Find out now, as the current passes through the crystal with the p-n junction. In figure. 21.6 schematically shows such a crystal. In the absence of external voltage (rice. 21.6, α) all flows of mobile charge carriers across the junction is balanced and the current is zero.

Turn the crystal in a circuit so, the external field was directed opposite to the field of transition (rice. 21.6, b). Field in p-n junction will be weakened and diffusion flow of majority carriers (holes from p-region and electrons from n-region) will rush through the transition. Counter the flow of minority carriers is almost unchanged. As a result, after the transition will flow a lot of current. The applied voltage and the current in this case called direct. Current by increasing the voltage increases very quickly (rice. 21.7), and Ohm's law is totally inapplicable here.

Submit now to the crystal a voltage of reverse polarity (rice. 21.6, b). In this case, the external voltage is identical in sign to the contact potential difference. The external field enhances the field R-n-junction and diffusion flows of the main carrier across the junction is significantly reduced. The flow of minority carriers, about the same, as in the absence of external fields, create a weak current through the junction. The applied voltage and the current in this case is called reverse.

It turns out, so the direct voltage, the current through the p-n junction in a million times more, than in a reverse (rice. 21.7). This means, that p-n junction works like the valve, t. e. current flows in one direction (transition open) and don't miss it in the opposite direction (the transition is closed). Therefore, if you include the c crystal p-n junction in the circuit of alternating current in series with a load resistance R (rice. 21.8), the current in the resistance will be practically constant in the direction. Therefore, a crystal with p-n-transition is called semiconductor rectifier or a semiconductor diode. (In figure. 21.7 shown voltage-current characteristic of a silicon diode average power; the magnitude of the current and voltage for forward and reverse direction are different.)

It is interesting to see, how is the voltage in the circuit of Fig. 21.8 between the diode and the load resistance R. (In the conventional image of the diode in the circuits the edge indicates the direction of the direct current.)

Graph of AC voltage in the network shown in figure. 21.9, α. The diode current flows only in the forward direction (rice. 21.9, b). Remember, that in series the voltage is divided proportionally to the resistances. In the first half of the period, when the current goes through the diode in the forward direction, the resistance of the diode is very small and almost all the voltage falls on load R. In the second half period, the diode resistance is very high and all the stress has been on the diode. The change in voltage across the diode is shown in figure. 21.9, in, and the voltage change across the load R in figure. 21.9, g.

Semiconductor diodes have a high to. p. d. (to 98%), small size and long life. The disadvantages of semiconductor diodes is the deterioration of their work when the temperature. As said above, that reverse current through the p-n junction, creates minority carriers, the concentration of which is small at ordinary temperatures, but increases rapidly with increasing temperature due to the generation of electron — hole pairs. Therefore, the reverse current of a semiconductor diode increases rapidly with increasing temperature: silicon rectifying diodes stop the current at a temperature of about 200°C, and the maximum temperature for germanium diodes even less.

Note, that semiconductor diode can not be included in the network without terminating resistors. If the load in the diagram Fig. 21.8 remove, all voltage will be applied to the diode. When the diode will be included in the forward direction, external voltage exceeds the contact potential difference, the p-n junction virtually disappear, through the diode will flow a very large current and the led will be damaged.