Semiconductors
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A conductor is a substance that can conduct electricity or heat through it. In other words, we can say that a conductor is a substance that has a very high electrical or thermal conductivity (a measure of a substance to conduct heat or electricity). Conductors are usually metals, with a high number of free electrons. The energy (electrical or thermal) is transferred through a conductor predominantly by the means of collisions between these free electrons and ions. In the case of electrical conductors, this is because when an electric field is established inside a conductor the free electrons are subjected to a force due to this and start moving with a certain drift speed. This finally results in the flow of electric current. The amount of flow of current is governed by the conductivity of the conductor. Amount of flow of current is directly proportional to its conductivity. Mathematically conductivity is given by the formula;
s = j / E
Where j is the current density and e the applied electric field.
INSULATORS
Insulators are substances that are poor conductors of heat and electricity. This property occurs due to the lack of free or mobile electrons.
Semiconductors are crystalline solids that have electrical conductivity between that of a conductor and an insulator (of the order of 10-5 to 10-7 siemens per metre). Another special feature of a semiconductor is that its conductivity changes with a change in temperature; as the temperature increases the conductivity increases and vice-versa. For the further study of semiconductors we should keep the following two terms in mind.
- ENERGY LEVELS
- ENERGY BANDS
Energy level is defined as the definite or fixed amount of energy that a molecule, atom, electron or a nucleus can have. The atom has a fixed energy governed by the orbital in which it is travelling around the nucleus. The atom passes from one energy level to another without passing through fractions of that energy transition (quantum theory).
Energy band is a range of energies that an electron can have in a solid. Electrons exist in discrete energy levels in a single atom. These isolated electrons can have certain definite energies labeled as 1s, 2s, 2p, 3s etc. The maximum number of electrons which can be accommodated in each energy level are governed by Pauli’s exclusion principle. The electrons in the case of a crystal are influenced by a number of adjacent nuclei, because of a closely packed lattice, thus the sharply defined levels of the atoms become bands of allowed energy. Large numbers of allowed energy is represented by each band. Between the bands are forbidden bands. A valence band in the case of solids represents the outermost electrons of the atoms. This band has the highest energy.
The band structure in the case of solids governs their electrical properties. Electrons change from one quantum state to another, when travelling through the solid. This is possible only if there are empty quantum states, close to the electrons. In other words, we can say that the electrons cannot change to new quantum states in the same band if the valence band is full. Thus, for conduction to occur the electrons must be in an unfilled band or the conduction band. In the case of metals either the valence band and conduction band are half filled or because there is an overlapping of the conduction band with the valence band. In the case of insulators, these two bands are separated by a large number of forbidden band and the electrons do not have much energy to jump from one to the other.
In the case of semiconductors, as the atoms of the crystalline solid are close together, the orbitals of their electron overlap and their individual energy levels are spread out into energy bands. Conduction in the case of semiconductors occurs as a result of the net movement of electrons in the conduction band and empty states, called holes, in the valence band, under the application of an electric field. A hole can be imagined as an electron with a positive charge, because it behaves just like that. Electrons and holes are responsible for the movement of charge inside a semiconductor, and are thus called charge carriers. The charge carrier that is mainly present in a substance is called the majority charge carrier and the other whose concentration is a little lower than these are called minority charge carriers. On the basis of the charge carriers semiconductors are classified into two types;
An intrinsic semiconductor is one in which the concentration of the charge carriers is a characteristic of metal itself. In this type of semiconductor, the charge carriers are equally divided between electrons and holes. Or in other words in an intrinsic semiconductor the number of electrons equals the number of holes. Thus, we can say that in an intrinsic semiconductor, only a small fraction of the valence electrons are able to jump from the valence band to the conduction band.
NOTE: In the case of intrinsic semiconductors, the forbidden gap is narrow and the electrons at the top of the valence band can move by thermal agitation into the conduction band because of the forbidden gap being very low.
2. Extrinsic semiconductors
If some impurity is added to the pure or intrinsic semiconductor then its conduction properties can be changed. These types of semiconductors are called extrinsic semiconductors and this method known as doping. To explain the working of an extrinsic semiconductor we can take the following example. Suppose that in a silicon crystal a small amount of phosphorous is diffused. Since a phosphorous atom has five electrons in its outermost shell, four of its electrons are shared with the neighboring four silicon atoms. The silicon atoms were earlier covalently bonded with neighboring four silicon atoms. Now the fifth electron moves around the phosphorous ion in a large radius. Thus, an extra electron is made available for conduction, increasing the number of conduction electrons. In this case, the number of negative charge carriers increases and thus these type of extrinsic semiconductors are called n-type semiconductors. We could also have added a trivalent impurity such as aluminium. In this case, the same mechanism would have taken place, except that in the case of an extra electron, a extra hole would have been generated in the valence band. In this case, the number of holes increases and these type of extrinsic semiconductors are called p-type semiconductors.
p-n JUNCTION OR A SEMICONDUCTOR DIODE
A p-n junction consists of a semiconductor crystal, doped in such a way that half is p-type and half is n-type. Thus one side has a large number of acceptor impurities and the other side has a large number of donor impurities. Since there is a large number of holes on one side and a large number of electrons on the other side, this does not remain in equilibrium. Because of this some of the valence electrons, on the side of their greater concentration, jump from their side onto the other side and fill some of the vacancies. This happens very close to the junction. Similarly, some conduction electrons also diffuse to the other side. This results in one of the sides, from where electrons have jumped, getting positively charged and the other half negatively charged. Thus, an electric field is created near the junction, whose direction is given by n-type to p-type. The layer in which an electric field exists is called as depletion layer. These electrons and holes continue moving until the electric field exerts a force on the holes as they come in the depletion layer. As a result of this only the holes with a high kinetic energy are able to cross over to the other side. Similarly, the electrons also experience a force due to the same electric field and are forced back to stay in the n-region, except a few who have a high kinetic energy. A potential barrier is thus created, which tends to keep the remaining electrons in the n-region and remaining holes in the p-region. Some electrons and holes, with a high energy, do cross this potential barrier and some current does flow from the p-region to the n-region. This current is called diffusion current.