 Namaste. Myself Dr. Mrs. Preeti Sunil Joshi working as assistant professor in Valshan Institute of Technology, Solapur. This session is related with the study of extrinsic semiconductors. Learning outcomes are by the end of this session students will be able to state formation properties and classification of extrinsic semiconductors. The contents include extrinsic semiconductor and classification. We are going to have a discussion on classification of semiconductors which will help us understand the basics of semiconductors better. So let us recall intrinsic semiconductor. In order for a semiconductor crystal to be formed, atoms share electrons with neighboring atoms. For example, in a silicon crystal, every silicon atom inside has four neighboring atoms. Central silicon atoms share their four valence electrons with four neighboring silicon atoms to have eight electrons in its valence orbit which produces a state of chemical stability. This sharing of valence electrons produces covalent bonds that hold the atoms together in a silicon crystal. Now if a crystal is purely made of silicon atoms, that silicon crystal is classified as an intrinsic semiconductor. This is because an intrinsic semiconductor is a pure semiconductor. At room temperature, the heat energy in the air allows the valence electrons in an intrinsic semiconductor to jump into the conduction band and become free electrons. In every electron that jumps into the conduction band, there is also a vacancy which is known as hole that is left in the valence band of the intrinsic semiconductor material. So in an intrinsic semiconductor, the thermal energy just creates an equal number of free electrons and holes. Intrinsic semiconductors act more like an insulator at room temperature. This is because the thermal energy at room temperature only produces a few free electrons and holes which are the current carriers in a semiconductor material. Increasing the operating temperature might produce more free electrons and holes but of course operating at high temperature is not ideal and it will still just create an equal number of free electrons and holes. Yet we say that a semiconductor can be a very useful device in electronics by controlling its conductivity. However, it is not possible in an intrinsic semiconductor since they have poor conductivity. To increase its conductivity, intrinsic semiconductors must be doped with impurity atoms. A doped semiconductor is classified as an extrinsic semiconductor. So a cautious introduction of impurity atoms in a perfect semiconductor crystal produces useful modifications of its electrical conductivity. A controlled amount of impurity added into an intrinsic semiconductor is known as doping. The impurity which is introduced is known as a dopant and a semiconductor that is doped with impurity atoms is called an extrinsic semiconductor. The impurity produced electrons are not temperature dependent but are voltage dependent and they will be under our control. The pentavalent elements from group 5th or trivalent elements from group 3rd are used as dopants. The atoms belonging to these two groups are nearly of the same size as silicon or germanium atoms and easily substitute themselves in place of some of the host atoms in the semiconductor crystal. Thus they are the substitutional impurities and do not cause any distortion in the original crystal structure. Conducting can either increase the number of free electrons or holes in a semiconductor. Because of this, extrinsic semiconductors have two types that is n-type and p-type semiconductors. The advantages of extrinsic semiconductors include the conductivity is high, the conductivity can be tailored to the desired value through the control of doping concentration and here the conductivity is not a function of temperature. Let us now see n-type semiconductor in detail. An n-type semiconductor is produced when a pure semiconductor is doped with a pentavalent impurity such as phosphorus. A phosphorus atom has 5 valence electrons and out of 5 electrons only 4 participate in bonding with 4 host silicon atoms while the 5th electron remains loosely bound. We can see in the diagram. So as a result, the Coulomb force between the phosphorus nucleus and the 5th electron is smaller than that it would be in free space. Therefore, the ionization energy of the 5th electron is very small. Hence the thermal energy can easily liberate the 5th electron from the nucleus. It means that the energy levels corresponding to phosphorus atoms are nearer to the bottom edge of the conduction band. The energy band diagram of n-type semiconductor is shown in the figure here. The energy level due to phosphorus is known as donor level and it represents the ground state of the 5th electron of phosphorus atom. As even a small amount of thermal energy can readily liberate 5th electron from the atom and send it into the conduction band, the donor levels are expected to be located very near to the bottom edge of the conduction band. In n-type semiconductor, the total number of conduction electrons is due to the electrons contributed by donors and those generated intrinsically, while the total number of holes is only due to the holes from the intrinsic source. But the art of recombination of holes would increase the number of electrons. As a result, electrons are the majority carriers and holes are the minority carriers in case of n-type semiconductor. A p-type semiconductor is produced when a pure semiconductor is doped with a trivalent impurity such as aluminum. Aluminium atom has three valence electrons therefore it falls short of one electron for completing the four covalent bonds with its neighbors. When an electron from a neighboring atom acquires energy and jumps into the vacancy to form the fourth bond, it leaves behind a hole. The aluminum atom having acquired an additional electron becomes a negative ion. The hole can move freely in the valence band whereas the impurity ion is fixed in position by the covalent bonds. As the aluminum atom accepted an electron from the valence band, it is called an acceptor atom. The acceptor impurity atom produce holes without the simultaneous generation of electrons in the conduction band. The energy band diagram of p-type semiconductor we can see in the figure. The acceptor level represents the ground level of the hole. As even small amount of thermal energy can make an electron in the valence band jump into the acceptor level, the acceptor levels are expected to be located very near to the top edge of the valence band. A hole is said to have moved from the acceptor atom to the valence band. Therefore in p-type semiconductor holes are the majority charge carriers and electrons are the minority charge carriers. Students now please pause the video and try to write down few comparing points between n and p-type semiconductors. Please check for the correct answers. Now in the doping process manufacturers melt a pure silicon crystal. This will break the covalent bonds and turns the solid silicon crystal to a liquid meaning the atoms don't have any set order or structure. Then impurity atoms are added. If we take pentavalent atoms and add them to liquefied silicon it increases the number of free electrons and creates an n-type semiconductor and going the other way we can add a trivalent atoms which will increase the number of holes creating a p-type semiconductor. Manufacturers control the conductivity of a doped semiconductor through the amount of impurity atoms added. In this case a semiconductor can be lightly or heavily doped. Lightly doped semiconductors have high resistance while low resistance semiconductors are heavily doped. But with careful manipulation of how semiconductor materials of different types interface and the application of voltage in specific ways the resistance can be further increased or decreased. Then let us see the uses of extrinsic semiconductors. Extrinsic semiconductors are components of many common electrical devices. A semiconductor diode the device that allow current only in one direction consists of p-type and n-type semiconductor placed in junction with one another. Currently most semiconductor diodes use doped silicon or germanium. Then transistors they also make use of extrinsic semiconductors bipolar junction transistors that is BJT which amplify current are one type of transistor. The most common BJTs are NPN and PNP type. Field effect transistors are another type of transistor which amplify current implementing extrinsic semiconductors. As opposed to BJTs they are called unipolar because they involve single carrier type operation either n-channel or p-channel. When other devices implementing the extrinsic semiconductor are laser, solar cells, photo detectors, light emitting diodes, thyristors etc. Thank you.