EST 130 BASICS OF ELECTRONICS ENGINEERING || ENERGY BAND THEORY || KTU

Hi all, welcome to the 6th lecture of basic electronics. Today we are going to discuss about energy band theory. If we take a single isolated atom, each of its orbit and electrons have a definite energy. But in the case of solids, all the atoms are close to each other. All the atoms are packed close to each other. So energy levels of outermost orbit electrons are affected by the neighboring atoms. Two single or isolated atoms are brought close to each other. Outermost orbit electrons of two atoms will interact. Therefore, the outermost orbit of an atom in the outermost orbit will experience attractive forces from the nearest neighboring atomic nucleus. Therefore, these electrons will never have the same energy level. So the energy levels of electrons are changed to a value which is higher or lower than the original energy level of electrons. The energy level of this electron will be higher or lower than the actual energy level. The electrons in the same orbit will exhibit different energy levels. This grouping of different energy levels is called energy band. Outermost orbit electrons will exhibit different energy levels. This grouping of different energy levels is called energy band. This is an individual atom. This is the outermost shell of the individual atom. There are electrons in the outermost shell. In the case of solids, all the atoms are packed close to each other. The electrons in the outermost orbit of each individual atom are attracted to other electrons or nucleus. The energy levels of each electron in the outermost shell will be different. These energy levels are represented as energy bands. Discrete energy levels or range of energy levels can be represented as bands. Even though we can draw a number of bands, we mainly use three bands to explain the behavior of a solid. Let us see what these bands are. These are valence band, conduction band, forbidden band or forbidden gap or energy gap. We have classified the materials based on the energy band. Each material will exhibit a different nature depending on the energy band. As I said earlier, we mainly use three bands. These are valence band, conduction band, forbidden band or forbidden gap. Let us see what these are. The range of energies possessed by valence electrons is known as valence band. Valence band electrons are not tightly packed in the nucleus of a conduction band. Hence, they can become free electrons. In the case of metals or conductors, valence band electrons are not tightly held by the nucleus. These free electrons are responsible for conduction. In the case of current conduction, valence band electrons are not tightly held by the nucleus. This can be seen in conductors or metals. Hence, these electrons are known as conduction electrons. The range of energies possessed by these conduction electrons is known as conduction band energy. The range of energies possessed by valence electrons is known as valence band energy. The range of energies possessed by conduction electrons is known as conduction band energy. Let us see what the forbidden energy gap is. This is the energy level of valence electrons. This is the energy level of free electrons or conduction electrons. This difference is called the forbidden energy gap. This energy gap plays a major role. Based on this forbidden gap, we can divide materials into three categories. Insulators, conductors and semiconductors. When electrons from valence band reaches the conduction band, current conduction happens. So, when the gap increases, the electrons here will have less chance to reach here. If the gap is less, the electrons here can easily reach the condition band. If the gap is medium, the electrons here can easily reach the condition band. At the same time, if the gap is very large, the electrons here will not be able to reach the condition band. So, you can understand from this that if the gap is large, the electrons here will not be able to reach the condition band. What will happen then? It will be an insulator. If the gap is small, the electrons here can easily reach the condition band. We call this a semiconductor. It conducts a little. If the gap is small, the electrons here can easily reach the condition band. We call this a conductance. So, there are three types of insulators. An insulator has a very wide forbidden gap. The forbidden gap will be very large in the case of an insulator. It will be more than 5 electrons in the case of an insulator. Hence, it is difficult for an electron to jump from valence band to condition band. It is very difficult for an electron to jump from valence band to condition band in the case of an insulator. In the case of a conductor, the valence band and condition band will be overlapped. Since the valence band and condition band are overlapped, it is easy for free electrons to jump from valence band to condition band. In the case of a semiconductor, the forbidden gap will be around 1 electron volt. Hence, electrons can move from valence band to condition band in room temperature. If the temperature is below room temperature, the electrons can never move from valence band to condition band in a semiconductor even at 0 degree Kelvin. It will behave as a perfect semiconductor insulator. This is what is shown here. The band gap here is very large. Hence, it is difficult for the electrons to move from valence band to condition band. The band gap here is medium. The electrons here can reach here. If you try, you can reach here. There is no band gap here. It is almost overlapped. Electrons can easily reach the conduction band. Okay. Now, the main thing we learn is about semiconductors. The specialty of semiconductors is that conductors will always conduct. Insulators will never conduct. But we can adjust its conductivity in the case of semiconductors. A semiconductor is materials having conductivity between conductors and insulators. Materials. Conductivity. The material which has conductivity between the conductor and the insulator is called a semiconductor. The specialty of a semiconductor is in very low temperature, the valence band will be almost full. The conduction band will be empty. Why? Because electrons in the valence band will move to the conduction band in very low temperature. At the same time, in room temperature, the valence band and the conduction band will be partially filled. The electrons in the valence band will move to the conduction band in room temperature. Because there is only a small energy gap. So, if you get some energy, it will move from the valence band to the conduction band. There are two types of semiconductors. Intrinsic semiconductor and extrinsic semiconductor. Let's see what is an intrinsic semiconductor. A semiconductor in its pure form is called an intrinsic semiconductor. A semiconductor in an extremely pure form is known as an intrinsic semiconductor. Pure form means there is no impurity in the semiconductor. Moreover, a semiconductor has no crystal defects. Such a semiconductor is known as a pure semiconductor. Such a semiconductor is known as an intrinsic semiconductor. Silicon and germanium are the intrinsic semiconductors. Outermost shell of silicon and germanium has 4 electrons. Outermost shell has 4 valence electrons. Valence electrons have 4 electrons. Only if there are 8 electrons, stability can be achieved. So, these 4 valence electrons are in the covalent bond. In room temperature, or in absolute zero, all valence electrons will be tightly held as a parent atom. In room temperature, all valence electrons will be tightly held as a parent atom. Outermost shell of silicon has 4 electrons. Silicon outermost shell has 4 electrons. In all, all the electrons go in the covalent bond to acquire stability. In all, all the electrons go in the covalent bond to acquire stability. In all, all electrons There are no free electrons, so there is no conductivity. That is why we can say that an intrinsic semiconductor behaves as an insulator at absolute zero temperature. If it is at room temperature, some covalent bonds in it will break. At room temperature, thermal energy will come in it. Zero Kelvin is the absolute zero temperature. At room temperature, it is 300 Kelvin. At 300 Kelvin, the heat energy in it will be absorbed. The covalent bond in it will break and free electrons will come. Such a few charging areas will cause conductivity. Conductivity will occur at a very small scale in an intrinsic semiconductor. That is, it is in pure form. Normally, it is an insulator at absolute zero temperature and does not conduct. At room temperature, it breaks with covalent bonds and conducts slightly. Next is the extrinsic semiconductor. Intrinsic semiconductors are pure semiconductors. It has very little conductivity at room temperature. So we can't use it much. So we add some impurity to it. By adding impurity, we vary the conductivity characteristics. By adding impurity, we can either add donors or acceptors. If we add donors, free electrons will be produced. If we add acceptors, free holes will be produced. Doping is the process of adding impurity to it. Doping involves 10 raised to 6 to 10 raised to 10 semiconductor atoms. Each impurity is added to it. As I said earlier, we can either add donors or acceptors. If we add donors, electrons will be produced. If we add acceptors, holes will be produced. Based on the material used for doping, we can divide the semiconductors into two types. N-type and P-type semiconductors. First, let's see what a N-type semiconductor is. We can either use silicon as an intrinsic semiconductor or we can use germanium as an intrinsic semiconductor. To form an N-type semiconductor, we add a pentavalent impurity to silicon or germanium. Pentavalent impurity is the fifth group of elements. Fifth group elements are phosphorus, antimony and arsenic. Pentavalent impurity is the fifth group of elements. We add this impurity to germanium or silicon. That means, our intrinsic semiconductor. We add this impurity to our intrinsic semiconductor. To add this impurity, we form an N-type semiconductor. This pentavalent impurity has 5 electrons in its outermost shell. If we use germanium or silicon, it has 4 electrons in its outermost shell. Here is germanium, 1, 2, 3. There is one here. I didn't draw the bond there, so I didn't show it. It has these 4 electrons. When we add arsenic atom, it has 5 electrons in its outermost shell. 1, 2, 3, 4. This is the fifth electron. The 4 electrons in this atom form a covalent bond with germanium. These are the 4 electrons of arsenic. This forms a covalent bond. This fifth electron does not form a covalent bond. It remains free. This free electron is responsible for current conduction. Because it does not form a covalent bond. It is loosely bound to the parent atom. So, we can consider this as a free electron. This is what we use as a current carrier. This free electron is responsible for current conduction. One more thing to remember. If we are an n-type semiconductor, Now think of the increase in value of the pentavalent impurity added now think of the increase in the pentavalent impurity added Oneこと Now think of the increase in value of the pentavalent impurity added one proton That is a lot of arsenic atoms. So, in turn The electron concentration in the particle will increase. The Hall concentration will be less. So, the majority carriers are electrons. The minority carriers would simply be Halls. Hall is the positive charge The majority carriers are electrons and the minority carriers are holes. Even though this is a very low temperature, the impurity atom in it gives an electron conduction band. Each impurity atom, when we drop one arsenic here, it will give one free electron to that one arsenic. Similarly, each impurity atom gives one electron to the conduction band. Because each electron is given to it, we call it a donor. When we add this pentavalent impurity, we call it a donor type impurity. We are donating each electron. That is why it is called a donor type impurity. If we take a look at the conduction band here, we can see the electrons coming from the valence band and the electrons we donated. So, when we consider the carrier concentration of this, we saw earlier, when we consider the carrier concentration, my semiconductor will have more electrons. That is why electrons are the majority carriers. The number of holes will be less. That is why holes are the minority carriers. Then, as part of this doping, we are donating electrons here. Then a donor energy level also comes there. Because the energy level of this electron is different. The energy level of a closely packed electron is different. Each electron has its own energy. The energy of this free electron is a different energy. This energy is the energy coming close to the conduction band. It is the energy of the donor electrons. So, a different energy level is coming. That energy level is coming through this impurity, donor or doping. That energy level will be close to the conduction band. If you look here, valence band, valence band, conduction band. Another energy level will form close to the conduction band. That energy level is what we call as the donor level. The electrons seen there will be the free electrons that came when we were doping. Okay. Next, let's look at the p-type semiconductor. We used the pentavalent impurity in the n-type semiconductor. In the p-type semiconductor, we use trivalent impurity. That is, third group elements. Third group elements are boron, aluminum, gallium, indium. These are the third group elements we use as trivalent impurity. Okay. When these elements are added to an intrinsic semiconductor, a p-type semiconductor is formed there. Trivalent impurity has three electrons in its outermost shell. I added gallium here. One, two, three electrons in the outermost shell of gallium. The deficiency of one electron. These three electrons were added to the covalent bond. Here, the deficiency of an electron cannot be added to the covalent bond. What do we call the deficiency of this electron? We call it hole. We call the deficiency of an electron hole. When trivalent impurity is added, what is formed there? A hole is formed. Or, a deficiency of an electron is formed. So, the current conduction happens here through the deficiency of this electron or through holes. In the p-type semiconductor, the deficiency of the electron is produced. The other one is donated. Since it is a deficiency, what do we call it here? We call it accepted. We can call this type of impurity as accepted type impurity. If you look here, the number of electrons is less here. The number of holes will be more. It will be holes. Majority carriers are electrons. It will be a minority carrier. Another peculiarity is that the donor level is below the conduction band. This is because it is the level of electrons. The accepted level is above the valence band. This is the theory of the band. I hope everyone understood this lecture. See you in the next class.