The diode is an electronic component whose conductivity depends on its polarity. To put it more simply, the diode conducts the current in only one direction. How is this possible? What is the usage of this fact? To answer these questions, we have to get acquainted with the term semiconductor.
Surprisingly, some elements of the periodic system cannot be classified as conductors that conduct the current, nor as isolators that prevent its flow. These elements are considered semiconductors because they conduct the current under certain conditions. The duality of their nature can be used very intelligently in electronic circuits, which makes them the foundation of modern electronics. Therefore, the diode has many applications, from which we can mention the conversion from alternating to direct current, which we need in order to build a simple power supply. So, let’s try to understand how it works.
For example, let us consider the chemical element silicon (Si). Silicon is a semiconductor and has 4 valence electrons, which connect themselves in a pyramidal structure, inside a crystal grid. In that way, the covalent bonds are created which enable the atoms to share pairs of electrons. Since electrons are bound, they cannot travel, which makes silicon nonconducting.
However, if we inject the phosphorous (P) atom with 5 valence electrons in the silicon grid, we are effectively giving it one more electron which is not bound with silicon atoms and is able to move through the grid. This process is called doping, and it creates a N-type (negative cathode) of conducting silicon with electrons as carriers. Silicon is still stable because the phosphorous atom sits nicely in the grid. It is also neutral because the ratio of protons and electrons is uniform.
On the other hand, if we inject the boron (B) atom with 3 valence electrons in the silicon grid, we will create a cavity that is missing an electron. That way, we created P-type (positive anode) of conducting silicon with cavities as carriers.
N-types and P-types are not particularly interesting unless we try to connect them. Creating an PN junction, some of the additional electrons of N-type cross over to the cavities of the P-type and they form a depletion layer. It is worth mentioning that the junction is still neutral because the ratio of protons and electrons is uniform.
Now, if we connect the PN junction to the direct current of diverse polarities, we will notice interesting things. If we connect the source of direct current in a manner where the N-type is on the negative part of the source, and the P-type on the positive side of the source, the depletion layer gets smaller and the current starts flowing. This connection is called forward bias.
If we connect the source of direct current in opposite direction, each of the sources attracts the opposite charge and depletion becomes wider. This connection is called reverse bias and the current cannot flow.
The PN junction that we created is called a diode and we can fully understand why it conducts current in only one direction. It is important to notice that the diode needs to be exposed to a certain forward voltage in order to conduct. For silicon diode, the voltage is ~0.7 V, and for germanium diode ~0.3 V. However, in the reverse bias connection, the diode will block the current until the reverse voltage reaches the value of its breakdown, which is very important information and needs to be consulted in the technical specification.
References:
https://www.halbleiter.org/en/fundamentals/doping/
https://www.youtube.com/watch?v=IcrBqCFLHIY
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