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In 1947 was created the world's first transistor. In our days, annually produces more than 10 000 000 000 000 000 000 transistors that are 100 times greater than the number of rice grains absorbed annually seven billion inhabitants of the Earth. The world's first transistor computer was assembled in 1953 in Manchester and contained 92 of the transistor. Today you can buy more than 100,000 transistors for the price of a grain of rice, and in your mobile phone them about a billion. In this Chapter, we will describe the operation of the transistor, which can certainly be considered the most important application of quantum theory.
As we have seen in the previous Chapter, the conductor because the conductor, some electrons are in the conduction band. For this reason, they are quite mobile and can "flow" through the wire when battery is connected. An analogy with flowing water; the battery makes the current flow. To illustrate the ideas you can use even the concept of "capacity": the battery creates a potential within which the electrons move to the conduction band, and the potential in a sense, creates a "slope". This slope in the conduction band of the material electron "slips", finding the motion energy. This is another way of presenting small shocks, about which we spoke in the last Chapter, in which the battery pushes the electrons accelerated through the wire, and formed something like a water fall from the hill. This is a good way of visualizing the conduction of electricity by electrons, they are what we'll use until the end of this Chapter. In semiconductors such as silicon, there is something very interesting: the current is carried not only by electrons in the conduction band. Electrons in the valence band also contribute. Look at the pic. 1. The arrow shows how an electron, initially resting supinely in the area of valence, absorbs some energy and jumps to conduction band.
Fig. 1. The pair electron-hole in the semiconductor
of Course, after that electron is much more mobile, but mobility is also becoming something else: in the area of valence holes, and it gives you the ability to maneuver electrons from the valence to the fact equally inert. As we can see, connect the battery to this field will make an electron from the conduction band energy to make the jump, thereby causing movement of the electric current. What happens to that hole? The electric field created by the battery could cause an electron in the valence band are in some lower energy state to jump into this vacant hole. Now the hole is filled, but there is a hole "deeper" — at a lower energy level in the valence band. When the electrons in the valence band jump to a free hole, that rotates. Instead of having to track the movement of all electrons in almost full valence band, we can track the location of a hole, forgetting about the electrons. Such optimization of counting routinely use experts in the physics of semiconductors. We also make life easier.
the Applied electric field drives the electrons of the conduction band, creating a current, and we would like to know what is happening in this case with holes in the valence band. We know that electrons in valence band cannot move, since they were almost completely inhibits the Pauli principle, but under the influence of an electric field, they move a little, and the hole moves along with them. It's probably counterintuitive, so if you can't accept the fact that when electrons in the valence band shifted to the left, and the hole also moves to the left, consider the following analogy. Imagine a normal turn. The distance between people is 1 meter, but somewhere in the middle of a queue of one person is not enough. These people are the equivalent of electrons, and the missing people — the analog hole. Now imagine that all these people have moved a meter forward, so that each of them was there, where it stood in front of him in the queue. It is obvious that a gap in the queue is also moving on the meter. So behave and holes. In addition, you can imagine how water flows in a pipe: bubble water will move in the same direction as the jet, and this "missing water" corresponds to the hole in the valence band. As you remember, the electrons moving in the upper part of the filled energy band, are accelerated from the electric field in the opposite direction relative to the electrons moving in the bottom of the same strip. This means that the holes are at the top of the valence band move in the opposite direction relative to the electrons in the bottom of the conduction band.
the Result is that we can represent the flow of electrons in one direction and a corresponding flow of holes in the other. We can assume that the hole has an electric charge opposite to the charge of an electron. Remember that the material through which the flow of our electrons and holes, on average, electrically neutral. In any given region, the material has no charge because the negative charge of the electrons cancels the positive charge is carried by atomic nuclei. But if we create a pair electron-hole, electron moving from the valence band to the conduction band (as we have already described), is formed of a freely moving electron, which creates an excess negative charge compared to normal conditions in this region of the material. Similarly, a hole is lack of electron, and in the place where it is dominated by positive charge. The electric current is, by definition, is a value which is moving positive charges, so electrons contribute to the current negative vklad42 and holes — positive if moving in the same direction. If, as in the case of our semiconductor, electrons and holes move in opposite directions, they add up, the result is a larger charge and consequently a large current.
Although it all seems quite confusing, the results are clear as day: we have to imagine that the flow of electricity through the semiconductor is within charge, and it consists of electrons in the conduction band moving in one direction and holes in the valence band moving in the opposite direction. This situation differs from the movement of current in a conductor, when current is determined by the movement of many electrons in the conduction band, and the additional strength of the current generated in the formation of pairs of electron-hole, is negligible. Understand the use of semiconductors means to know that the current going through the semiconductor, it is impossible to call the uncontrolled movement of electrons through a wire as the conductor. This is a much more complex combination of movements of electrons and holes, which if properly set can be used to create microscopic devices able to provide full control over the movement of current in the circuit.
the Following statement is an inspiring example of applied physics and technology. The idea is to deliberately contaminate a piece of pure silicon or germanium to create some new available energy levels of electrons. These new levels will allow you to control the flow of electrons and holes passing through the semiconductor, we can use the valves to control the movement of water through the pipes. Of course, to control the current going through the wire, in principle, easy: you pull the switch. But we now not about it, and how to create a more subtle switches and dynamically control the current in the circuit. These switches — the building blocks of logic circuits, and logic circuits, in turn, consist of microprocessors. So how does this all work?
the Left part of Fig. 2 shows what happens if a piece of silicon contaminated with phosphorus. The level of contamination can be precisely controlled, which is very important. Imagine that in a crystal of pure silicon, each atom is sequentially replaced by a phosphorus atom. The phosphorus atom gets replaced by atom of silicon, and the only difference is that have phosphorus on one electron more than silicon. This extra electron is very weak, but bound to its atom, it is not free and is an energy level just below the conduction band. At low temperatures the conduction band is empty, and the extra electrons emerging from phosphorus atoms located in the donor energy level, marked in the figure.
Fig. 2. New energy levels introduced in n-type semiconductor (left) and p-doped (right)
At room temperature, the pair electron-hole in silicon creates an extremely rare. Only one of about a trillion electrons gets enough energy from thermal vibrations of the lattice to jump from the valence band to the conduction band. On the contrary, since the donor electron in phosphorus is very weakly bound to the atom, the probability that he will be able to make a small jump from the donor level into the conduction band.
So, at room temperature at the pollution level is higher than one phosphorus atom in a trillion of silicon atoms in the conduction band are mainly present, the electrons released by the atoms of phosphorus. This means that it is possible with very high accuracy to monitor the presence of mobile electrons which can conduct electricity, simply varying the degree of phosphorus pollution. Since the current in this case, as electrons moving freely in the conduction band, we say that this type of contaminated silicon is called n-type (negative — negative).
the Right part of Fig. 2 shows what happens if, instead of phosphorus pollute the silicon atoms of aluminium. The atoms of aluminium are again among the silicon atoms and replace them perfectly. The difference is that the aluminium one electron less than silicon. So in clear crystal see holes, while when phosphorus pollution appeared superfluous electrons. These holes are close to atoms of aluminium and can be filled with electrons, which jump from the valence band of the neighboring silicon atoms. "Perforated" acceptor level shown in the figure. It is located directly above the valence band, because the electron from the atom of silicon in the valence band can easily jump into the hole left by the aluminum atom. In this case, it is natural to assume that the electric current is carried by holes, so this type of contaminated silicon is called p-type (positive — positive). As in the previous case, at room temperature, the level of aluminum contamination may not be more than one trillion before the movement of holes from the aluminium to be electrified.
so, we have just proved that you can make such a piece of silicon that will conduct current — giving the opportunity for either the electrons from the phosphorus atoms to move in the conduction band or holes from the atoms of aluminium to move in the valence band. So what?
In Fig. 3 shows that we are on the way to something important: it shows what happens if you add together two pieces of silicon, one n-type and one p-type. Initially in the field of n-type electrons move from the phosphorus, and p-type electrons of aluminum.
Fig. 3. The connection of two pieces of silicon of n-type and p-type
In the result, the electrons from region n-type region flow into the p-type and electrons from the region of the p-type region n-type. There is no mystery; the electrons and holes snakes across the joint of two materials, like the drop of ink is dissolved in the bath water. But since electrons and holes move in opposite directions, they leave behind a region of positive charge (n-type) and a region of negative charge (p-type). This arrangement of charges prevents further migration by the rule "like charges repel", eventually there comes a balance and migration ends.
the second illustration of Fig. 3 shows how it is possible to describe this phenomenon in the language of potentials. Demonstrates how the electric potential changes around the joint. In the deep region of n-type effect of the joints is small, and since it is the condition of equilibrium, the current is absent. So, in this region the potential is constant. Before going any further, we must once again explain why we are important potential: it just shows what forces act on electrons and holes. If the potential is smooth, the electron will not move, like not moving the ball lying on a level floor.
If the potential goes down, it can be assumed that the electron near this falling potential will also "roll down". Sorry, made a rather awkward solution assume that the reduction potential means "increase" of the electron, i.e. electrons will flow up. In other words, the falling potential is used for the electron barrier, which we depicted in the figure. This is the force pushing the electron away from the area of the p-type as a consequence of the negative charge due to the earlier migration of electrons. This force prevents further movement of electrons from n-type silicon in silicon p-type. The use of reduction potential to illustrate the ascent of an electron is actually not as silly as it seems, because now the most clarity is achieved for holes, as they naturally flow down. We can assume that our representation of the potential (moving from a high point left to low point right) correctly describes the fact that the potential drop does not allow the holes to leave the area of the p-type.
the Third illustration in the drawing the analogy with flowing water. The electrons on the left are ready and willing to leak down the wire, but a barrier prevents them from doing so. Similarly, holes in p-type accumulate on the wrong side of the barrier; water barrier and the potential drop is two different ways to represent the same thing. So, if you just staple together two pieces of silicon — n-type and p-type. However, their bond requires more effort than you might think: it's not the glue, because such an articulation will not allow the electrons and holes freely flow from one area to another. The most interesting if to connect this pn-transition to the battery, this will allow you to raise or lower a potential barrier between the regions of n-type and p-type. If lower potential region of the p-type, he will fall even more, so that electrons and holes will be harder to move the joint. But the enhancement of the capacity region of p-type (or a weakening of the capacity region, n-type) like lowering the dam holding back the water. The electrons in region n-type immediately begin to flood the region of the p-type and the holes move as massive, but in the opposite direction. Thus pn-transition can be used as a diode: it can provide the current movement, however, only one napravlenii43. But the diodes are not the main subject of our interest.
Fig. 4 is a sketch of the device has changed the world — of the transistor. It shows what happens if you make a kind of sandwich layer of silicon p-type to place between two layers of n-type silicon. Here us a good service will serve the explanation about the diode, because the ideas about the same. The electrons move from regions of n-type region p-type, the holes move in the opposite direction, while the fall of potential in the joints between the layers is the interpenetration does not stop. In isolation it is possible to imagine the existence of two electron reservoirs separated by a barrier and a reservoir of holes, sandwiched between them.
Fig. 4. Transistor
the Most interesting happens when we apply voltage to the area of n-type on one side and to the area of the p-type in the middle. The application positive voltage makes up the flat part of the curve to the left (by the amount Vc) and flat plot in the area of the p-type (the value Vb). This is shown by the solid line in the Central chart. This way of locating potential has serious consequences: it creates a waterfall of electrons that overcome the Central barrier is reduced and sent to the region n-type to the left (recall that the electrons flowing "uphill"). If Vc is greater than Vb, the electron flow will be one-sided and electrons to the left will not be able to cross the area of the p-type. Matter how harmless nor heard these phrases, but what you've just described the electronic valve. So, by applying voltage to the region of the p-type we can turn on and off an electric current.
here is the conclusion: we are ready to fully realize the potential of the humble transistor. In Fig. 5 again demonstrate the action of the transistor through parallel with the moving water. The situation "valve closed" is completely analogous to what happens in the region of the p-type without any tension. The application of the voltage corresponds to the valve opening. Under the two tubes, we designate the symbol usually used for the transistor, and with a certain degree of imagination it can be argued that it is even similar to valve.
Fig. 5. The analogy transistor water tubes
What can be done with valves and tubes? We can create a computer, and if pipes and valves are small enough, it is a serious computer.
Fig. 6 is a conceptual illustration of how you can use the pipe with two valves and create something called a "logic gate". The tube on the left both valves are open, the result of bottom-water flows. The tube in the center and right tubes, one valve open and one valve closed, so obviously the water is not poured from the bottom. We decided not to represent the fourth scenario is when both valves are closed. If we denote the flow of water from the bottom of the tubes the number 1, the absence of such leakage is the number 0, and also set to-open valve figure 1, and the closed figure of 0, can be represented by the four tubes (three drawn and one nenarisovannoy) equations 1 and 1 = 1, 1 and 0 = 0, 0 and 1 = 0 and 0 and 0 = 0.
The word "and" is a logical operator that is used here in a technical sense: the system of pipes and valves that we have just described, is called "the valve". This valve allows two inputs (as the two valves) and returns a single value (water flowing or not), the only way to output 1 is to introduce both times 1. Hopefully, now understand how you can use a pair of transistors connected to make a "valve and" — schematic diagram given in this figure.
Fig. 6. "The valve and" created with water pipe and two valves (left) and a pair of transistors (right). The second option is much better suited for the creation of computers.
, We see that the current starts flowing only if both transistors on (that is, if you apply a positive voltage to the areas of the p-type, Vb1 and Vb2), and that is what leads to "vent".
the second logic circuit shown in Fig. 7. Here the water will flow out from the bottom, if you open any of the valves, and will not flow if both valves are closed. This is called "valve or", and it can be described similarly: 1 or 1 = 1 1 or 0 = 1, 0 or 1 = 1 and 0 or 0 = 0. The corresponding circuit of the transistor is also shown in the figure. The current would flow in all cases except when both transistors are off.
Fig. 7. "The valve or" created with two water pipes and two valves (left) or a pair of transistors (right)
on such logic based power electronic devices. These humble building blocks provide a combination of logic circuits that can be used to create arbitrarily complex algorithms. You can assign inputs to certain logic (a set of zeros and ones), to drive these values through some sophisticated configuration of transistors and to output a list of the other values (different set of zeros and ones). Thus we create a chain for performing complex mathematical calculations or making decisions based on what keys are pressed on the keyboard. Then we implement this information a device that displays the appropriate symbology on the screen, or launching an alarm, if someone breaks into your house, or send the stream of text characters via fiber optic cable (in this case they are represented by binary code) to the other end of the world, or... well, anything, because almost any electronic device at our disposal is chock-full of transistors.
the Potential is infinite, and we already use transistors to change the world. It is no exaggeration to say that the transistor is the most important invention in the last 100 years: modern world is built on semiconductor technology and formed them. From a practical point of view these technologies have saved millions of lives in particular is to point out the use of computing devices in hospitals, the advantages of fast, reliable and widely used around the world communication systems, the use of computers in scientific research and to control complex industrial plants.
William Shockley, John Bardeen and Walter Brattain in 1956 received the Nobel prize in physics "For the study of semiconductors and the discovery of the transistor effect." May never the Nobel prize was not awarded for work that would be to the extent directly affected the lives of countless people.
the Excerpt from the book by Brian Cox, Jeff Forshaw "the Quantum universe. How does what we can't see"
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