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HISTORY OF TECHNOLOGY, TECHNOLOGY, OBJECTS AROUND US
Transistor. History of invention and production
Directory / The history of technology, technology, objects around us A transistor, a semiconductor triode, is an electronic component made of a semiconductor material, usually with three terminals, which allows the input signal to control the current in an electrical circuit. Typically used to amplify, generate and convert electrical signals. In the general case, a transistor is any device that imitates the main property of a transistor - signal changes between two different states when the signal on the control electrode changes.
The invention of the transistor in the late 40s was one of the biggest milestones in the history of electronics. Vacuum tubes, which until then had been an indispensable and main element of all radio and electronic devices for a long time, had many shortcomings. As the complexity of radio equipment and the increase in the general requirements for it, these shortcomings were felt more and more acutely. These include, first of all, the mechanical fragility of the lamps, their short service life, large dimensions, low efficiency due to large heat losses at the anode. Therefore, when semiconductor elements that did not have any of the listed flaws replaced vacuum tubes in the second half of the XNUMXth century, a real revolution took place in radio engineering and electronics. It must be said that semiconductors did not immediately reveal their remarkable properties to man. For a long time, only conductors and dielectrics were used in electrical engineering. A large group of materials that occupied an intermediate position between them did not find any application, and only a few researchers, studying the nature of electricity, from time to time showed an interest in their electrical properties. So, in 1874, Brown discovered the phenomenon of current rectification at the point of contact between lead and pyrite and created the first crystal detector. Other researchers have found that the impurities contained in them have a significant effect on the conductivity of semiconductors. For example, Beddecker in 1907 found that the conductivity of copper iodide increases 24 times in the presence of an admixture of iodine, which in itself is not a conductor. What explains the properties of semiconductors and why have they become so important in electronics? Take such a typical semiconductor as germanium. Under normal conditions, it has a resistivity 30 million times that of copper and 1000000 million times that of glass. Therefore, in terms of its properties, it is still somewhat closer to conductors than to dielectrics. As you know, the ability of a substance to conduct or not conduct an electric current depends on the presence or absence of free charged particles in it.
Germanium is no exception in this sense. Each of its atom is tetravalent and must form four electronic bonds with neighboring atoms. But due to thermal action, some of the electrons leave their atoms and begin to move freely between the nodes of the crystal lattice. That's about 2 electrons for every 10 billion atoms. One gram of germanium contains about 10 thousand billion atoms, that is, it has about 2 thousand billion free electrons. This is a million times less than, for example, in copper or silver, but still enough for germanium to pass a small current through itself.
However, as already mentioned, the conductivity of germanium can be significantly increased by introducing impurities into its lattice, for example, a pentavalent atom of arsenic or antimony. Then four arsenic electrons form valence bonds with germanium atoms, but the fifth one will remain free. It will be weakly bound to the atom, so that a small voltage applied to the crystal will be enough for it to break off and turn into a free electron (it is clear that the arsenic atoms become positively charged ions in this case). All this noticeably changes the electrical properties of germanium. Although the impurity content in it is small - only 1 atom per 10 million germanium atoms, due to its presence, the number of free negatively charged particles (electrons) in a germanium crystal increases many times. Such a semiconductor is usually called an n-type semiconductor (from negative - negative).
A different picture will be in the case when a trivalent impurity (for example, aluminum, gallium or indium) is introduced into the germanium crystal. Each impurity atom forms bonds with only three germanium atoms, and in place of the fourth bond there will be a free space - a hole that can easily be filled by any electron (in this case, the impurity atom is ionized negatively). If this electron passes to an impurity from a neighboring germanium atom, then the latter will, in turn, have a hole. By applying a voltage to such a crystal, we obtain an effect that can be called "hole displacement". Indeed, let from the side where the negative pole of the external source is located, the electron will fill the hole of the trivalent atom. Therefore, the electron will move closer to the positive pole, while a new hole is formed in the neighboring atom closer to the negative pole. Then the same phenomenon occurs with another atom. The new hole, in turn, will be filled with an electron, thus approaching the positive pole, and the hole thus formed will approach the negative pole. And when, as a result of such a movement, the electron reaches the positive pole, from where it will go to the current source, the hole will reach the negative pole, where it will be filled with an electron coming from the current source. The hole moves as if it were a particle with a positive charge, and we can say that here the electric current is created by positive charges. Such a semiconductor is called a p-type semiconductor (from positiv - positive). In itself, the phenomenon of impurity conductivity is not yet of great importance, but when two semiconductors are connected - one with n-conductivity and the other with p-conductivity (for example, when n-conductivity is created in a germanium crystal on one side, and p-conductivity on the other -conductivity) - very curious phenomena occur. Negatively ionized atoms of the p region will repel free electrons of the n region from the transition, and positively ionized atoms of the n region will repel the hole of the p region from the transition. That is, the pn junction will turn into a kind of barrier between the two areas. Due to this, the crystal will acquire a pronounced one-sided conductivity: for some currents it will behave like a conductor, and for others - like an insulator. Indeed, if a voltage greater than the "shut-off" voltage of the pn junction is applied to the crystal, and in such a way that the positive electrode is connected to the p-region, and the negative electrode to the n-region, then an electric current will flow in the crystal formed by electrons and holes moving towards each other. If the potentials of the external source are changed in the opposite way, the current will stop (or rather, it will be very insignificant) - there will only be an outflow of electrons and holes from the boundary between the two regions, as a result of which the potential barrier between them will increase. In this case, the semiconductor crystal will behave in exactly the same way as a diode vacuum tube, so devices based on this principle are called semiconductor diodes. Like tube diodes, they can serve as detectors, that is, current rectifiers. An even more interesting phenomenon can be observed when not one, but two pn junctions are formed in a semiconductor crystal. Such a semiconductor element is called a transistor. One of its outer regions is called the emitter, the other is called the collector, and the middle region (which is usually made very thin) is called the base. If we apply voltage to the emitter and collector of the transistor, no current will flow, no matter how we reverse the polarity.
But if you create a small potential difference between the emitter and the base, then the free electrons from the emitter, having overcome the pn junction, will fall into the base. And since the base is very thin, only a small number of these electrons will be enough to fill the holes located in the p region. Therefore, most of them will pass into the collector, overcoming the locking barrier of the second transition - an electric current will appear in the transistor. This phenomenon is all the more remarkable because the current in the emitter-base circuit is usually ten times less than that which flows in the emitter-collector circuit. From this it can be seen that in its action the transistor can in a certain sense be considered an analogue of a three-electrode lamp (although the physical processes in them are completely different), and the base here plays the role of a grid placed between the anode and cathode. Just as in a lamp, a small change in the grid potential causes a large change in the anode current, in a transistor, small changes in the base circuit cause a large change in the collector current. Therefore, the transistor can be used as an amplifier and an electrical signal generator. Semiconductor elements began to gradually replace vacuum tubes from the beginning of the 40s. Since 1940, a point germanium diode has been widely used in radar devices. Radar in general served as a stimulus for the rapid development of electronics for high-power sources of high-frequency energy. Increasing interest was shown in decimeter and centimeter waves, in the creation of electronic devices capable of operating in these ranges. Meanwhile, vacuum tubes, when used in the region of high and ultrahigh frequencies, behaved unsatisfactorily, since their own noise significantly limited their sensitivity. The use of point germanium diodes at the inputs of radio receivers made it possible to drastically reduce intrinsic noise, increase the sensitivity and range of detection of objects.
However, the real era of semiconductors began after World War II, when the point transistor was invented. It was created after many experiments in 1948 by employees of the American company "Bell" Shockley, Bardeen and Brattain. By placing two point contacts on a germanium crystal at a short distance from each other and applying a forward bias to one of them and a reverse bias to the other, they were able to control the current through the second one using the current passing through the first contact. This first transistor had a gain of about 100. The new invention quickly became widespread. The first point transistors consisted of a germanium crystal with n-conductivity, which served as a base, on which two thin bronze points rested, located very close to each other - at a distance of several microns. One of them (usually beryllium bronze) served as the emitter, and the other (made of phosphor bronze) served as the collector. In the manufacture of the transistor, a current of about one ampere was passed through the tips. The germanium melted, as did the tips of the points. Copper and the impurities present in it passed into germanium and formed layers with hole conductivity in the immediate vicinity of the point contacts. These transistors were not reliable due to the imperfection of their design. They were unstable and could not work at high powers. Their cost was great. However, they were much more reliable than vacuum tubes, were not afraid of dampness, and consumed hundreds of times less power than analogous vacuum tubes. At the same time, they were extremely economical, since they required a very small current of the order of 0,5-1 V for their power supply and did not need a separate battery. Their efficiency reached 70%, while the lamp rarely exceeded 10%. Since the transistors did not require heating, they began to work immediately after applying voltage to them. In addition, they had a very low level of intrinsic noise, and therefore the equipment assembled on transistors turned out to be more sensitive.
Gradually, the new device was improved. In 1952, the first planar doped germanium transistors appeared. Their manufacture was a complex technological process. First, germanium was purified from impurities, and then a single crystal was formed. (An ordinary piece of germanium consists of a large number of crystals spliced in disorder; such a material structure is not suitable for semiconductor devices - here an exceptionally regular crystal lattice is needed, the same for the entire piece.) For this, germanium was melted and a seed was lowered into it - a small crystal with a correctly oriented lattice. Rotating the seed around the axis, it was slowly raised. As a result, the atoms around the seed lined up in a regular crystal lattice. The semiconductor material solidified and enveloped the seed. The result was a single-crystal rod. Simultaneously, an impurity of the p or n type was added to the melt. Then the single crystal was cut into small plates, which served as a base. The emitter and collector were created in various ways. The simplest method was to place small pieces of indium on both sides of the germanium plate and quickly heat them up to 600 degrees. At this temperature, the indium fused with the germanium underneath. Upon cooling, the regions saturated with indium acquired p-type conductivity. Then the crystal was placed in the case and the leads were attached. In 1955, the Bell System company created a diffusion germanium transistor. The diffusion method consisted in placing semiconductor plates in an atmosphere of gas containing impurity vapor, which was supposed to form an emitter and collector, and heating the plates to a temperature close to the melting point. In this case, impurity atoms gradually penetrated into the semiconductor. Author: Ryzhov K.V.
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