
• This process is repeated over and over again until the space charging area acquires the same power potential as the battery.

In this situation, the diode should not carry current; however, due to the effect of temperature, electron-hole pairs (see semiconductor) will form on both sides of the junction, producing a small current (of the order of 1 μA) called inverse saturation current . In addition, there is also a so-called surface leakage current which, as its name suggests, conducts a small current across the surface of the diode; since on the surface, the silicon atoms are not surrounded by enough atoms to make the four covalent bonds necessary for stability. This means that the atoms on the surface of the diode, both in the n and p zones, have holes in their valence orbital, so that the electrons circulate without difficulty through them. However, like the reverse saturation current, the surface leakage current is usually negligible.

Diode characteristic curve

Curve characteristic of the diode.

• Threshold tension, elbow or departure (V)_γ ).
  The threshold voltage (also called the potential barrier) of direct polarization coincides in value with the voltage of the non-polarized diode space load zone. By polarizing the diode directly, the initial potential barrier is shrinking, increasing the current slightly, about 1% of the nominal. However, when external tension exceeds the threshold tension, the potential barrier disappears, so that for small increases of tension there are large variations of current intensity.
• Maximum current (I)_max ).
  It is the maximum current intensity that can lead the diode without melting by the Joule effect. Since it is function of the amount of heat that can dissipate the diode, it depends mainly on the design of the diode.
• Reverse current of saturation (I)_s ).
  It is the small current that is established by reverse polarizing the diode by the formation of electron-bone pairs due to temperature, admitting that it is duplicated by each increase of 10 °C in temperature.
• Surface flow of leaks.
  It is the small current that circulates through the surface of the diode (see reverse polarization), this current is the function of the voltage applied to the diode, thus increasing the surface current of leaks.
• Breakdown voltage (V)_r ).
  It is the maximum reverse tension that the diode can endure before giving the avalanche effect.

Theoretically, by reverse biasing the diode, it will conduct reverse saturation current; In reality, from a certain voltage value, in the normal or abrupt junction diode, the breakdown is due to the avalanche effect; However, there are other types of diodes, such as Zener diodes, in which the breakdown can be due to two effects:

• Avalanche effect (doughly doped) Converse polarization generates electron-bone pairs that cause the reverse current of saturation; if the reverse tension is high the electrons are accelerated by increasing their kinetic energy so that by hitting with valence electrons can cause their leap to the driving band. These released electrons, in turn, are accelerated by voltage effect, colliding with more valence electrons and releasing them in turn. The result is a avalanche of electrons that causes a large current. This phenomenon occurs for voltage values above 6 V.
• Zener effect (very doped fingers). The more doping the material is, the less the width of the loading area. Since the E electric field can be expressed as a quotient of voltage V between distance d; when the diode is very doped, and therefore d is small, the electric field will be large, of order 3·10⁵V/cm. In these conditions, the field itself can be able to boot electrons of valence by increasing the current. This effect is produced for 4 V or lower voltages.

For inverse voltages between 4 and 6 V, the breakdown of these special diodes, such as Zener diodes, can be produced by both effects.

Mathematical models

The most widely used mathematical model is the Shockley model (in honor of William Bradford Shockley) which allows us to approximate the behavior of the diode in most applications. The equation that relates the current intensity and the potential difference is:

I=IS(eVD/(nVT)− − 1),{displaystyle I=I_{mathrm {S} }left(e^{V_{mathrm {D}/(nV_{mathrm {T} })}-1right),,}

Where:

• I is the intensity of the current that passes through the diode
• V_D is the difference of tension between its ends.
• I_S is the current of saturation (approximately 10− − 12A{displaystyle 10^{-12}A})
• n is the emission coefficient, dependent on the diode manufacturing process and which usually adopts values between 1 (for germanium) and 2 (for silicon).

The Thermal Voltage V_T is approximately 25.85 mV at 300 K, a temperature close to room temperature, widely used in circuit simulation programs. For every temperature there is a known constant defined by:

VT=kTq{displaystyle V_{mathrm {T} }={frac {kT}{q}{}},}

Where k is the Boltzmann constant, T is the absolute temperature of the pn junction, and q is the magnitude of the charge on an electron (the elementary charge).

Shockley's ideal diode equation or diode law is derived by assuming that only the processes that give current to the diode are by flux (due to the electric field), diffusion, and thermal recombination. It also assumes that the recombination current in the depletion region is negligible. This means that Shockley's equation does not take into account the processes related to the breakdown region and induction by photons. Additionally, it does not describe the stabilization of the I-V curve in active bias due to internal resistance.

Under negative voltages, the exponential in the diode equation is negligible. and the current is a negative constant of the value of I_s. The breakdown region is not modeled in the Shockley diode equation.

For large voltages, in the forward bias region, the 1 can be eliminated from the equation, leaving as a result:

I=ISeVD/(nVT){displaystyle I=I_{mathrm {S} }e^{V_{mathrm {D} }/(nV_{mathrm {T} })}}

In order to avoid the use of exponentials, sometimes even simpler models are used, which model the zones of operation of the diode by straight sections; They are called continuous models or Ram-signal. The simplest of all is the ideal diode.

Types of semiconductor diodes

Several semiconductor diodes, below: a rectifying bridge. In most diodes, the cathode terminal is indicated by painting a white or black strip.

There are several types of diodes, which can differ in their physical appearance, impurities, use of electrodes, which have particular electrical characteristics used for a special application in a circuit. The operation of these diodes is based on principles of quantum mechanics and band theory.

Normal diodes, which operate as described above, are usually made of doped silicon or germanium. Before the development of these silicon rectifier diodes, cuprous oxide and selenium were used: their low efficiency gave them a very high voltage drop (from 1.4 to 1.7 V) and they required a lot of heat dissipation. larger than a silicon diode. The vast majority of pn diodes are found in CMOS ICs, which include two diodes per pin and many other internal diodes.

• Diodo avalancha (TVS): Diodes that drive in the opposite direction when the reverse voltage exceeds the rupture voltage, are also known as TVS diodes. They are usually similar to Zener diodes, but it works under another phenomenon, the avalanche effect. This happens when the reverse electric field crossing the p-n union produces an ionization wave, similar to an avalanche, producing a current. Avalanche diodes are designed to operate on a defined reverse voltage without being destroyed. The difference between avalanche diode (which has a reverse voltage of approximately 6.2 volts) and the Zener diode is that the channel width of the first one exceeds the "free association" of the electrons, so collisions occur among them on the way. The only practical difference is that both have opposite polarity temperature coefficients (maximum heat dissipation is greater in a Zener diode, which is why these are mainly used in voltage regulator circuits). This type of diodes are used to remove transient voltages and currents that could cause a malfunction of a data bus connecting two devices sensitive to transient voltages.

• Silicio Diode: They usually have a millimetric size and, aligned, they constitute multichannel detectors that allow to obtain spectra in milliseconds. They are less sensitive than photomultipliers. It is a p-type semiconductor (with holes) in contact with a n-type semiconductor (electrons). Radiation communicates the energy to release the electrons that move to the holes, establishing an electric current proportional to the radiant power.

• Glass Diode: It's a kind of contact diode. The crystal diode consists of a sharp metal cable pressed against a semiconductor glass, usually galena or a part of coal. The cable forms the anode and the crystal forms the cathode. Glass diodes have a great application on the galena radius. Glass diodes are obsolete, but can still be obtained from some manufacturers.

• Constant current diode: It really is a JFET, with its gate connected to the source, and it functions as a power limiter of two terminals analogous to the Zener diode, which limits the voltage. They allow a current through them to achieve an adequate value and thus stabilize in a specific value. Also called UNCCDs (for its English acronym) or current regulator diode.

• Diodo tunnel or Esaki: They have a region of operation that produces negative resistance due to the tunnel effect, allowing to amplify very simple signals and circuits that have two states. Due to high load concentration, tunnel diodes are very fast, they can be used in very low temperatures, large magnetic fields and in high radiation environments. Because of these properties, they are usually used in space travel.

• Diodo Gunn: Similar to the tunnel diode are constructed of materials such as GaAs or InP that produces a negative resistance. Under appropriate conditions, the dipole domain forms and propagation through the diode, allowing high frequency microwave wave oscillators.

Ledes of different colors.

• Diodo emitter of light or LED of the English acronym, light-emitting diode: It is a diode formed by a semiconductor with holes in its energy band, such as arseniuro de galio, the load carriers that cross the union emit photons when recombined with the majority carriers on the other side. Depending on the material, the wavelength that can be produced varies from the infrared to wavelengths close to the ultraviolet. The potential of these diodes depends on the wavelength they emit: 2.1V corresponds to the red, 4.0V to the violet. The first LEDs were red and yellow. White LEDs are actually combinations of three different color LEDs or a blue led coated with a yellow sparkler. LEDs can also be used as low-efficiency photodiodes in signal applications. A LED can be used with a photodiode or phototransistor to form an optocoupler.

• Laser Diode: When the structure of a led is introduced into a resonant cavity formed when polishing the faces of the ends, a laser can be formed. Laser diodes are frequently used in optical storage devices and for high-speed optical communication.

• Thermal Diode: This term is also used for conventional diodes used to monitor temperature to voltage variation with temperature, and for thermoelectric refrigerators for thermoelectric cooling. Thermoelectric refrigerators are made of semiconductors, although they do not have any correctional union, they take advantage of the different behavior of load carriers of the type P and N semiconductors to transport the heat.

• Photo: All semiconductors are subject to optical load carriers. It is usually an unwanted effect, so many of the semiconductors are packed into materials that block the passage of light. Photodids have the function of being sensitive to light (photocelda), so they are packed in materials that allow the passage of light and are usually PIN (type of diode more sensitive to light). A photodiode can be used in solar cells, photometry or optical communication. Several photodiodes can be packed on a device like a linear arrangement or as a two-dimensional arrangement. These arrangements should not be confused with the attached loading devices.

• Diode with contact tips: They work as the semiconductor diodes of union mentioned above although their construction is simpler. A section of semiconductor type n is manufactured, and a sharp end conductor is made with a group 3 metal to make contact with the semiconductor. Some of the metal migrates to the semiconductor to make a small region of type p near the contact. The very used 1N34 (German manufacture) is still used in radio receivers as a detector and occasionally in specialized analog devices.

• Diodo PIN: A PIN diode has a central section without bending or in other words an intrinsic layer forming a p-intrinseca-n structure. They are used as high frequency switches and attenuators. They are also used as high volume ionizing radiation detectors and as photodetctors. PIN diodes are also used in power electronics and their central layer can withstand high voltages. In addition, the PIN structure can be found in semiconductor power devices, such as IGBTs, power MOSFETs and thristors.

• Diodo Schottky: The Schottky diode are built from a metal to a semiconductor contact. It's got a much less break-up tension than pn diodes. Its rupture voltage in 1mA currents is in the range from 0.15V to 0.45V, which makes them useful in saturation fixing and prevention applications in a transistor. They can also be used as low-loss grinders although their leak current is much higher than that of other diodes. Schottky diodes are carriers of wholesale loads so they do not suffer from problems of storage of minority load carriers that slow most other diodes (so this type of diodes has a faster reverse recovery than pn binding diodes). They tend to have a much lower binding capacitance than pn diodes that work as fast switches and are used for high-speed circuits such as switched sources, frequency mixer and detectors.

• Stabistor: The stabistor (also called Direct Reference Diode) is a special type of silicon diode whose direct tension characteristics are extremely stable. These devices are specially designed for low voltage stabilization applications where the tension is required to keep very stable within a wide range of current and temperature.

• Diodo Varicap: Varicap diode known as variable capacity diode or varactor, is a diode that takes advantage of certain constructive techniques to behave, with variations of the applied tension, as a variable capacitor. Conversely polarized, this electronic device features features features that are extremely useful in tuned circuits (L-C), where capacity changes are needed.

Diode Applications

• Medium wave rectifier
• Fullwave rectifier
• Parallel counter
• Tension Duplicator
• Zener Stabilizer
• Led
• Limiter
• Fixed circuit
• Tension multiplier
• Tension divider