So far, we have studied the case of pure semiconductors. It means perfect semiconductors without defects. In reality, such semiconductors don't exist. Now, we'll treat the case of impurities, which are intentionally incorporated. This is called doping, and we'll see that doping is one of the main characteristics of semiconductors. The doping capability allows to prepare electronic devices from semiconductors. Generally, characterized by the presence of electric fields. On the contrary, semiconductor that cannot be doped as new particle applications. The concept of doping can be explained quite naturally by chemical issues. Silicone like diamond carbon oximanion belongs to the fourth column of the periodic table as shown here. We now consider the incorporation of elements from column five on elements from column three, which are located in close proximity of the column four. Let's start with the column five, the silicon lattice, valence 4 is shown schematically in the left figure. Suppose that phosphorus atoms in a small amount are incorporated into bulk silicon. It is necessary that this incorporation of phosphorus atoms will be performed in the weak concentration. Typically, 10 to the minus three, even less, 10 to the minus four or five, so that the silicon network is not activated. This weak phosphorus incorporation prevents any in allowed formation. First, we can use the word substitution because an atom of silicon is replaced by a phosphorus atom with the valence 5. So, one can imagine that four of these electrons will be shared with four neighboring silicon atoms in order to create four valence bones. The fifth electron will be therefore weakly bonded. Thus, this fifth electron will be easy to ionize, even at ambient much easier than the silicon atom. So, this fifth electron will be almost free. It will therefore be able to contribute to the condition. This phosphorus atom will therefore create a donor effect together with the formation of the B plus ion. Then, as we can see in more detailed is an Ec the donor level will be closed to the minimum of the conduction bond, so that Ec minus Ed is small as compared to the bond gap. If the level is filled, it means that electron is still bonded to a phosphorus. Ionisation means that enough energy was given to this electron in order to induce or transfer into the condition bond of silicon. Later then, now, with the column three atoms like bond. Suppose that we substitute a silicon atom by a boron atom, with the same former assumption that is to say, that the boron nickel partial is weak enough in order to prevent any pertubation of the silicon lattice. The boron atom are three valence electrons. Therefore, this boron atom will easily accept a force electron in order to be covenantly bonded to the neighboring silicon atoms. In other words, its acceptor energy level will be close to the valence bone of silicon. Therefore, the energy EA minus EV should be provided to the electron forms of valence bond, to compensate for the whole bonding to the acceptor. More precisely, you ensure in the 3 that the energy level EA minus EV are of the order of a few tenths of meV, that is to say of the order of magnitude of KT at home temperature. In summary the incorporation in a substitutional way of an atom of colon 5 creates a free electron which is related to donor level close to the condition bond. On the other hand, the incorporation of the colon 3 atom creates an acceptor level close to valence bonds. I would like to add some remarks about doping mechanism. I emphasize on the fact that the main hypothesis is related to the stability of the crystal lattice. It means as we incorporated impurities do not induce pertubation on the crystalline lattice. So the lattice stability is a dominant characteristic. For instant, from the chemical point of view boron on phosphorus depart from the valence 4 behaviors, with gases, phosphate VHC B2H6 dealing with neighboring atoms of silicon or germanium, nitrogen also belong to the column 5. However, the behaviors of nitrogen on phosphorus differ because of chemical affinities such as sea and bone formation. Furthermore, this doping mechanism is correlated to the presence of covalent bonds. Such a mechanism cannot be extended to 2,6, semiconductor which are ionic crystals. Finally, as we shall see later, this doping mechanism does not apply to these other semiconductor such as amorphous silicon. Indeed the solid lattice of this other material can relax in order to incorporate valence 5 or 3 atoms without creating any donor or acceptor effect. Thank you.
