When an n-doped and a p-doped semiconductor are brought together, some electrons from the n-type region automatically diffuse into the p-type region due to thermal motion and recombine with the holes present there. As a result, a thin boundary layer forms between them, called the p-n junction. A component with this characteristic is called a diode.
If a voltage is applied such that the positive pole is connected to the n-side and the negative pole to the p-side, the depletion region becomes wider, blocking current flow. This orientation is called reverse bias.
If the polarity is reversed, the depletion region becomes narrower. Once a certain potential is exceeded, charges can move continuously across the junction, and a steady current flows. In this orientation, the diode operates in forward bias.
Thus, a diode can be compared to a valve that allows current to flow only in one direction — much like the valve on a bicycle tire.
One must note that the relationship between current and voltage is not linear but follows an exponential law. A diode is therefore a non-ohmic component, i.e., it does not follow Ohm’s law.
The required voltage to overcome the potential barrier in forward direction is called the forward voltage.
- For silicon, this is typically about 0.7 V.
- For germanium, it is about 0.3 V.
This forward voltage drops across the diode and is not available to the load.
The work done to overcome the potential barrier for each charge carrier is released as discrete energy:
- Either as heat (temperature increase),
- Or as visible light in the case of a light-emitting diode (LED).
The reverse of the LED principle is the solar cell.
Here, the energy needed to overcome the p-n junction is provided by external light (from the sun).
The stronger the illumination, the greater the current flow, while the voltage mainly depends on the potential difference of the p-n junction.
By connecting many solar cells in photovoltaic systems, voltages of typically 12 V or 24 V can be generated, which are then stepped up using inverters and fed into the public power grid.
Example problem:
A light-emitting diode (LED) with a forward voltage of 3.5 V is operated with a 5 V supply.
What value must the series resistor have to limit the current to 20 mA?
Solution: 75 Ω