Preparation of the
Semiconductors for electronic use |
Culture and doping of the
monocrystals |
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Created it, 05/10/15
Update it, 06/01/13
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SEMICONDUCTORS 2 “4th PART”
The explanation on the flow of the current in a semiconductor of the type P can seem complex, owing to the fact that it supposes a conduction due to positive loads. However, the phenomenon is justified by measurements relating to the Hall effect which highlight the displacement of positive loads (holes) in a semiconductor of the P. type.
Hall effect (of the name of the scientist which discovered it), can be emphasized by the schematically shown experiment figure 6.
Let us suppose initially that a tension continues is applied to a conducting block out of ordinary matter using two plates AA' and BB'. In this case, the current crossing this block is made of electrons circulating of BB' towards AA'.
Let us place then the block in a uniform magnetic field, created by a magnet, so that the lines of flow are perpendicular to the block.
The electrons circulating in the block are influenced by the magnetic field and tend to move in a direction perpendicular to that of their own movement and lines of the magnetic field.
In case illustrated figure 6 where the electrons move BB' towards AA' whereas the tension fields go from the top (North Pole) to the bottom (South Pole), the loads tend to move on the line of their initial movement. It results from it that these loads (electrons) are not uniformly any more distributed in the block, but concentrate on the side A'B'.
This accumulation of the electrons produces an effect which can be highlighted by a very sensitive voltmeter. Indeed, by connecting the apparatus as indicated figure 6, one measures a potential difference electric: the side A'B' is negative compared to side AB. This potential difference comes obviously from the concentration from the electrons on the side A'B'.
The Hall effect consists precisely of this deformation of the threads of current (flow of electrons), in a conducting plate under the effect of a magnetic field perpendicular to this plate. This effect checks a semiconductor block still in the case of N, owing to the fact that in fact the electrons form the flow of the most important current.
On the other hand, with a semiconductor block of the type P (figure 7), the tension indicated by the voltmeter is of polarity opposed to that which one had with the ordinary conducting block or the semiconductor N.
This result can be explained only if it is admitted that the live loads are positive in the case of the semiconductor of the P. type.
They will be shifted, under the action of the magnetic field, towards the left of the direction of their run which is contrary with that of the electrons of figure 6. This concentration of holes on the face A'B' involves, it also, an opposite polarity of that which was present in the case of an ordinary driver or of a semiconductor. Thus, it is confirmed in experiments that the holes (positive loads) form the current circulating in the semiconductors of the P. type.
Using the Hall effect, one also could measure the mobility of the loads in the two types of semiconductor.
Mobility expresses, in centimetre a second (cm/S), the speed of the loads which move in a cube of semiconductor of 1 cm of edge when the tension applied to two opposite faces is 1 volt.
This measurement made it possible to determine that the electrons have a mobility higher than that of the holes (still called cavity or gap).
The average of life of the holes and the free electrons is a parameter important not to neglect; it corresponds to the interval of time which runs out between the moment when in the semiconductor the concentration of the holes and the free electrons undergoes a variation, and that when it returns to the conduction of balance (recombination of the loads).
3. - PREPARATION
OF THE SEMICONDUCTORS FOR ELECTRONIC USE
Germanium and silicon are the semiconductors most largely used in the manufacture of the electronic components.
One finds germanium in the form of rock salt in certain types of rock: coal (or fossil coal), ores of zinc and cadmium.
Most of germanium intended for the electronic uses is extracted from the by-products of the industrial transformations of the sphalerite (ore of zinc) and of fossil carbon (coal).
By subjecting to convenient treatments soots of carbon and slags of the zinc ore, one obtains a compound of germanium called germanium dioxide. The dioxide thus extract is far from having the required purity. It indeed contains many traces of foreign elements which it is advisable to eliminate by chemical means before extracting germanium. The purified germanium dioxide is placed in a crucible which passes in a furnace to atmosphere of hydrogen and at exit, one obtains germanium (figure 8).
This one has a white-silver plated aspect, shining ; it is already relatively pure, but not enough for the use in the electronics industry. To obtain the wanted purity, one puts germanium in a graphite crucible, which passes slowly in a special continuous pipe still, schematically illustrated figure 9-a.
The furnace consists of a long quartz tube, on which reels traversed by a high frequency very intense current are. This current produces inside the furnace a variable magnetic field particularly powerful under each reel. Thus, in the germanium which is under the reels, there is a strong heating and a fast fusion. The fusion of germanium remains limited to the zones placed under the reels. Consequently, while the crucible slowly advances from one end to another of the furnace, the zones in fusion move in germanium in the direction opposed to that of the crucible.
The face of solidification which advances from left to right (fig 9-b) while following the zone in fusion, can be compared with a porous filter which lets pass germanium, and maintains the impurities dissolved in the molten zone.
The operation is repeated under each reel, and on the outlet side of the furnace one obtains a germanium of a high degree of purity.
Like germanium, the silicon intended for electronic uses must be very pure. Normally, silicon is extracted from silicon dioxide while carrying a sand-coal mixture at a temperature of 3 000°C.
The silicon which one obtains by this process contains 2 to 3 % of impurities, whereas for the use in electronics this value must be lower than 0,05 %.
One can obtain a certain degree of purity by chemically treating silicon with various acids.
For some time, one however obtains better results starting from the chemical treatment of a salt of silicon instead of sand.
For the physical refining of the silicon crystals, one uses an alternative of the illustrated method appears 9-a.
The silicon bar is placed in the position driving in the center of a continuous pipe still HF.
Along the external wall of the furnace, one reduces a whorl traversed by an intense current HF. In correspondence with this whorl, one forms in silicon a molten zone which moves in the bar from one end to another. The filter action is exerted thus in the same way that that described for germanium (figure 9-b).
The process is repeated several times in order to obtain the degree of purity necessary.
4. - CULTURE
AND DOPING OF THE MONOCRYSTALS
After the refining, the semiconductor is presented in the form of a solid aggregate, consisted innumerable crystals, very small and laid out in all the directions.
The semiconductor thus formed, although being very pure, is not yet usable for the realization of the diodes and the transistors. For these components, it is necessary to transform the aggregate into monocrystal, i.e. in single and large crystal. Figure 10 illustrates two methods of formation of the germanium monocrystals.
The first method (figure 10-a) constitutes a new application of the furnace HF (High frequency) and mobile zone in fusion.
The germanium bar obtained at the end of the refining is placed in a quartz crucible with a germanium monocrystal called seed and a layer of graphite. The crucible advances slowly in the furnace so that the zone in fusion regularly moves from one end to another of the bar.
The molten part is solidified in the form of a single crystal directed according to the crystalline reticle of the seed.
During the formation of the monocrystal, one introduces rigorously proportioned quantities of foreign substances, in order to transform the intrinsic semiconductor into semiconductor of the type P or N.
The substances most frequently used to form the semiconductors of the type P are indium, aluminum and gallium. For the semiconductors of the type N, one uses arsenic, phosphorus and antimony.
For the formation of the monocrystal, one can also proceed according to the illustrated method appears 10-b. The installation includes/understands primarily a heater, a crucible and a tree of rising.
In the crucible, one dissolves the germanium and materials of impurity necessary to the realization of the types P or N. Initially, the seed is placed at the lower part of the tree of rising, and adheres thus thereafter to surface germanium in fusion contained in the crucible.
By regulating the temperature, one can obtain that germanium starts to crystallize at the point of contact with the seed. At this time, the tree of rising goes up very slowly so that the crystal in formation continues to grow.
In general at the end of this stage, the monocrystal is presented in the form of cylindrical ingot. This one, after some controls concerning the perfect distribution of the impurities, is cut in plates very fine called pastilles (figure 11-a). These pastilles are levelled on the two faces, polished with solvents, then cut out in plates (figure 11-b).
Another method of increase in a crystal largely diffused is that of the epitaxial growth (figure 12).
This technique makes it possible to deposit on the surface of a crystalline product another product which is brought to him in the form of vapor. The layer deposited on the surface of the product said, epitaxial layer, has same doping as the vaporized product.
The epitaxial crystalline growth makes it possible to obtain the formation of very fine crystalline mono layers a thickness about the micron (thousandths of millimetre).
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