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Transistor


Transistor
The invention in the late 40-ies of the XX century, a transistor has become one of the major milestones in the history of electronics. Vacuum lamps that have to be essential and central to all the radio for a long time - and electronic devices have many shortcomings. As the complexity of the radio equipment, and improve the general requirements to it, these shortcomings were felt all the more acutely. These must be attributed primarily mechanical fragility of lamps, light the battery life, large size, low efficiency due to the large heat loss at the anode. Therefore, when replacing vacuum tubes in the second half of the XX century came the semiconductor elements do not possess any of these flaws, in radio and electronics was a real coup.

I must say that semiconductors are not immediately opened to man their remarkable properties. For a long time used exclusively for electrical conductors and insulators. A large group of materials, occupies an intermediate position between them, did not find any use, and only a few researchers studying the nature of electricity, from time to time showing interest in their electrical properties. So, in 1874, Brown discovered the phenomenon of rectification at the site of contact and lead pyrite and created the first crystal detector. Other investigators have found that a significant impact on the conductivity of semiconductors have impurities contained therein. For example, in 1907 Beddeker year found cuprous iodide conductivity that increases by 24 times in the presence of iodine impurities, which itself is not a conductor.

What explains the properties of semiconductors and why they have become so important in electronics? Consider this typical semiconductor as germanium. Under normal conditions, it has a resistivity at 30 million times greater than that of copper, and 1000000 million times less than that of glass. Consequently, according to its properties it is still somewhat nearer to conductors than to insulators. It is known that the ability of a substance to conduct or not conduct an electric current depends on the presence or absence in it of free charged particles.

Germany in this sense, is no exception. Each atom is tetravalent and it should form with four neighboring atoms of electronic communication. But due to the thermal effects of some of the electrons leave their atoms and begins to move freely between the crystal lattice sites. It is about 2 electron for every 10 billion atoms. One gram of germanium contained about 10 thousand billion atoms, that is, it has about 2 thousand billion free electrons. It is a million times lower than, for example, copper or silver, but still enough to germanium could pass through a small current. However, as already mentioned, the conductivity of germanium can be significantly improved by introducing a part of its lattice impurities such as pentavalent arsenic or antimony atom. Then four arsenic electrons form covalent bonds with the germanium atoms, but the fifth will remain free. It is weakly bound to the atom, so that a small voltage applied to the crystal, will be enough for him to come off and become a free electron (it is clear that the arsenic atoms then become positively charged ions). This significantly changes the electrical properties of germanium. Although the content of impurities is small - only 1 to 10 atom millionov germanium atoms, due to its presence the amount of available negatively charged particles (electrons) in the germanium crystal repeatedly increases. Such a semiconductor is called n-type semiconductor (from negative - negative).

Another picture would be the case when the germanium crystal trivalent impurity is introduced (for example, aluminum, gallium or indium). Each impurity atom forms bonds with only three atoms of germanium, and the place of the fourth connection will remain free space - a hole that can easily fill any electron (the impurity atom is ionized negatively). If this electron to move from impurities adjacent germanium atom, a hole will be in turn the latter. With a voltage to such a crystal, we get an effect that can be called "motion of holes." Indeed, even on the other side, where there is a negative pole of an external source, the electron fills the hole trivalent atom. Consequently, the electron closer to the positive pole, whereas the new hole is formed in the adjacent atom located nearer to the negative pole. Then there is the same phenomenon with another atom. A new hole is in turn filled with electrons, thus approaching to the positive pole, and formed for the expense of the hole closer to the negative pole. And as a result of this motion, the electron reaches the positive pole, where it will go to the current source, the hole reaches the negative pole, where it is filled with electrons emitted from the power source. The hole moves as if it is a particle with a positive charge, and we can say that there is an electric current creates a positive charge. Such a semiconductor is called a p-type semiconductor (from positiv - positive).

By itself, a phenomenon impurity conduction still has a large value, but when the two semiconductors - one with n-conductivity and the other with a p-conductivity (for example, when the crystal germanium on the one hand formed n-conductivity, and on the other - p conductivity of) - occur very curious phenomenon. Negatively ionized atoms repel the field of p from the transition area of ​​free electrons n, and positively ionized atoms n pushed down the field from the transition area of ​​the holes p. That is, a pn junction turn into a kind of barrier between the two areas. This crystal will become pronounced unilateral conductivity: some currents it will behave as a conductor, but for others - as an insulator.

In fact, if applied to a crystal voltage greater in magnitude than the "locking" voltage pn junction in such a manner that the positive electrode is connected to the p-region, and a negative - with n-region, the crystal will leak electric current formed by electrons and holes, moving towards each other.

If the external source potential change in the opposite way, the current will stop (or rather, it will be very slight) - will only outflow of electrons and holes from the border separating the two areas, so that the potential barrier between them will increase.

In this case, the semiconductor chip behaves exactly the same way as diode vacuum tube, so instruments based on this principle is called semiconductor diodes. As tube diodes, they can serve as detectors, that is, current rectifiers.

Even more interesting phenomenon can be observed in the case where a semiconductor chip is formed is not one, but two pn junction. Such a semiconductor element was the name of the transistor. One of its outer regions called the emitter, the other - the collector, and the middle area (which usually make very thin) - base.

If a voltage is applied to the emitter and collector of the transistor, current does not flow, as if we did not change the polarity. But if you create a small potential difference between the emitter and the base, the free electrons from the emitter, breaking the pn junction, reach your base. And since the base is very thin, only a small number of these electrons will be enough to fill in the holes that are in the area p. Therefore, most of them take place in the reservoir, breaking the barrier of the locking of the second transition - occurs in the transistor current. This phenomenon is all the more remarkable that the current in the emitter-base is usually in the tens of times less than that which occurs in the emitter-collector This shows that its effect transistor can be in a certain sense be considered an analogue of a three-electrode lamp (although the physical processes in them It is quite different), and the base plays the role of a grid, is placed between the anode and the cathode. In the same way as in a lamp, a small change in grid potential causes a considerable change in anode current in a transistor in weak base circuit changes cause substantial changes in collector current. Therefore, the transistor can be used as a booster and a generator of electrical signals.

Semiconductor components began to gradually replace the vacuum tubes from the beginning of the 40s. Since 1940, widely used in radar devices was spot germanium diode. Radar generally served as an incentive for the rapid development of electronics powerful sources of high frequency energy. All the more interest was shown to the centimeter and decimeter waves to create electronic devices capable of operating in these bands. Meanwhile, electronic lamp when used in high and ultra-high frequencies behaved poorly as own noise significantly limits their sensitivity. The use of the inputs radio spot germanium diodes allowed to drastically reduce its own noise, increase sensitivity and detection range of objects.

However, the true era of semiconductors began after the Second World War, when the transistor was invented by a point. It was created after many experiments, in 1948, employees of the American company "Bell" Shockley, Bardeen and Brattain. Located on the germanium crystal, a short distance from each other, two point contact and submitting to one of them a direct offset, on the other - the opposite, they were able to help with the current passes through the first contact, to control the current through the second. This first transistor has a gain of about 100.

The new invention quickly became widespread. The first point of the germanium transistors consist of a crystal with n-conductivity, serves as a base on which rested two thin bronze island located very close to each other - at a distance of a few microns. One of them (usually beryllium bronze) served as the emitter, and the other (phosphor bronze) - collector. When making the tip passed through the transistor current of about one ampere. Germanium when it melted, as well as the tips of the island. Copper and impurities present in it and passed into the germanium formed in the vicinity of point contacts with the p-type layers.

These transistors are not reliable due to the imperfections of its construction. They were stable and could not work at high powers. their cost was high. However, they were much more reliable than vacuum tubes, are not afraid of moisture and power a hundred times smaller than their equivalent vacuum tubes. At the same time they were extremely economical, as required for your power is very small current of the order 0, 5-1, and did not need a separate battery. Their efficiency reached 70%, while the lamp it rarely exceeded 10%. Since the transistors do not require heating, they began to work immediately after the voltage on them. In addition, they have a very low noise level, and therefore the equipment assembled on transistors, are more sensitive.

Gradually, a new device was improved. In 1952 came the first flat impurity germanium transistors. Their production was a complex process. First germanium purified from admixtures, and then form a single crystal. (Normal piece of germanium consists of a large number of spliced ​​in disarray crystals for semiconductor devices, such a structure of the material is not suitable - there need only correct, unified for the whole piece of the crystal lattice.) To do this, germanium is melted and dropped into his seed - a small crystal correctly oriented bars. By rotating around the axis of the seed, it slowly lifted the. As a result, the atoms around the seed lined up in a regular crystal lattice. The semiconductor material is solidified and enveloped seed. The result was a single crystal rod. Simultaneously, the melt was added p or n-type impurity. Then, a single crystal was cut into small plates, which served as a base. Emitter and collector are created in various ways. The most simple method consists in the fact that on both sides of the superimposed plates of germanium and indium small pieces quickly heated them to 600 degrees. At this temperature, the indium is alloyed with germanium below it. At cooling indium rich region acquired the conductivity of p-type. Then, the crystal was placed in the housing and connected terminals.

In 1955, the company "Bell Systems" was created by diffusion germanium transistor. diffusion method was that the semiconductor plate was placed in a gas atmosphere containing impurities pair that was to form the emitter and collector, and the plate was heated to a temperature close to the melting point. impurity atoms at the same time gradually penetrated into the semiconductor.

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