CONDUCTIVITY OF SEMICONDUCTORS

Semiconducting material (semiconductor) has remarkable properties that allow you to create a variety of devices for solving important problems in the field of rectifying, amplifying, photoelectric, thermoelectric engineering and in many other technical areas.

Semiconductors occupy an intermediate position between metals (conductive materials) and dielectrics (insulating materials). Semiconductors differ from metals by a significantly smaller number of current carriers (electrons) per unit volume, and consequently, by a lower conductivity. In contrast to metal, a semiconductor has, as a rule, a negative temperature coefficient, i.e. with increasing temperature, its conductivity increases (resistance decreases). The semiconductor material has, as a rule, a crystalline structure. If each crystal consists of identical atoms, then the bond between the atoms is carried out with the help of electrons moving around each pair of neighboring atoms and combining them into one system. It is this pattern that is characteristic of elementary semiconductors (for example, germanium, silicon, as well as arsenic, phosphorus, and other semiconducting materials that are chemical elements).

Physics and technology of semiconductors achieved great success due to the fact that a number of substances could be obtained in an extremely pure form. For the first time, ultrapure germanium was obtained. When a crystal is formed from a melt, the impurities are displaced, and with repeated repetition of this operation, the crystal becomes cleaner and cleaner. After germanium, the production of ultrapure silicon and some other semiconductors was achieved. The content of impurities in ultrapure material should be less than one hundred millionth of a percent.

How should one imagine the mechanism of electric current in a super-pure semiconductor (in which the role of impurities can be neglected)? Inside the crystal, atoms make collisions near the equilibrium positions and, therefore, possess some energy. The energy of such colts *** depends on temperature, and at a given temperature this energy is sufficient to transfer from a bound state to a free state (inside the crystal lattice of a semiconductor) a certain number of electrons. Free electrons can be current carriers.

When an electron leaves a certain atom, an empty space appears in that atom, which is not occupied by an electron, and therefore a positive charge arises. When an electron moves from a neighboring atom to a given one, this free space will appear in the neighbor’s, etc. This means that a positive charge in a semiconductor can move. In other words, the movement of an electron is replaced by a reverse movement of a positively charged particle with a specific mass. This imaginary "particle" got the name "hole". The number of free electrons is equal to the number of holes in a pure crystal. Under the action of an electric field applied to a semiconductor from an external source, the holes will move in the direction of the lines of force, participating in an electric current. In fact, of course, in this case too, the current is the movement of electrons, but the movement is abrupt - from a given atom to a neighboring one, i.e. with less "mobility".

So, in a pure semiconductor current is created by the movement of two types of carriers: electrons and holes. The directions of their movement are opposite, and the quantities are the same.

Suppose further that we deliberately introduced some foreign atoms into the pure semiconductor, i.e. impurity. Let impurity atoms give away electrons more easily than atoms of the main substance. In this case, at a given temperature, there can be significantly more free electrons in the crystal than holes. In contrast to intrinsic conductivity, a semiconductor will have impurity electronic conductivity. The electric current in it will mainly be created by the movement of negative charges (electrons), and therefore this conductivity is "negative", and the semiconductor with such an impurity is called a n -type semiconductor.

But you can choose an impurity with such atoms that are capable of capturing an electron from the main substance. When an electron from a neighboring crystal atom "sticks" to an impurity atom, a hole will appear at the neighboring atom, which can travel inside the crystal. With such an impurity, the number of holes will prevail over the number of free electrons and holes will become the main current carriers. A semiconductor with such an impurity has “positive” conductivity and is called a p -type semiconductor.

So, by introducing impurities of one sort or another, it is possible to change the mechanism of electric current in a semiconductor, i.e. create conditions under which electrons or holes will play an active role in this mechanism. Science allows measuring the number of free electrons and holes per unit volume and studying the behavior of these current carriers in crystals.

Of practical interest are systems consisting of two semiconductors of types n and p , which are in contact with each other. A n -type semiconductor contains many free electrons — the main current carriers, and few holes — minor current carriers. In the p type semiconductor, the opposite is true.

Semiconductors of two different types will begin to diffuse ("wade") across the boundary of contact between their main current carriers: p material - electrons, n material holes -. For a contacting surface, a positive type arises in a n -type semiconductor, and a negative potential arises in a p -type semiconductor. In other words, an electric field is formed at the point of contact, which prevents further increase in diffusion; the charge transfer due to diffusion is compensated by the reverse transition due to the contact field. After the establishment of an equilibrium of charges, the potential jump will remain ( Fig. 8-27 , a ).

Fig. 8-27. Explanation of the current mechanism in the electron-hole transition:
a is a potential barrier;
b - current in the transmission direction;
in - current in the opposite direction.

Now suppose that semiconductors of types n and p are superimposed on the outer sides of metal electrodes, the contact of which with the semiconductor has a very low resistance. Let's connect to the electrode of the semiconductor type p positive, and to the electrode of the semiconductor type n the negative pole of the external battery ( Fig. 8-27 , b ). In this case, a current flows through the pn junction; this current will be in the external circuit. From fig. 8-27b , it can be seen that in this case the external electric field is opposite to the field that exists in the transition layer. In other words, the potential jump of the pn junction will be more or less compensated by the action of the external field.

The main current carriers of each of the semiconductors (large circles), moving towards each other under the action of an external field, will be able to overcome the remaining potential jump (potential barrier) and slip through it. With increasing voltage, the number of main carriers crossing the boundary will increase, and hence the amount of current in the circuit.

Change the polarity of the electrodes to reverse by switching the current source ( Fig. 8-27 , c ). Then to the transition will rush on each side of minority current carriers (small circles). These carriers can cross the border even without an external field, since the potential difference for them is "passing". However, minority carriers are few, and therefore the amount of current generated by them is negligible. For the main carriers, the external field is like an addition to the potential barrier.

So, the resistance of the pn junction depends on the direction of the current: when the polarity ensures the movement of the main carriers across the boundary, the resistance is small and decreases with increasing voltage; with the opposite polarity, the resistance is large and slightly depends on the applied voltage. This property of the pn junction provides the possibility of creating various technical devices, and first of all, rectifiers. Rectifiers created on the basis of germanium or silicon crystals will be called semiconductor (crystalline) diodes. It should be noted that in reality the processes in the pn junction are more complex; we outlined them only in a simplified representation