At Normal room temperature, few of the covalent bonds in pure semiconductor material break, setting up free electrons. Under the effect of the electric field, these free electrons constitute an electric current. At the same time, another current – the hole current – also flows in the semiconductor. When a covalent bond is broken because of the thermal energy, the removal of one electron leaves an area i.e. a missing or leaved electron in the covalent bond. This missing electron is known as a hole that behaves as a positive charge. For one electron set free, one hole is created. Therefore, thermal energy creates hole-electron pairs; there being as many holes as the free electrons. The current conduction by holes can be explained as follows :
The hole shows a missing electron. Suppose the valence electron at L (See Fig. 5.8) has become free electron due to thermal energy. This creates a hole in the co-valent bond at L. The hole is a strong centre of attraction **for the electron. A valence electron (say at M) from nearby co-valent bond comes to fill in the hole at L. This results in the creation of hole at M. Another valence electron (say at N) in turn may leave its bond to fill the hole at M, thus creating a hole at N. Thus the hole having a positive charge has moved from L to N i.e. towards the negative terminal of supply. This constitutes hole current. It may be noted that hole current is due to the movement of ***valence electrons from one covalent bond to another bond. The reader may wonder why to call it a hole current when the conduction is again by electrons (of course valence electrons !). The answer is that the basic reason for current flow is the presence of holes in the covalent bonds. Therefore, it is more appropriate to consider the current as the movement of holes.
Energy band description. The hole current can be beautifully explained in terms of energy bands. Suppose due to thermal energy, an electron leaves the valence band to enter into the conduction band as shown in Fig. 5.9.This leaves a vacancy at L. Now the valence electron at M comes to fill the hole at L. The result is that hole disappears at L and appears at M. Next, the valence electron at N moves into the hole at M. Consequently, hole is created at N. It is clear that valence electrons move along the path PNML whereas holes move in the opposite direction i.e. along the path LMNP.
A semiconductor in an extremely pure form is known as an intrinsic semiconductor. In an intrinsic semiconductor, even at room temperature, hole-electron pairs are created. When electric field is applied across an intrinsic semiconductor, the current conduction takes place by two processes, namely ; by free electrons and holes as shown in Fig. 5.10. The free electrons are produced due to the breaking up of some covalent bonds by thermal energy. At the same time, holes are created in the covalent bonds. Under the influence of electric field, conduction through the semiconductor is by both free electrons and holes. Therefore, the total current inside the semiconductor is the sum of currents due to free electrons and holes.
It may be noted that current in the external wires is fully electronic i.e. by electrons. What about the holes ? Referring to Fig. 5.10, holes being positively charged move towards the negative terminal of supply. As the holes reach the negative terminal B, electrons enter the semiconductor crystal near the terminal and combine with holes, thus cancelling them. At the same time, the loosely held electrons near the positive terminal A are attracted away from their atoms into the positive terminal. This creates new holes near the positive terminal which again drift towards the negative terminal.
The intrinsic semiconductor has little current conduction capability at room temperature. To be
useful in electronic devices, the pure semiconductor must be altered so as to significantly increase its conducting properties. This is achieved by adding a small amount of suitable impurity to a semiconductor. It is then called impurity or extrinsic semiconductor. The process of adding impurities to a semiconductor is known as doping. The amount and type of such impurities have to be closely controlled during the preparation of extrinsic semiconductor. Generally, for 108 atoms of semiconductor, one impurity atom is added. The purpose of adding impurity is to increase either the number of free electrons or holes in the semiconductor crystal. As we shall see, if a pentavalent impurity (having 5 valence electrons) is added to the semiconductor, a large number of free electrons are produced in the semiconductor. On the other hand, addition of trivalent impurity (having 3 valence electrons) creates a large number of holes in the semiconductor crystal. Depending upon the type of impurity added, extrinsic semiconductors are classified into:
When a small amount of pentavalent impurity is added to a pure semiconductor, it is known as
n-type semiconductor. The addition of pentavalent impurity provides a large number of free electrons in the semiconductor crystal.
Typical examples of pentavalent impurities are arsenic (At. No. 33) and antimony (At. No. 51). Such impurities which produce n-type semiconductor are known as donor impurities because they donate or provide free electrons to the semiconductor crystal. To explain the formation of n-type semiconductor, consider a pure germanium crystal. We know that germanium atom has four valence electrons. When a small amount of pentavalent impurity like arsenic is added to germanium crystal, a large number of free electrons become available in the crystal. The reason is simple. Arsenic is pentavalent i.e. its atom has five valence electrons. An arsenic atom fits in the germanium crystal in such a way that its four valence electrons form covalent bonds with four germanium atoms. The fifth valence electron of arsenic atom finds no place in co-valent bonds and is thus free as shown in Fig. 5.11. Therefore, for each arsenic atom added, one free electron will be available in the germanium crystal. Though each arsenic atom provides one free electron, yet an extremely small amount of arsenic impurity provides enough atoms to supply millions of free electrons. Fig. 5.12 shows the energy band description of n-type semi-conductor. The addition of pentavalent impurity has produced a number of conduction band electrons i.e., free electrons. The four valence electrons of pentavalent atom form covalent bonds with four neighbouring germanium atoms. The fifth left over valence electron of the pentavalent atom cannot be accommodated in the valence band and travels to the conduction band.
The following points may be noted carefully :
(i) Many new free electrons are produced by the addition of pentavalent impurity.
(ii) Thermal energy of room temperature still generates a few hole-electron pairs. However, the
number of free electrons provided by the pentavalent impurity far exceeds the number of holes. It is due to this predominance of electrons over holes that it is called n-type semiconductor (n stands for negative). n-type conductivity. The current conduction in an n-type semiconductor is predominantly by free electrons i.e. negative charges and is called n-type or electron type conductivity. To understand n-type conductivity, refer to Fig. 5.13. When p.d. is applied across the n-type semiconductor, the free electrons (donated by impurity) in the crystal will be directed towards the positive terminal, constituting electric current. As the current flow through the crystal is by free electrons which are carriers of negative charge, therefore, this type of conductivity is called negative or n-type conductivity. It may be noted that conduction is just as in ordinary metals like copper. Fig. 5.11
When a small amount of trivalent impurity is added to a pure semiconductor, it is called p-type
semiconductor. The addition of trivalent impurity provides a large number of holes in the semiconductor. Typical examples of trivalent impurities are gallium (At. No. 31) and indium (At. No. 49). Such impurities which produce p-type semiconductor are known as acceptor impurities because the holes created can accept the electrons. To explain the formation of p-type semiconductor, consider a pure germanium crystal. When a small amount of trivalent impurity like gallium is added to germanium crystal, there exists a large number of holes in the crystal. The reason is simple. Gallium is trivalent i.e. its atom has three valence electrons. Each atom of
gallium fits into the germanium crystal but now only three co-valent bonds can be formed. It is because three valence electrons of gallium atom can form only three single co-valent bonds with three germanium atoms as shown in Fig. 5.14. In the fourth co-valent bond, only germanium atom contributes one valence electron while gallium has no valence electron to contribute as all its three valence electrons are already engaged in the co-valent bonds with neighbouring germanium atoms. In other words, fourth bond is incomplete; being short of one electron. This missing electron is called a hole. Therefore, for each gallium atom added, one hole is created. A small amount of gallium provides millions of holes. Fig. 5.15 shows the energy band description of the p-type semiconductor. The addition of trivalent impurity has produced a large number of holes. However, there are a few conduction band electrons due to thermal energy associated with room temperature. But the holes far outnumber the conduction band electrons. It is due to the predominance of holes over free electrons that it is called p-type semiconductor ( p stands for positive).
p-type conductivity. The current conduction in p-type semiconductor is predominantly by holes
i.e. positive charges and is called p-type or hole-type conductivity. To understand p-type conductivity, refer to Fig. 5.16. When p.d. is applied to the p-type semiconductor, the holes (donated by the impurity) are shifted from one co-valent bond to another. As the holes are positively charged, therefore, they are directed towards the negative terminal, constituting what is known as hole current. It may be noted that in p-type conductivity, the valence electrons move from one co-valent bond to another unlike the n-type where current conduction is by free electrons.
As discussed before, in n-type semiconductor, current conduction is due to excess of electrons whereas in a p-type semiconductor, conduction is by holes. The reader may think that n-type material has a net negative charge and p-type a net positive charge. But this conclusion is wrong. It is true that n-type semiconductor has excess of electrons but these extra electrons were supplied by the atoms of donor impurity and each atom of donor impurity is electrically neutral. When the impurity atom is added, the term “excess electrons” refers to an excess with regard to the number of electrons needed to fill the co-valent bonds in the semiconductor crystal. The extra electrons are free electrons and increase the conductivity of the semiconductor. The situation with regard to p-type semiconductor is also similar. It follows, therefore, that n-type as well as p-type semiconductor is electrically neutral.
It has already been discussed that due to the effect of impurity, n-type material has a large number of free electrons whereas p-type material has a large number of holes. However, it may be recalled that even at room temperature, some of the co-valent bonds break, thus releasing an equal number of free electrons and holes. An n-type material has its share of electron-hole pairs (released due to breaking of bonds at room temperature) but in addition has a much larger quantity of free electrons due to the effect of impurity. These impurity-caused free electrons are not associated with holes. Consequently, an n-type material has a large number of free electrons and a small number of holes as shown in Fig. 5.17 (i). The free electrons in this case are considered majority carriers — since the majority portion of current in n-type material is by the flow of free electrons — and the holes are the minority carriers.
Similarly, in a p-type material, holes outnumber the free electrons as shown in Fig. 5.17 (ii).
Therefore, holes are the majority carriers and free electrons are the minority carriers.\
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Diode | Types | Properties | Applications (Hole Current | Intrinsic & Extrinsic Semiconductor)
Chapter Review Topics |Problems |Discussion Questions (Hole Current | Intrinsic & Extrinsic Semiconductor)
MCQ’s| Electrons|Atomic | Voltage |Thevenin’s (Hole Current | Intrinsic & Extrinsic Semiconductor)
Maximum Power Transfer Theorem |Applications (Hole Current | Intrinsic & Extrinsic Semiconductor)
Thevenin’s Theorem | Properties | Problems (Hole Current | Intrinsic & Extrinsic Semiconductor)
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Reference: Principles Of Electronics By V K Mehta And Rohit Mehta
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