Semiconductors are substances that electrical current is generated by the movement of electrons, and the magnitude of specific resistance between conductors and insulators. Semiconductors are the chemical elements IV, and VI groups of periodic system of D. I. Mendeleev - graphite, silicon, germanium, selenium and others, and many oxides and other compounds of various metals. The number of mobile charge carriers in semiconductors in normal conditions is small, but it increases in hundreds and thousands of times when some external influences (heating, light and so on), and also in the presence of P. certain impurities.
Semiconductors are divided into electronic (type n) and p-type (type p). In the letter type, and as charge carriers discusses the electrons in the formation of the current move across the Isthmus like free electrons in metals. In P. p type as charge carriers discusses the so-called holes (under the holes refers to free space in the atom, which can be busy outsiders him electron). Holes are considered to be equivalent to a positive charge equal to the electron. In the formation of the current in P. p electrons are committed only to switch between adjacent atoms; when jumping over an electron from one hole to another hole moves in the opposite direction, which is regarded as education current.
The main applications of semiconductors. Semiconductors, the resistance which when heated, through the release of charge carriers is significantly reduced, are used as electric thermometers or thermistors; compared with mercury thermometers they differ significantly higher sensitivity and lack of thermal inertia. Thermistor (Fig. 1, a) usually has the form of a ball 1, which closed up conclusions 2 from a thin wire. The thermistor is surrounded by a thin plastic insulation 3 and strengthened at the end of the measuring stick 1 (Fig. 1,6). The wires from the thermistor included in one shoulder measuring circuit (Wheatstone bridge), in the other shoulder which included microammeter 2 (Fig. 1, b). The scale of the instrument is calibrated in degrees Celsius. In one case the instrument is placed dry cells and other details of the measuring circuit. Thanks to the small size of the thermistor can be used to measure skin, abdominal and even interstitial temperature; in the latter case it is closed up inside the needle, which vylivaetsya in the fabric.
If you heat up one end of the rod of the semiconductor, then released it to the charge carriers with high kinetic energy (electrons or holes) will diffuse to the other end of the rod, forming it an excess charge of the appropriate sign. Between the hot and cold ends P. formed the potential difference is directly proportional to the temperature difference of these ends. Usually constitute a pair of electron and hole P. When heating their junction between the cold ends formed the thermoelectric power equal to the sum of the difference of potentials generated in each of semiconductors. It hundreds of times thermoelectric power metal thermocouples.
Thermoelectric phenomena reversible: if after the junction of electron and hole P. flowing in a particular direction, the current from an external source, the junction will be cooled to the temperature of loose ends P. This phenomenon is used when the device cooling elements. In Fig. 2 shows semiconductor laboratory refrigerator. Refrigeration units are allocated in the form of a ring, junctions inside. In this ring is inserted into the vessel with liquid cooled. The opposite ends of the items equipped with radiators in which they supported the ambient temperature. The direct current from the battery is supplied to the terminals.
In close contact semiconductor with electron and hole conductivity (such contact is called electron-hole transition) is the diffusion of electrons from the electronic point fall in the hole and the hole of the hole in P. e. When this adjacent to contact layers P. the number of fixed charge carriers decreases, and the electrical conductivity of them decreases. If electron-hole transition applied external potential difference that causes the movement of the main charge carriers in a semiconductor towards each other, the boundary layers are enriched by them, conductivity them increases and the current in this direction is formed freely. If the external potential difference causes the movement of the main charge carriers in Petrograd in the opposite side of the contact, the conductivity boundary layers is reduced to a minimum. The current in this direction is not formed. In this regard, electron-hole transition is called "locking layer and is used for AC rectification. Cuproxat or selenium rectifier elements consist of the support washers with a layer of Peterhof, which was formed barrier layer. Necessary (depending on the size straighten voltage) the number of items collected on the web in the form of a column (figure 3). Area elements correspond with the power to straighten current.
Solar cells - semiconductor devices, which under the action of light formed independent potential difference. Selenium cell (Fig. 4) consists of a layer P., located between two electrodes: reference 1 and the second 3 in the form of thin transparent to light layer of metal. Inside P. formed barrier layer 2.
The action of light in semiconductors is the release of electrons and holes, which tend to be distributed throughout P. However, through the barrier layer can take charge of only one of any character. This results in a semiconductor is a charge separation between adjacent electrodes is formed the potential difference. Spectral sensitivity curve of the selenium cell close to the same curve for the eyes. In this regard, it is widely used instruments for objective photometry (light meters) and colorimetry (photocolorimetric).
Electron-hole transition is also used when the device crystal diodes and triodes devices, similar to the properties of the electronic lamps, and in many cases used instead.

Fig. 1. Diagram of the thermistor.
Fig. 2. Semiconductor laboratory refrigerator.
Fig. 3. Kproxy rectifier element.
Fig. 4. Diagram of the solar cell.