WHAT IS A SEMICONDUCTOR?
In previous lessons, I talked about conductors and dielectrics and briefly mentioned that there is a certain intermediate form of conductivity, which under certain conditions can adopt the properties of a conductor or a dielectric. This type of substance is called a semiconductor. In this lesson, we will learn in considerable detail what a semiconductor is and what an essential role it plays in radio electronics.
Semiconductors and their properties
Let me remind you: in terms of electrical properties, semiconductors occupy an intermediate position between conductors and non-conductors of current.
To what has been said, we can add that the group of semiconductors includes many more substances than the groups of conductors and non-conductors combined. Semiconductors that have found practical application in engineering include germanium, silicon, selenium, cuprous oxide, and some other substances. But for semiconductor devices, mainly only germanium and silicon are used. What are the most characteristic properties of semiconductors that distinguish them from conductors and non-conductors of current? The electrical conductivity of semiconductors strongly depends on the ambient temperature. At very low temperatures close to absolute zero (-273°C), they behave towards electric current as insulators. Most conductors, on the contrary, become superconductive at such a temperature, i.e., they offer almost no resistance to the current. As the temperature of conductors increases, their resistance to electric current increases, while the resistance of semiconductors decreases. The electrical conductivity of conductors does not change when exposed to light. The electrical conductivity of semiconductors under the influence of light, the so-called photoconductivity, increases.
Semiconductors can convert light energy into electric current.
This is completely uncharacteristic of conductors. The electrical conductivity of semiconductors increases sharply when atoms of certain other elements are introduced into them. The electrical conductivity of conductors, however, decreases when impurities are introduced. These and some other properties of semiconductors have been known for a relatively long time, but they began to be widely used relatively recently.
Germanium and silicon, which are the base materials for many modern semiconductor devices, have four valence electrons in the outer layers of their shells.
In total, the germanium atom has 32 electrons, and the silicon atom has 14. But 28 electrons of the germanium atom and 10 electrons of the silicon atom, located in the inner layers of their shells, are firmly held by the nuclei and do not detach from them under any circumstances. Only the four valence electrons of the atoms of these semiconductors can, and even then not always, become free. A semiconductor atom that has lost at least one electron becomes a positive ion. In a semiconductor, atoms are arranged in a strict order: each atom is surrounded by four identical atoms. They are also located so close to each other that their valence electrons form single orbits passing around all neighboring atoms, binding them into a single substance. Such an interconnection of atoms in a semiconductor crystal can be represented as a flat diagram, as shown in Fig. 1, a. Here, large circles with a "+" sign conventionally represent the nuclei of atoms with the inner layers of the electron shell (positive ions), and small circles represent valence electrons. Each atom, as you can see, is surrounded by four exactly identical atoms. Any of the atoms is connected to each neighbor by two valence electrons, one of which is its own, and the second is borrowed from the neighbor.
This is a two-electron, or covalent (valence), bond. The strongest bond!
In turn, the outer layer of the electron shell of each atom contains eight electrons: four of its own and one from each of the four neighboring atoms. Here it is no longer possible to distinguish which of the valence electrons in the atom is its own and which is foreign, since they have become common. With such an atomic bond throughout the mass of a germanium or silicon crystal, it can be considered that the semiconductor crystal is one large molecule. For clarity, the diagram of the interaction of atoms in a semiconductor can be simplified by depicting it as shown in Fig. 1, b. Here, the nuclei of atoms with inner electron shells are shown as circles with a plus sign, and the interatomic bonds as two lines symbolizing valence electrons.
Electrical conductivity of semiconductors
At a temperature close to absolute zero, a semiconductor behaves like an absolute non-conductor because it has no free electrons.
If there is no temperature increase, the bond of valence electrons with the atomic nuclei weakens, and some of them, due to thermal motion, can leave their atoms. An electron that has broken out of the interatomic bond becomes free (in Fig. 1, b - a black dot), and an empty space is formed where it was previously. This empty space in the interatomic bond of a semiconductor is conventionally called a hole (in Fig. 1, b - a broken electron line). The higher the temperature of the semiconductor, the more free electrons and holes appear in it. Thus, the formation of a hole in the mass of a semiconductor is associated with the departure of a valence electron from the atom's shell, and the appearance of a hole corresponds to the appearance of a positive electrical charge equal to the negative charge of an electron.
Now look at Fig. 2. It schematically depicts the phenomenon of the generation of current in a semiconductor.
The cause of the current is the voltage applied to the semiconductor (in Fig. 2, the voltage source is symbolized by the "+" and "-" signs).
Due to thermal phenomena, a certain number of electrons are released from interatomic bonds throughout the entire mass of the semiconductor (in Fig. 2 they are indicated by dots with arrows). Electrons released near the positive pole of the voltage source are attracted by this pole and leave the semiconductor mass, leaving holes behind. Electrons that have left the interatomic bonds at some distance from the positive pole are also attracted by it and move in its direction. But, encountering holes on their way, the electrons seem to jump into them (Fig. 2, a), filling some interatomic bonds. And the holes closest to the negative pole are filled with other electrons that have broken out of atoms located even closer to the negative pole (Fig. 2, b). As long as an electric field acts in the semiconductor, this process continues: some interatomic bonds are broken — valence electrons leave them, holes appear — and other interatomic bonds are filled — electrons released from some other interatomic bonds jump into the holes (Fig. 2, b-c).
At temperatures above absolute zero, free electrons and holes continuously appear and disappear in a semiconductor even when there are no external electric fields. But electrons and holes move randomly in different directions and do not leave the boundaries of the semiconductor.
In a pure semiconductor, the number of electrons released at any given moment in time is equal to the number of holes formed in the process.
Their total number at room temperature is relatively small. Therefore, the electrical conductivity of such a semiconductor, (called intrinsic conductivity), is low. In other words, such a semiconductor offers a fairly high resistance to electric current. But if even a tiny amount of impurity in the form of atoms of other elements is added to a pure semiconductor, its electrical conductivity will increase sharply. Moreover, depending on the structure of the impurity element atoms, the electrical conductivity of the semiconductor will be electronic or hole-type. How do these two types of electrical conductivity of a semiconductor differ?
If any atom in a semiconductor crystal is replaced by an antimony atom, which has five valence electrons in the outer layer of its electron shell, this alien atom will connect with four neighboring semiconductor atoms using four of its electrons. The fifth valence electron of the antimony atom will turn out to be superfluous and will become free. The more antimony atoms are introduced into the semiconductor, the more free electrons will be in its mass. Consequently, a semiconductor with an antimony impurity approaches a metal in its properties: for an electric current to pass through it, its interatomic bonds do not necessarily have to be destroyed.
Semiconductors possessing such properties are called semiconductors with n-type conductivity, or n-type semiconductors. Here the Latin letter "n" is the initial letter of the Latin word negativ (negative). This term in this case should be understood in the sense that in an n-type semiconductor the main current carriers are negative charges, i.e., electrons.
A completely different picture will emerge if atoms with three valence electrons, such as indium atoms, are introduced into a semiconductor. Each indium metal atom will fill the bonds with only three neighboring semiconductor atoms with its three electrons, and it lacks one electron to fill the bond with the fourth atom. A hole is formed. It can, of course, be filled by some electron that has broken out of a valence bond with other semiconductor atoms. However, regardless of where the holes are, there will not be enough electrons in the mass of the semiconductor with an indium impurity to fill them. And the more impurity indium atoms are introduced into the semiconductor, the more holes are formed in it. In order for electrons to move in such a semiconductor, the valence bonds between atoms must absolutely be destroyed. Electrons breaking out of them, or electrons entering the semiconductor from the outside, move from hole to hole. And in the entire mass of the semiconductor at any given moment, the number of holes will be greater than the total number of free electrons.
Semiconductors possessing this property are called semiconductors with hole conductivity, or p-type semiconductors. The Latin letter "p" is the first letter of the Latin word positiv (positive). This term in this case should be understood in the sense that the phenomenon of electric current in the mass of a p-type semiconductor is accompanied by the continuous appearance and disappearance of positive charges — holes.
Moving in the mass of the semiconductor, the holes essentially act as current carriers. P-type semiconductors, just like n-type semiconductors, have many times better electrical conductivity compared to pure semiconductors. It must be said that practically there are neither perfectly pure semiconductors nor semiconductors with absolute n-type or p-type electrical conductivity. In a semiconductor with an indium impurity, there is necessarily a small amount of atoms of some other elements that give it electronic conductivity, and in a semiconductor with an antimony impurity, there are atoms of elements that create hole conductivity in it. For example, in a semiconductor that generally has n-type conductivity, there are holes that can be filled by free electrons of the impurity antimony atoms. As a result, the electrical conductivity of the semiconductor will slightly deteriorate, but overall it will retain electronic conductivity. A similar phenomenon will be observed if free electrons enter a semiconductor with a hole-type electrical conductivity character.
Therefore, n-type semiconductors are considered to be those semiconductors in which the main current carriers are electrons (electronic conductivity prevails), and p-type semiconductors are those in which the main current carriers are holes (hole conductivity prevails).
Now that you have some understanding of the phenomena occurring in semiconductors, it will not be difficult to understand the principle of operation of semiconductor devices. The next lesson will be more intensive, with a lot of material to memorize. We will start with the predecessors of the transistor — semiconductor diodes.
Moving on to the next lesson!
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