1.1 Semiconducting Materials and pān Junctions
As is known, all substances, depending on their electroconductivity, are divided into three groups: conductors (usually metals) with a resistance of 10ā6ā10ā3 Ī©cm, dielectrics with a resistance of 109ā1020 Ī©cm, and semiconductors (many native-grown and artificial crystals) covering an enormous intermediate range of values of specific electrical resistance.
The main peculiarity of crystal substances is typical, well-ordered atomic packing into peculiar blocksācrystals. Each crystal has several flat symmetric surfaces and its internal structure is determined by the regular positional relationship of its atoms, which is called the lattice. Both in appearance and in structure, any crystal is like any other crystal of the same given substance. Crystals of various substances are different. For example, a crystal of table salt has the form of a cube. A single crystal may be quite large in size or so small that it can only be seen with the help of a microscope. Substances having no crystal structure are called amorphous. For example, glass is amorphous in contrast to quartz, which has a crystal structure.
Among the semiconductors that are now used in electronics, one should point out germanium, silicon, selenium, copper-oxide, copper sulfide, cadmium sulfide, gallium arsenide, and carborundum. To produce semiconductors, two elements are mostly used: germanium and silicon.
In order to understand the processes taking place in semiconductors, it is necessary to consider phenomena in the crystal structure of semiconductor materials, which occur when their atoms are held in a strictly determined relative position to each other due to weakly bound electrons on their external shells. Such electrons, together with electrons of neighboring atoms, form valence bonds between the atoms. Electrons taking part in such bonds are called valence electrons. In absolutely pure germanium or silicon at very low temperatures, there are no free electrons capable of creating electric current, because under such circumstances all four valence electrons of the external shells of each atom that can take part in the process of charge transfer are too strongly held by the valence bounds. That is why this substance is an insulator (dielectric) in the full sense of the word: It does not let electric current pass at all.
When the temperature is increased, due to the thermal motion some valence electrons detach from their bonds and can move along the crystal lattice. Such electrons are called free electrons. The valence bond from which the electron is detached is called a hole. It possesses properties of a positive electric charge, in contrast to the electron, which has a negative electric charge. The higher the temperature is, the more free electrons that are capable of moving along the lattice and the higher the conductivity of the substance.
Moving along the crystal lattice, free electrons may run across holesā valence bonds missing some electronsāand fill up these bonds. Such a phenomenon is called recombination. At normal temperatures in the semiconductor material, free electrons occur constantly, and recombination of electrons and holes takes place.
If a piece of semiconductor material is put into an electric field by applying a positive or negative terminal to its ends, for instance, electrons will move through the lattice toward the positive electrode and holes to the negative one. The conductivity of a semiconductor can be enhanced considerably by applying specially selected admixtures (metal or nonmetal) to it. In the lattice, the atoms of these admixtures will replace some of the atoms of the semiconductors. Let us remind ourselves that external shells of atoms of germanium and silicon contain four valence electrons and that electrons can only be taken from the external shell of the atom. In their turn, the electrons can be added only to the external shell, and the maximum number of electrons on the external shell is eight.
When an atom of the admixture has more valence electrons than required for valence bonds with neighboring atoms of the semiconductor, additional free electrons capable of moving along the lattice occur on it. As a result the electroconductivity of the semiconductor increases. As germanium and silicon belong to the fourth group of the periodic table of chemical elements, donors for them may be elements of the fifth group, which have five electrons on the external shell of atoms. Phosphorus, arsenic, and stibium belong to such donors (donor admixture).
If admixture atoms have fewer electrons than needed for valence bonds with surrounding semiconductor atoms, some of these bonds turn out to be vacant and holes will occur in them. Admixtures of this kind are called p-type ones because they absorb (accept) free electrons. For germanium and silicon, p-type admixtures are elements from the third group of the periodic table of chemical elements, the external shells of atoms that contain three valence electrons. Boron, aluminum, gallium, and indium can be considered p-type admixtures (accepter admixture).
In the crystal structure of a pure semiconductor, all valence bonds of neighboring atoms turn out to be fully filled, and occurrence of free electrons and holes can be caused only by deformation of the lattice arising from thermal or other radiation. Because of this, conductivity of a pure semiconductor is quite low under normal conditions.
If some donor admixture is injected, the four electrons of the admixture, together with the same number in the filled valence, bond with the latter. The fifth electron of each admixture atom appears to be āexcessiveā or āredundantā and therefore can freely move along the lattice.
When an accepter admixture is injected, only three filled valence bonds are formed between each atom of the admixture and neighboring atoms of the semiconductor. One electron is lacking to fill up the fourth. This valence bond appears to be vacant. As a result, a hole occurs. Holes can move along the lattice like positive charges, but instead of an admixture atom, which has a fixed and permanent position in the crystal structure, the vacant valence bond moves.
It goes like this. An electron is known to be an elementary carrier of an electric charge. Affected by different causes, the electron can escape from the filled valence bond, leaving a hole that is a vacant valence bond and that behaves like a positive charge equaling numerically the negative charge of the electron. Affected by the attracting force of its positive charge, the electron of another atom near the hole may ājumpā to the hole. At that point, recombination of the hole and the electron occurs, their charges are mutually neutralized, and the valence bond is filled. The hole in this place of the lattice of the semiconductor disappears.
In its turn a new hole, which has arisen in the valence bond from which the electron has escaped, may be filled with some other electron that has left a hole. Thus, moving of electrons in the lattice of the semiconductor with a p-type admixture and recombination of them with holes can be regarded as moving of holes. For better understanding, one may imagine a concert hall in which for some reason som...
