Understanding Diodes: Types, Function, and Applications
Diodes: An Introduction
A diode (Greek: two roads) is a semiconductor device that allows the passage of electric current in one direction, with characteristics similar to a switch. In simple terms, the characteristic of a diode (IV) consists of two regions: below a certain potential difference, it behaves like an open circuit (no leads), and above it as a closed circuit with a very small electrical resistance.
Because of this behavior, diodes are often referred to as rectifiers. They are devices that are capable of suppressing the negative part of any signal, as an initial step to convert alternating current into direct current. Its operation is based on the experiments of Lee De Forest.
The first diodes were large on-chip valves or vacuum tubes, also called thermionic valves, consisting of two electrodes surrounded by vacuum in a glass tube with a look similar to incandescent lamps. The invention was made in 1904 by John Ambrose Fleming, of the Marconi Company, based on observations made by Thomas Alva Edison. As with incandescent lamps, vacuum tubes have a filament (the cathode) through which current circulates, causing Joule heating. The filament is treated with barium oxide, so that when heated, it emits electrons into the surrounding vacuum. These electrons are driven electrostatically to a nameplate for a double spring positively charged (the anode), thus producing driving. Obviously, if the cathode is not heated, it cannot donate electrons. For this reason, the circuits using vacuum tubes required time for the valves to heat up before they could operate, and the valves were burnt very easily.
PN Diode or PN Junction
PN diodes are junctions of two p and n type extrinsic semiconductor materials, so they are also referred to as pn junctions. It should be noted that neither of the two crystals is electrically charged separately, because in each crystal, the number of electrons and protons is the same. We can say that the two crystals, both the p and the n, are neutral. (The net charge is 0).
Formation of the Space-Charge
By joining the two crystals, a diffusion of electrons occurs from the n crystal to the p crystal (Je).
In establishing these currents, fixed charges are created in an area on both sides of the junction. This area has different names, such as space-charge, depletion, depletion region, etc.
As the diffusion process progresses, the space-charge width increases deeper into the crystal on both sides of the junction. However, the accumulation of positive ions in the n area and negative ions in the p area creates an electric field (E) acting on free electrons in the n area with a given displacement force that will oppose the electron current and eventually stop them.
This electric field is equivalent to saying that there is a voltage difference between the p and n areas. This potential difference (VD) is 0.7 V for silicon and 0.3 V when the crystals are made of germanium.
The width of the space-charge, once equilibrium is reached, is typically on the order of 0.5 microns. However, when one of the regions is more doped than the other, the space-charge is much larger.
The device thus obtained is called a diode. In the case described, where it is not under an external potential difference, we say that it is not polarized. Since electrons flow from the n zone into the p area, the p point is called the anode (represented by the letter A), while the n end is called the cathode (represented by the letter C or K).
There are also thermal protection diodes, which are capable of protecting cables.
A (p) |
| C or K (n) |
When the diode is subjected to an external voltage difference, it is said that the diode is polarized. The polarization may be direct or reverse.