Today we will tell you what the intrinsic and impurity conductivity of semiconductors is, how it arises, and what role it plays in modern life.
Atom and band theory
At the beginning of the twentieth century, scientists found that the atom is not the smallest particle of matter. It has its own complex structure, and its elements interact according to special laws.
For example, it turned out that electrons can only be at certain distances from the nucleus — orbitals. Transitions between these states occur jerkily with the release or absorption of a quantum of the electromagnetic field. To explain the mechanism of intrinsic and impurity conductivity of semiconductors, we must first understand the structure of the atom.
The sizes and shapes of the orbitals are determined by the wave properties of the electron. Like a wave, this particle has a period, and when it revolves around the nucleus, it “superimposes” itself. Only where the wave does not suppress its own energy can an electron exist for a long time. The corollary follows: the further the level is from the nucleus, the smaller the distance between this and the previous orbital.
Solid Grate
Physics explains the intrinsic and impurity conductivity of semiconductors as a “collective” of identical orbitals that occurs in a solid. By solid means not an aggregate state, but a completely specific term. This is the name of a substance with a crystalline structure or an amorphous body, which can potentially be crystalline. For example, ice and marble are solid, but wood and clay are not.
There are a lot of similar atoms in a crystal, and identical electrons revolve around each in the same orbitals. And there is a little problem. An electron belongs to the class of fermions. This means that there cannot be two particles in exactly the same states. And what to do in this case, a solid?
Nature has found an amazingly simple way out: all the electrons that belong to the same orbital of one atom in a crystal differ slightly in energy. This difference is incredibly small, and all orbitals seem to be “compressed” into one continuous energy zone. Between the zones are large dips - such places where electrons cannot be. These spaces are called "prohibited."
How is a semiconductor different from a conductor and a dielectric?
Among all the zones of one solid, two stand out. In one (the highest) electrons can move freely, they are not “attached” to their atoms and move from place to place. This is called the conduction band. In metals, such an area is in direct contact with everyone else, and in order to excite electrons, it is not necessary to expend a lot of energy.
But for other substances, everything is different: electrons are located in the valence band. There they are bound to their atoms and cannot just leave them. The valence band is separated from the conduction band by a “dip”. So that the electrons can overcome the forbidden zone, the substance must be given a certain energy. Dielectrics differ from semiconductors only in the size of the "dip". In the former, it is greater than 3 eV. But on average for semiconductors, the band gap is from 1 to 2 eV. If the gap is larger, then the substance is called a wide-gap semiconductor and is used with caution.
Conductivity Types
To understand what are the features of the intrinsic and impurity conductivity of semiconductors, you must first find out what its types are.
We have already said that a semiconductor is a crystal. Therefore, its lattice consists of identical periodic elements. And its electrons must be "thrown" into the conduction band so that current flows through the substance. If it is electrons that move in the volume of the crystal, this is electronic conductivity. It is denoted as n-conductivity (from the first letter of the English word negative, that is, "negative"). But there is another type.
Imagine that in a certain periodic system one element is missing. For example, tennis balls are in a basket. They are located in even identical layers: each has an equal number of balls. If one ball is removed, a void, a hole is formed in the structure. All the surrounding balls will try to fill the gap: one element from the top layer will fall into place of the missing one. And so on, until equilibrium is established. But at the same time, the hole will also move - in the opposite direction, up. And if initially the surface of the balls in the basket was flat, then after moving in the upper row a hole will form in the place of one missing ball.
It is the same with electrons in semiconductors: if electrons move to the positive pole of the voltage, then the voids remaining in their place move to the negative pole. These opposite quasiparticles are called “holes” and they have a positive charge.
If holes prevail in the semiconductor, then the mechanism is called p-conductivity (from the first letter of the English word positive, that is, “positive”).
Impurity: chance or desire?
When a person hears the word "impurity", then most often it means something undesirable. For example, "an admixture of toxic substances in water", "an admixture of bitterness in the joy of triumph." But the admixture is also something small, insignificant.
In the case of semiconductors, this word has a second meaning rather than the first. To enhance one of the types of conductivity, an atom can be introduced into the crystal, which will give away the electrons (donor), or take them (the acceptor). Sometimes a small amount of a foreign substance is required in order to increase some kind of current.
Thus, the intrinsic and impurity conductivity of semiconductors are similar phenomena. The additive only enhances the existing quality of the crystal.
The use of doped semiconductors
The type of conductivity for crystals is important, but in practice they are used in combination.
At the junction of n- and p-type semiconductors, an interlayer of positive and negative particles is created. If the current is connected correctly, the charges will cancel each other out, and electricity will flow in the circuit. If the poles are connected in the opposite direction, then the uncharged particles will "lock" each other in their own half, and there will be no current in the system.
Thus, a small piece of doped silicon can become a diode for rectifying an electric current.
An element containing two types of semiconductor can also serve as a transistor for controlling and amplifying current.
As we have shown above, the intrinsic and impurity conductivity plays a key role in a semiconductor. Semiconductors have become much smaller than tube devices. This technological breakthrough made it possible to accomplish much of what scientists predicted theoretically, but for the time being it was not possible to put into practice due to the large size of the equipment.
Silicon and space
Flying into space has become one of the most important opportunities available thanks to semiconductors. Until the sixties of the twentieth century, this was not feasible for the simple reason that rocket control was contained in incredibly heavy and fragile tube devices. No way could lift such a colossus without vibration and stress. And the discovery of silicon and germanium conductivity made it possible to reduce the weight of the control elements and make them more solid and durable.