Bipolar transistor

A bipolar transistor is an electrical semiconductor device used to amplify a signal and a number of other purposes in which current is generated by the movement of carriers of both signs. In its current form, the product was proposed and patented in 1947 by William Shockley.

The history of the development of the first transistors

. The inclinations are inherited; this is illustrated by the example of William Bradford Shockley. The son of a mining engineer and one of the first women surveyor in the United States. Specific combination. At 22, he received a bachelor's degree, did not stop there, and in 1936 he became a doctor of philosophy. The title awarded by the Massachusetts Institute of Technology does not mean that Shockley studied Nietzsche and Aristotle. The degree indicates the presence of a thesis in the field of a large list of sciences. The bizarre name is a tribute to tradition, when philosophy in the Middle Ages dealt with a wide range of issues, rightly considered the progenitor of other directions of the course of scientific thought.

Laboratory Bell

The meaning of the work was to study the electronic levels of sodium chloride. The band theory, which explained the processes taking place in the materials, was just gaining popularity. According to the theory, any electron in a crystal is able to occupy a unique state peculiar to this particle, with a certain energy and spin direction. In accordance with the presentation of the gradation, there is a certain discreteness in the valence band( connected with the nucleus), in addition there is a forbidden region where the particles are not allowed to settle down. From the last thesis, the exception is considered to be impurity semiconductors, which have become the basis for the creation of solid-state electronics, including bipolar transistors.

Bella Shockley got into the Lab for curious ideas in the field of designing nuclear reactors. Uranus in its pure form was discovered long before that, for the first time by the example of the element Becquerel, discovered radioactivity. He tried to bombard the metal nuclei with neutrons in the early 30s( XX century) of Enrico Fermi, the goal was to obtain transuranium elements. Later it turned out that radioactive decay occurs simultaneously with the release of energy outside. Shockley decided to bomb U-235 in order to get a new source of high power. During the Second World War, engaged in research to assess the possible land invasion of Japan, the data collected largely contributed to Truman's decision to drop an atomic bomb on Hiroshima.

Bella Lab has set a direct task for Shockley - to find an alternative to bulky tube amplifiers. This would mean saving space and the birth of a new generation of devices capable of operating in war conditions. It is no secret that the military achievements of the USSR proved to be appreciated on the opposite side of the ocean. Shockley was appointed the brigade manager, who was beating the task, which included, among other things, the creators of the first point transistor:

  1. John Bardeen;
  2. Walter Hauser Brattein.

Readers already know about a point diode based on a crystal detector, but what did the transistor represent? This is a field device: two electrodes are applied to a p-type semiconductor area and separated by a dielectric wedge. The thickness of the barrier layer varies from base. The control electrode applied to the n-region under a positive potential greatly depletes the transition region, and no current flows. Historically, the first transistor is considered a field transistor.

The design turned out to be specific. For example, gold contact pads are pressed by a spring to a germanium pnn junction crystal, more like a laboratory setup, rather than a full-featured device for military equipment. Assembled with stationery clips and poisonous electrolyte glue. But the device in the future will give the name of Silicon Valley. There was contention between scientists, because Shockley’s field theory, used in the transistor, did not help to create the device, in addition, it was mentioned in the 1925 Canadian patent for Lilienfeld. As a result, Bell Lab throws William's name out of the list of creators when setting up papers.

It is noteworthy that the structure of the MESFET( field effect transistor) proposed by Lilienfeld was not functioning. But the ideas in the office were accepted, and Bell Labs had difficulty submitting applications. It's a paradox, but scientists could patent only the design of Bardeen and Brattein - nothing more. The rest has long existed as a concept at the time of 1946.Shockley decided that fate played with the inventor another joke after all the failures. However, Bella’s company is making concessions, and it is generally accepted that William figures as the first person for the press.

William Bradford Shockley

Shockley begins to work on his own direction, simultaneously trying to rectify the situation. The latter does not give positive results, but the first leads to the creation of a device, today known to the world as a bipolar transistor. Looking through a number of constructions, on January 1, 1948, he finds the right one, but does not immediately realize it. Subsequently, Shockley comes the idea that the current is formed not only by the main charge carriers.

The principle of operation of a bipolar transistor, temperature modes

The concept outlined by Shockley brings the team into a frenzy: for years he worked behind his colleagues' back! But the idea was successful. If the base semiconductor is thin, the injected minority charge carriers are partially captured by the collector field. There they are already becoming major, participating in the creation of electric current. The process is controlled by a base field, the number of charge carriers that have broken through is proportional to the applied voltage.

In fact, the pn-collector junction operates in the breakdown mode. Temperatures are entirely determined by the materials. Germanium transistors are not able to function at temperatures above 85 degrees Celsius, and once exceeding the reference value, the subsequent cooling of the device does not return to work. Silicon can withstand almost twice the heat. Frequent copies of transistors capable of functioning at 150 degrees Celsius, but a minus in a relatively large voltage drop on the pn-junction.

Bipolar Transistor

It turns out that the designer is looking for the most suitable transistors to create an electrical circuit according to the existing conditions. Calculation of power dissipation is carried out, if necessary, the elements are supplemented by massive radiators. The maximum temperature is selected with a fair margin to prevent overheating. Semiconductors have obvious resistance, are used in technology exclusively for solving specific problems. For example, when creating a pn-transition. Otherwise, the thicker the layer of material, the greater the loss of active ohmic resistance. We give a clear example: the resistivity of germanium exceeds the value of the analogous parameter of copper( metal) 30 million times. Consequently, losses will increase( and heat) according to the indicated figure.

So the semiconductor layer is small. How to put this into practice? Forget temporarily about the paper clips used in the first design, let's turn to modern technology. In the manufacture of a bipolar transistor, the following regularities are maintained:

  • The emitter material serves to inject the main carriers into the base, where they will be captured by the field. Therefore, semiconductors with a large proportion of impurities are used. This ensures the creation of a large number of free carriers( holes or electrons).The collector volume is slightly higher than that of the emitter, the power dissipation is assumed to be greater. This affects the cooling conditions of the device.
  • In the database, the concentration of impurities is lower, so that most of the injected stream does not recombine. The share of external atoms in the crystal lattice is minimal.
  • The collector in the proportion of impurities is located midway between the base and the emitter. The charge carriers that have broken through here must recombine. The difference in the concentration of impurities becomes the reason why the collector and emitter cannot be interchanged in the electric circuit of the device. The second reason is the fact that the areas of pn-junctions are not the same. From the side of the collector - more.

Transistor Action

The width of the barrier layer of the pn-junction depends on the fraction of the impurity( increases with increasing).Moreover, its penetration into the emitter, collector and base is not the same. To the minimum depth, the barrier layer extends into the material with a maximum proportion of impurities. That is, the emitter. Germanium bipolar transistors are a thing of the past, silicon and on the basis of gallium arsenide come to replace it. Today, two technologies for the production of semiconductor devices dominate: emit:

  1. Fused transistors are made, for example, by melting germanium into a thin plate( mostly made of the specified material) of two indium drops of various sizes. The materials show a different liquidus temperature, it becomes possible to process the furnaces. Due to the diffusion of atoms, indium is firmly melted into germanium( melting point 940 degrees Celsius).Then electrodes are soldered to the emitter, collector and base.
  2. Planar transistors are closest to the original idea of ​​Shockley, his devices just called flat. Unlike the famous before. The desired layers are applied on a flat substrate using various methods. Masks of various configurations are actively used to create drawings. The advantage in the possibility of mass production of transistors on a single substrate, then it is cut into pieces, each becomes a separate semiconductor device.

In the course of the above-described technological manipulations, the production cycle steps are actively used:

  1. The diffusion method allows precise control of the geometrical dimensions of the pn-junction, which results in better repeatability and accuracy. To create a semiconductor transistor in an atmosphere of "noble" gas is heated to the liquidus point, impurities floating around are easily deposited on the surface. Diffusion occurs. By the dosage of the partial vapor pressure of impurities and the duration of the operation, the depth of penetration of atoms into the base material( substrate) varies. Sometimes diffusion occurs during the fusion process. The moment is determined by the exact selection of the temperature regime.
  2. Epitaxy is the process of growing a crystal of the desired type on a substrate. Deposition may occur from solution or gas. Vacuum sputtering also belongs to this class of technologies; electrolysis is a little apart, based on the principle of building up layers under the action of current.
  3. Lithography techniques are often used to obtain a given mask. For example, a photoresist is applied onto the substrate, the islands of which disappear under the action of the developer. Formative radiation is filtered by a mask of opaque material. The process of photolithography recalls familiar to every professional photographer, independently leading the processing of the film.

The directories often indicate two or more key terms describing the production cycle of a bipolar transistor.

transistor notation

transistor notation system OCT 11-0948 is issued for semiconductor devices, setting standards for bipolar transistors as well. In the first place is indicated the material, which largely determines the temperature modes of operation and parameters, then the digital marking, which determines the power, frequency and other qualities of the bipolar transistor. The volt-ampere characteristic and current gain are among the main parameters in the reference books.

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