 Namaste. Myself Dr. Mrs. Preeti Sunil Joshi working as assistant professor in Valshan Institute of Technology, Sallapur. In this session we are going to study intrinsic semiconductors in detail. Learning outcomes are by the end of this session students will be able to state formation and properties of intrinsic semiconductors. The contents include intrinsic semiconductors, current conduction in semiconductor and limitations of intrinsic semiconductor. In last session we have seen the classification of solids depending on band theory. Solids are classified into three categories. That is, semiconductors are the materials having electrical conductivity considerably greater than that of an insulator but significantly lower than that of a conductor. Of all the elements in the periodic table, eleven elements are semiconductors. Germanium and silicon are the most widely used semiconductors in device manufacturing applications and they are known as elemental semiconductors. Besides these, there are certain compound semiconductors such as gallium, arsenide, indium phosphide, etc. which are formed from the combination of the elements of groups 3rd and 5th or 2nd and 6th. Silicon and germanium are elemental semiconductors. They belong to 4th A group in the periodic table. Silicon has 14 electrons and electronic configuration is 1S2, 2S2, 2P6, 3S2, 3P2. Similarly, germanium has 32 electrons so each of them has 4 valence electrons which are distributed among the outermost S and P orbitals. The unique and interesting feature of semiconductors is that they are bipolar and two charge carriers that is electrons and holes they transport current in these materials. The electrical conductivity of a pure semiconductor which is known as intrinsic conductivity is significantly low and is drastically influenced by the temperature. Then doped semiconductors are known as extrinsic semiconductors. Extrinsic semiconductors are widely used in fabrication of solid state devices. Let us now discuss these two types of semiconductors that is intrinsic and extrinsic semiconductor in detail. An intrinsic semiconductor is the purest form of a semiconductor that is without any impurities. Naturally available elements like silicon and germanium are the best examples of an intrinsic semiconductor. Here we can see a two dimensional representation of a silicon crystal in which each silicon atom forms covalent bonds with four surrounding atoms. The shaded circle here represents the cores of silicon atoms. The four valence electrons they are shown by the black dots which surrounds each shaded circle. The probability of valence electrons being in any place between the bonding atoms is indicated by the curves. Let us now see the current conduction in intrinsic semiconductor. Consider the first case when the temperature is equal to 0 Kelvin or close to 0 Kelvin. All the valence electrons are locked in covalent bonds as we can see in the figure. And these valence electrons spend most of the time between neighboring atoms. Since all the valence electrons are engaged in covalent bonds, the bonds are complete. The energy available at 0 Kelvin is not sufficient to break these covalent bonds. Therefore there are no free electrons within the material at absolute 0. This is the corresponding energy band diagram. There are no electrons left to go into the conduction band. At 0 Kelvin, electrons in the valence band do not possess enough energy to jump into the conduction band. As free electrons do not exist in the conduction band and externally applied electric field cannot cause flow of current through the crystal. Hence an intrinsic semiconductor behaves as a perfect insulator at 0 Kelvin. Now at temperature above absolute 0. At this temperature, the finite thermal energy causes each atom in the crystal to vibrate about its mean position. Whenever a covalent bond is ruptured by thermal energy, a valence electron becomes free. The higher the temperature, the more covalent bonds are broken. The electrons liberated from bonds move randomly in the void spaces between the atoms in the crystal. And now if an electric field is applied, these free electrons cause electrical conduction. This is the corresponding energy band diagram. When an electron from the valence band jumps to the conduction band, an empty state arises in the valence band. Thus, now both the bands are partially filled. The electrons in the conduction band and the electrons in the valence band can be excited to upper vacant levels within the respective bands. Therefore, when an electric field is applied, these electrons can move into the higher vacant levels and current flows in the crystal at ordinary temperature. Thus, in pure semiconductors, all available charge carriers that are electrons and holes arise due to thermally ruptured bonds and these thermally generated electron hole pairs cause electrical conduction. This thermal generation is an intrinsic process. Therefore, we can define intrinsic semiconductors also as an intrinsic semiconductor is a semiconductor crystal in which electrical conduction arises due to thermally excited electrons and holes. The band gap energy is the minimum energy required for breaking of a covalent bond. And this band gap energy for germanium crystal its value is 0.72 electron volt and for silicon crystal the value is 1.12 electron volt. Students, now please pause the video, think for a while and try to answer these questions. Check for the answers. Yes, silicon has 4 electrons in its outermost orbit. Therefore, option D is correct for the first question. And for the second question, as addition of impurity increases the number of electrons, so the collisions between them will also increase which leads to the increase in thermal and electrical resistance. Therefore, electrical and thermal conductivities both decrease. That's why option B is correct. Now let us see the limitations of intrinsic semiconductors. In the limitations it is seen that we cannot use intrinsic semiconductors as they are available in device manufacture. They possess low conductivity which strongly depends on temperature. So if we take a crystal of pure silicon or germanium and connect it in the circuit, the current in the circuit will increase with voltage. So increasing the voltage increases the current that is according to Ohm's law. We will also find that the current in the circuit will gradually increase as the temperature of the crystal is increased. Increasing the temperature increases the current at an exponential rate. Thank you.