Schottky diode is a semiconductor electrical rectifier element, where a metal-semiconductor transition is used as a barrier. As a result, useful properties are acquired: high speed and low voltage drop in the forward direction.
From the history of the discovery of Schottky diodes
The rectifying properties of the metal-semiconductor transition were first observed in 1874 by Ferdinand Brown using the example of sulfides. Passing the current in the forward and reverse directions, he noted a difference of 30%, which fundamentally contradicted Ohm’s famous law. Brown could not explain what was happening, but, having continued the research, he found that the resistance of the section is proportional to the current flowing. Which also looked unusual.
Experiments repeated by physicists. For example, Werner Siemens noted similar properties of selenium. Brown found that the properties of the structure appear most clearly with a small amount of contacts attached to the sulphide crystal. The researcher used:
- spring-loaded wire with a pressure of 1 kg;
- mercury contact;
- copper metallized pad.
So the point diode was born, in 1900 prevented our compatriot Popov from taking a patent for a radio detector. In his own work, Brown presents a study of manganese ore( psilomelane).By pressing the contacts to the crystal with a clamp and isolating the sponge from the current-carrying part, the scientist obtained excellent results, but no effect was found at that time. Describing the unusual properties of copper sulfide, Ferdinand marked the beginning of solid-state electronics.
For Braun practical use was found by like-minded people. Professor Jagdish Chandra Bose announced on April 27, 1899 the creation of the first detector / receiver to work in conjunction with a radio transmitter. He used galena( lead oxide) in a pair with a simple wire and caught millimeter wave waves. In 1901, he patented his brainchild. It is possible that under the influence of rumors about Popov. The Bosch detector is used in Marconi’s first transatlantic radio program. A similar kind of device on a silicon crystal was patented in 1906 by Greenleaf Witter Pickard.
In his speech at the Nobel Prize in 1909, Brown noted that he did not understand the principles of the phenomenon discovered by him, but he discovered a number of materials exhibiting new properties. This is the aforementioned galena, pyrite, pyrolusite, tetrahedrite, and a number of others. The listed materials attracted attention for a simple reason: they conducted an electric current, although they were considered compounds of elements of the Periodic Table. Before such properties were considered the prerogative of simple metals.
Finally, in 1926, the first transistors with a Schottky barrier appeared, and William Bradford Shockley in 1939 brought the theory under the phenomenon. At the same time, Neville Francis Mot explained the phenomena occurring at the junction of the two materials, calculating the diffusion current and the drift of the main charge carriers. Walter Schottky complemented the theory by replacing the linear electric field with a damping and adding an idea of the ion donors located in the surface layer of a semiconductor. The volume charge at the interface under the metal layer was named after the scientist.
Davydov made similar attempts to summarize the theory for the existing fact in 1939, but incorrectly gave the limiting factors for the current and made other errors. The most correct conclusions were made by Hans Albrecht Bethe in 1942, who linked the current to the thermionic emission of carriers through a potential barrier at the boundary of two materials. Thus, the modern name of the phenomenon and the diodes should be the name of the last scientist, the Schottky theory revealed flaws.
Theoretical studies rest on the difficulty of measuring the work function of electrons from a material into a vacuum. Even for a chemically inert and stable metal of gold, certain indications vary from 4 to 4.92 eV.With a high degree of vacuum, in the absence of mercury from a pump or oil film, values of 5.2 eV are obtained. With the development of technology in the future, the values are foreseen more accurately. Another solution would be to use information about the electronegativity of materials to correctly predict events at the transition boundary. These values (on the Polling scale) are known with an accuracy of 0.1 eV.From what has been said it is clear: today it is not possible to predict correctly the height of the barrier by the indicated methods and, therefore, the rectifying properties of Schottky diodes.
The best ways to determine the height of the Schottky barrier
It is permissible to determine the height by the known formula( see figure).Where C is a coefficient slightly dependent on temperature. The dependence on the applied voltage Va, despite its complex shape, is considered almost linear. The angle of the graph is q / kT.The barrier height is determined according to the plot of lnJ versus 1 / T at a fixed voltage. Calculation is carried out on the angle of inclination.
Calculations An alternative method is to irradiate the metal-semiconductor transition with light. The following methods are used:
- The light passes through the semiconductor.
- The light falls directly on the sensitive area of the photocell.
If the photon energy falls within the gap of energy between the forbidden zone of the semiconductor and the height of the barrier, electron emission from the metal is observed. When the parameter is higher than both of these values, the output current rises sharply, which is easily noticeable on the setup for the experiment. This method makes it possible to establish that the work function for the same semiconductor, with different types of conductivity( n and p), in total gives the width of the forbidden zone of the material.
A new method for determining the height of the Schottky barrier is to measure the junction capacitance depending on the applied reverse voltage. The graph shows the form of a straight line intersecting the abscissa axis at the point characterizing the desired value. The result of the experiments strongly depends on the quality of surface preparation. The study of technological processing methods shows that etching in hydrofluoric acid leaves a layer of oxide film 10–20 angstroms thick on a silicon sample.
The aging effect is consistently noted. Less characteristic of Schottky diodes formed by cleaving the crystal. Barrier heights differ for a particular material, in some cases they are strongly dependent on the electronegativity of metals. For gallium arsenide, the factor almost does not appear, in the case of zinc sulphide plays a crucial role. But in the latter case, the quality of surface preparation has a weak effect, for GaAs this is extremely important. Cadmium sulfide is in an intermediate position with respect to these materials.
In the study it turned out that most semiconductors behave like GaAs, including silicon. Mead explained this by the fact that a series of formations form on the surface of the material, where the electron energy lies in the region of one third of the band gap from the valence zone. As a result, in contact with the metal, the Fermi level in the latter tends to occupy a similar position. History repeats itself with any guide. At the same time, the barrier height becomes the difference between the Fermi level and the edge of the conduction band in the semiconductor.
. A strong influence of the electronegativity of the metal is observed in materials with pronounced ionic bonds. These are primarily tetravalent silica and zinc sulphide. This fact is explained by the absence of formations that affect the Fermi level in the metal. In conclusion, add that an exhaustive theory about the issue under consideration today is not created.
Advantages of Schottky Diodes
It is no secret that Schottky diodes serve as rectifiers at the output of switching power supplies. Manufacturers rest on the fact that power loss and heat in this case is much lower. It is established that the voltage drop in the direct connection on the Schottky diode is 1.5 - 2 times less than in any type of rectifiers. Let's try to explain the reason.
Consider the work of a normal pn-junction. When materials come into contact with two different types of conductivity, diffusion of the main carriers begins beyond the contact boundary, where they are no longer the main ones. In physics, this is called the barrier layer. If a positive potential is applied to the n-region, the main electron carriers will instantly be attracted to the output. Then the barrier layer expands, the current does not flow. With direct inclusion, the main carriers, on the contrary, attack the barrier layer, where they actively recombine with it. The transition opens, current flows.
It turns out that neither open nor close a simple diode instantly fails. There are processes of formation and elimination of the barrier layer, which requires time. The Schottky diode behaves slightly differently. The applied direct voltage opens the transition, but the injection of holes into the n-semiconductor practically does not occur, the barrier for them is large, there are few such carriers in the metal. With the reverse inclusion in a heavily doped semiconductor able to flow tunneling current.
Readers, familiar with the topic of LED lighting, already know that, originally in 1907, Henry Joseph Round made a discovery on a crystal detector. This is a Schottky diode in the first approximation: the boundary of the metal and silicon carbide. The difference is that today they use n-type semiconductor and aluminum.
Schottky diode can not only glow: for these purposes they use pn-junction. The metal-semiconductor contact does not always become rectifying. In the latter case, it is called ohmic and is included in most transistors, where its parasitic effects are superfluous and harmful. What the transition will be depends on the height of the schottky barrier. At large values of the parameter, exceeding the temperature energy, rectifying properties appear. Properties are determined by the difference in the work function of the metal( in vacuum) and semiconductor, or by electron affinity.
The transition properties depend on the materials used and on the geometrical dimensions. The volume charge in this case is less than when two semiconductors of different types are in contact, which means that the switching time is significantly reduced. In a typical case, it fits in the range from hundreds of ps to tens of ns. For conventional diodes at least an order of magnitude higher. In theory, this looks like the absence of an increase in the barrier level with an applied reverse voltage. It is easy to explain the small voltage drop by the fact that part of the transition is composed of a pure conductor. Actual for devices designed for relatively low voltages of tens of volts.
According to the properties of Schottky diodes, they are widely used in switching power supplies for household appliances. This allows to reduce losses, improve the thermal mode of operation of rectifiers. The small area of the transition causes low breakdown voltages, which is slightly offset by an increase in the metallization area on the crystal, which encompasses a portion of the silica-insulated region. This area, resembling a capacitor, when the diode is turned back on, impoverishes adjacent layers with main charge carriers, significantly improving performance.
Due to their speed, Schottky diodes are actively used in integrated circuits aimed at using high frequencies - operating and synchronization frequencies.