Initial question
What is written in wikipedia about the diode?
In wikipedia there are topics, branches of knowledge about diodes. Article gives sometimes brief, sometimes sufficient explanations of the physical phenomena. We concentrated on principle physical phenomena rather than taking into account history and engineering applications.
In electronics, a diode is a two-terminal electronic component with asymmetric conductance; it has low (ideally zero) resistance to current in one direction, and high (ideally infinite) resistance in the other.
A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material with a p–n junction connected to two electrical terminals.
Further questions T1 (occurred after wikipedia search)
How the diode is conducting current in one direction and not in another?
An ideal diode is a conductor in one direction and an insulator on the other, though in reality we do not seem to have a ‘perfect’ state. The reason that a PN junction diode conducts in one direction is because of the following. A PN junction diode has two parts the “p” and the “n” side. with excess of holes and electrons respectively. Initially a momentary conduction causes flow of electrons from the “n” and holes from “p” to the opposite side to occur but will stop soon because of electrostatic field that will be created. In the middle of this field a neutral space is created, which is called a depletion layer. Now whenever conduction is required, connection is made so that negative terminal is connected with the “n” side and positive to the “p” side; this will create repulsion which will break the neutral depletion layer and conduction occurs. This connection is called forward bias. On the other hand if positive terminal is connected to “n” side the charges will be pulled away from the neutral zone away. the depletion layer widens and the diode acts as an insulator. this connection is called reverse bias. as shown in the sketch
How does PN junction work?
There are free electrons on N-type, free holes on P-type . when bring them together and join them to make one piece of semiconductor, we can see free electrons -side and free holes on the p-side can initially wander across the junction.When near the junction jump from N to P, holes near the junction jump from P to N . This phenomenon creates a space charge region This movement of electrons and holes in space region gives rise to diffusion current .Now the current flows across the circuit and this current is called the drift current. In this process the current flowing through the PN junction is very high and ultimately the PN junction gets damaged due to overheating caused by the excess flow of current .
Why is there depletion?
Depletion is an insulating region within a conductive, doped semiconductor material where the mobile charge carriers have diffused away, or have been forced away by an electric field. It is formed from a conducting region by removal of all free charge carriers, leaving none to carry a current.No free electrons exist on the N side in the depletion region and no free holes exist on the P side .
What is valence band and conduction band?
Valence band is the highest range of electron energies in which electrons are normally present at absolute zero temperature.
Conduction band quantifies the range of energy required to free an electron from its bond to an atom. Once freed from this bond, the electron becomes a 'delocalized electron', moving freely within the atomic lattice of the material to which the atom belongs. Various materials may be classified by their band gap: this is defined as the difference between the valence and conduction bands.
What is diode reverse-recovery effect?
Diode has sort of internal capacitance in the form of pn layer mix in the pn contact place. When voltage is across diode, some p material goes to n side and some n material goes to p side. This creates "small diode" inside of the main diode. That "blurs" diode orientation and makes diode conduct current in the opposite direction. This lasts for small period of time because then voltage across diode is reversed and this brings diode structure back to normal.
LED? Why diodes emit light?
When electron jumps into hole, the electron is on high level and hole is on low level inside of corresponding atom. Energy difference is lost in the form of photon. As there is certain levels, the energy, wavelength and color of the photon are the same.
Also, it turns out that all diodes should emit light. Probably the ones that do not emit light in visible spectrum, emit it to their package wall. (?)
New questions:
- Do all semiconductor diodes emit light?
- Why semiconductor diodes are widely used instead of other types of diodes?
How semiconductors make up P and N type?
Conduction of a material is highly dependent on the most external free electrons. Silicon as a typical semiconductor materials has full outer most
electrons. When materials other substances are added as impurity conduction property changes. this adding of impurities is known as doping. When a silicon is doped with phosphorus extra electrons which can move freely through the silicon bonds emerges. The result is known as N-type material. On the other hand when a silicon is doped with substances such as germanium; a substance which has extra ‘hole’ in its outermost electron orbit. The resulting material is called a P type.
What is the difference between diode and laser diode?
In laser diode electron and hole can exist near each other for long time, up to microseconds. This creates sort of population inversion. When resonant photon passes by, electron jumps into hole and during this emits coherent photon. In opposite, conventional LED does not wait until photon and emits constantly. Why then some diodes become conventional LEDs, and some - laser diodes?
New questions:
- Why some diodes are constant LEDs, and some - laser diodes?
Further questions T2
Do all semiconductor diodes emit light?
Initial hypothesis: Yes. The ones that emit light in non-visible spectra are covered with package and light is emitted into plastic.
Evaluation of initial hypothesis: there may be somehow different physical mechanism of non-LED diodes work, which doesn’t lead to light emission.
It appears so that all semiconductor diodes emit light. To be exact, all p-n semiconductor junctions emit photons. (Check the picture) Yes yes, but these are not emitted to plastic, they are absorbed back to semiconductor die. This energy is mostly dissipated as heat. LEDs and laser diodes are made so that energy does escape the junction really well.
New questions
- Why some materials are transparent, and others - aren't?
Why semiconductor diodes are widely used instead of other types of diodes?
Initial hypothesis: They are cheapest and the smallest. Which implies that at some point engineers decided that physical principles behind semiconductor diodes allow cheaper and smaller diodes in the next few years. Which means that known non-semiconductor diode’s physical working mechanism contains obstacles that are hard to avoid.
Evaluation of initial hypothesis: On the one side, this is not very good question as it enclosures current state of technological development. On the other side, I tried to make it in the way that it leads to study of physical phenomena and principle, which is good.
Solid-state diodes is used mostly because of its size. Modern mobile phone radio modulator would definitely be of size of the living room, if contained
vacuum tubes. We do not need this huge currents and voltages to calculate and transmit data. However, some radio transmitter towers are using vacuum diodes: this is still very effective solution for very large current applications, where solid state device would just start to melt. There are also differences in current-voltage characteristics (see picture)
Why some diodes are constant LEDs, and some - laser diodes?
Initial hypothesis: Probably to the use of different materials. Somehow some materials lead to population inversion and some - not. Why? To answer this question, additional study of the materials and outcoming properties are required.
Evaluation of initial hypothesis: Not only materials might be different, but overall geometry and principles may differ as well. However, the word laser diode implies that diode principle stays intact (i.e. it should conduct current in one direction etc.)
To keep it simple, so-called “constant LEDs” are emitting light, when electrons from n-junctions fill the holes in the p-junction. The depletion layer area
between two doped layers is mostly exposed to the plate mirror. Photons are shot here and there, and are focused to one direction with optical cavity.
In the laser diode, optical trap is sandwiched right between p- and n-materials. The junction itself becomes the gain medium. Stimulated emission happens at electron-hole pair: incident photon is forcing the electrons and holes in the junction to annihilate and release photons of light with equal phase and energy.
click on image to go to original webpage
Further questions T3
Why some materials are transparent, and others - aren't?
When photons go through and near atoms without absorbtion, then the material is transparent. Photons don't interact with atom's electrons when band gap energy doesn't match photon energy. Materials are opaque only to the color that they absorb.
New questions:
- Why some materials are reflective and othes are mat?
- Why materials have different reflection coefficients?
- Which angle absorbed and then reemitted photon goes?
- Should absorbed photon be exactly band gap energy?
- Why do we consider that photons interact with only electrons, but not with atom core?