Semiconductor:- Materials such as germanium, silicon, carbon, etc. are not good conductors like copper nor insulators like glass. In other words, the resistivity of these substances lies in between conductors and insulators. Such materials are known as semiconductors. Semiconductors have some useful characteristics and are being extensively used in electronics devices. For instance, a transistor—a semiconductor device is very fast replacing to huge vacuum tubes in almost all [quads id=1]applications. Transistors are only one of the semiconductors devices family; and thousands of other semiconductor devices are becoming increasingly popular. In this article, we shall focus our attention on the different parameters of semiconductors.
It is not easy to explain a semiconductor if we want to take into account all its physical properties. However, normally, a semiconductor is elaborate on the basis of current conductivity as under: A semiconductor is a material which has resistivity (10−4 to 0.5 Ωm) in between insulators and conductors e.g. germanium, selenium, carbon, silicon, etc. The reader may wonder, when a semiconductor is not a good conductor nor an insulator, then why not to classify it as a resistance material? The answer shall be readily here if we study the following table :
Comparing the resistivities of the above substance, it is apparent that the resistivity of germanium (semiconductor) is large as compared to copper (conductor) but it is a little bit low when compared with glass (insulator). This shows that the semiconductor resistivity lies in mid of conductor resistivity and insulators resistivity. However, it will be wrong if we take the semiconductor as a resistance substance. For example, nichrome, which is one of the highest resistance matter, has resistivity very lower as compare to germanium. This shows that electrically germanium can’t be cotegraized as a conductor neither insulator nor a resistance material. This gave such materials like germanium the name of semiconductors. It is interesting to note that it is not only the resistivity alone which decides whether material is semiconductor or not. For example, it is just possible to make an alloy whose resistivity falls within the range of semiconductors materials but the alloy cannot become in contrary of semiconductor. In fact, semiconductors have a huge number of peculiar chargecterstics that distinguish them from conductors, insulators, and resistance subistance.
Properties of Semiconductors
(i) The resistivity of a semiconductor material is less than an insulator material but more than a conductor materail.
(ii) Semiconductors materail has a negative temperature co-efficient of resistance i.e. the resistance
of a semiconductor materails decreases with the increase in temperature and vice-versa. For example, germanium is actually an insulator materail at low temperatures but it becomes a good conductor at more temperatures.
(iii) When a suitable metallic impurity (e.g. arsenic, gallium etc.) is combine with a semiconductor material, its electricity conducting properties change appreciably. This charectersitc is most important and is discussed later in detail.
The atoms of all element are bound together by the bonding action of valence electrons. This
bonding is because of the fact that it is the tendency of every atom to complete its last orbit by acquiring 8 electrons in it. However, in most of the materials, the last orbit is not complete i.e. the last orbit does not have 8 electrons. This thing makes the atom active to enter into bargain with other atoms to complete its 8 electrons in the last orbit. To do so, the atom may share, lose or gain valence electrons with other atoms. In semiconductors materails, bonds are made by sharing of valence electrons. Such bonds are known as co-valent bonds. In the creation of a co-valent bond, each atom shares equal number of valence electrons and the contributed electrons are shared by the atoms engaged in the formation of the bond.
Fig. 5.1 shows the co-valent bonds between germanium atoms. A germanium atom has four electrons in valence orbit. It is the tendency of germanium atom to have in last orbit 8 electrons To do so, each germanium atom itself positions between four other germanium atoms as shown in Fig.5.1 (i). Each neighbouring atom contribute one valence electron with the central atom. In this business of sharing electrons, the central atom completes its last orbit by having Eight electrons revolving around the nucleus. In this way, the central atom sets up co-valent bonds. Fig. 5.1 (ii) shows the bonding diagram. The following points may be keep in mind regarding the co-valent bonds :
(i) Co-valent bonds are created by sharing of valence electrons.
(ii) In the formation of co-valent bond, every valence electron of an atom creats direct bond with the valence electron of an adjacent atom. In other words, valence electrons are associated with specified atoms. For this thing, valence electrons in a semiconductor materials are not free.
A material in which the atoms or molecules are arranged in an orderly pattern is called a crystal. All semi-conductors materails have crystalline structure. For example, referring to Fig. 5.1, it is clear that each atom is surrounded by neighbouring atoms in a repetitive manner. Therefore, a little piece of germanium is generally known as germanium crystal.
There are many semiconductors meterials available, but very little of them have a practical application in electronics world. The two most frequently used subistance are germanium (Ge) and silicon (Si). It is due to the energy needed to break their co-valent bonds (i.e. energy required to release an electron from their valence bands) is very little; being nearl 0.7 eV for germanium and nearly 1.1 eV for silicon. Therefore, we shall discuss these two semiconductors materials in detail.
Germanium has become the model materials among the semiconductors world; the main reason being that it can be purified relatively very well and crystallised very easily. Germanium is an earth element and in 1886 was discovered. It is recovered from the ash of some coals or from the flue dust of zinc smelters. Normally, recovered germanium is in the shape of germanium dioxide powder which is then converted to pure germanium.
The atomic number of germanium element is 32. Therefore, it has 32 electrons and 32 protons. 2
electrons are in the 1st orbit, 8 electrons in the 2nd, 8 electrons in the 3rd and 4 electrons in the outer or valence orbit [See Fig. 5.2 (i)]. It is clear that germanium atom has 4 valence electrons i.e., it is a tetravalent element. Fig. 5.2 (ii) shows how the many germanium atoms are held through co-valent bonds. As the atoms are arranged in an orderly form, therefore, germanium has a crystalline structure.
Silicon is an element which is most of the common rocks. In reality, sand is silicon dioxide. The silicon compounds are chemically decreases to silicon which is 100% pure for use as a semiconductor materai.
The atomic number of silicon is fourteen. Therefore, it has fourteen protons and fourteen electrons. 2 electrons are in the 1st orbit, 8 electrons in the 2nd orbit and 4 electrons in the 3rd orbit [See Fig. 5.3 (i)]. It is clear that silicon atom has 4 valence electrons i.e. it is a tetravalent element. Fig. 5.3
(ii) shows how many silicon atoms are bound through co-valent bonds. such as germanium, silicon
atoms are also arranged in an orderly patteren. Therefore, silicon has crystalline structure materail.
It has already been explaind that a semiconductor material is a material whose resistivity lies mid of the conductors and insulators. The resistivity is of the 10−4 to 0.5 ohm meter. However, a semiconductor materail can be explain much more comprehensively on the basis of energy bands/levels as under :
A semiconductor is a material that has an almost complete filled valence band and nearly zero conduction band with a very little energy gap (j 1 eV) separating the two. Figs. 5.4 and 5.5 show the energy band diagrams of silicon and germanium. It may be observed that the forbidden energy gap is very short; being 0.7 eV for germanium and 1.1 eV for silicon. Therefore, relatively little energy is required by their valence electrons to move over to the conduction band. Even at normal room temperature, few of the valence electrons may acquire sufficient energy to move into the conduction band and thus become free electrons. However, at this room temperature, the number of free electrons available is very *small. Therefore, at normal room temperature, a piece of germanium or silicon is not a good conductor material nor an insulator material. For this point, such materials are known as semiconductors. The energy band description is very extremely helpful in understanding or learning the current flowing through a semiconductor material. Therefore, we shall mostly use this concept in our further discussion.
The electrical conductivity of a semiconductor material changes appreciably with changing of temperature. This is a very important point.
At absolute zero temperature, all the electrons are very tightly bound by the semiconductor material atoms. The inner orbit electrons of semiconductor will bound whereas the valence electrons are engaged in covalent bonding. At this temperature, the covalent bonds are very tight and there are zero free electrons. Therefore, the semiconductor material behaves as a perfect insulator [SeeFig. 5.6 (i)]. In terms of energy band description, the valence band is completely filled and there is a huge energy gap between the valence and conduction band. Therefore, zero valence electron can be moved to the conduction band to become a free electron. It is because of the non-availability of free electrons that a semiconductor material behaves as an insulator material.
When the temperature is increased, some of the covalent bonds in the semiconductor material break because of the thermal energy provided. The breaking of bonds sets those electrons free which are engaged in the creating of these bonds. The result is that some free electrons exist in the semiconductor material. These free electrons can constitute a small electric current if the potential difference is
applied across the semiconductor material [See Fig. 5.7 (i)]. This describes that the resistance of semiconductor material reduces with the increase in temperature i.e. it has a negative temperature coefficient of resistance. It may be added that at normal room temperature, the current through a semiconductor material is too small to be of any practical current value.
Fig. 5.7 (ii) describes the energy band diagram. As the temperature is increased, few of the valence electrons acquire sufficient energy to move into the conduction band and thus become free electrons. Under the influence of the electric field, these free electrons will constitute an electric current. It may be pointed that every time a valence electron moves into the conduction band, a hole is a generator in the valence band. As we shall read in the next article (whatswho) , holes also play major to flowing current. In fact, hole current is the most significant concept in semiconductors materials.
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Reference: Principles Of Electronics By V K Mehta And Rohit Mehta
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