What is the voltage diode
Similar to a tube diode, a semiconductor diode represents an "electrical one-way street"; Electric current can only pass through a semiconductor diode in one direction.
A diode has two connections called an anode and a cathode. Current can only flow through a diode if the anode points to the plus pole and the cathode to the minus pole; it locks in the opposite direction. The cathode side is marked on the component by a black or white ring. From an applied voltage of approx with silicon diodes respectively with germanium diodes, current begins to flow in the forward direction.
When passing through a silicon diode, the voltage drops relatively constantly (in contrast to ohmic resistors, which always require a higher electrical voltage to pass a higher current intensity) from - largely independent of the strength of the flowing current. Ohm's law is therefore not applicable to diodes.
If you put an opposite tension on, a diode behaves like an isolator up to a certain voltage value - the diode "blocks". If the voltage value, which depends on the type and material of the diode, is exceeded, the current intensity (also in the opposite direction) increases rapidly to; the diode can quickly overheat or be destroyed.
Two characteristic values are printed on each diode:
- The voltage specified in volts indicates the maximum voltage with which the diode may be operated against the forward direction (in the "reverse direction").
- The current in (milli-) amperes indicates the maximum current that is allowed to flow through the diode (in the forward direction).
Both values must not be exceeded, otherwise the diode can be destroyed.
- For the diode are the values specified; the maximum voltage in the reverse direction must therefore be at most , the maximum current in the forward direction at most be.
The Shockley equation
From a mathematical point of view, the -Characteristic curve of a diode above the breakdown voltage can be described by the so-called Shockley equation:
The following parameters occur here:
As can be seen from equation (1), the Characteristic curve of a diode has an exponential course. Such curves can often be better represented with the help of a logarithmic scale. 
Light emitting diodes ("Light Emitting Diods", short: LEDs) are special diodes that are built into a transparent housing and light up when current flows through them. The usual operating voltage of a light emitting diode is usually included ; A maximum voltage of can be created.  The amperage is between and .
The anode of the light emitting diode, which is identified by a longer connecting wire, must be connected to the positive pole and the cathode to the negative pole of the power source. The anode and cathode side of an LED can also be identified by their internal structure, as shown in the figure of the light-emitting diode design.
Light-emitting diodes have a large number of important properties: They only need a low operating voltage, they are insensitive to impacts, require little space and only have a low power requirement. In addition, light-emitting diodes have a very fast response time: They can be switched on and off thousands of times in one second and therefore, similar to the "Morse keys" in the past, can be used for signal transmission with suitable coding.
If light hits a photodiode, an electric current is triggered in it, which is called a photocurrent referred to as. Depending on the version, the light sensitivity of the photodiode is in the infrared, ultraviolet or visible range of light.
In principle, a solar cell also consists of a large-area photodiode. Often, solar cells consist of thin silicon wafers on the front -doped and on the back - are doped. Both sides are provided with grid-like electrical contacts.
Gets light through the very thin -doped layer through to the -doped layer, electrons are released from their bonds there; so electron-hole pairs are generated. In the -doped layer accumulates an oversupply of electrons. These electrons are, however, prevented by the barrier layer of the diode from immediately ensuring a charge equalization again. Rather, the electrons flow through the external circuit to the -doped layer back.
If a solar cell is illuminated, a voltage of approx. on ("open circuit voltage"). This voltage drops when a consumer is connected.
With normal diodes, the effect is used that the current can only pass through the diode in one direction, i.e. the diode blocks in the opposite direction. Zener diodes (sometimes also called "Zener diodes" after the inventor Clarence Zener), on the other hand, are deliberately built in such a way that they become conductive in the opposite direction from a certain (breakdown) voltage.
A Zener diode behaves in the reverse direction essentially the same as a normal diode in the forward direction. For example, a Zener diode has a breakdown voltage of , a current can only flow in the reverse direction from this voltage onwards. At higher voltages, the current strength (according to the figure characteristic of a diode in reverse direction) increases strongly; however, the maximum current indicated on the diode should not be exceeded. 
Examples of the use of these diodes can be found in the section Voltage regulation with Zener diodes.
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