semiconductor
diodes

The original tube diode used an evacuated glass tube, a heated cathode filament, and an anode plate to rectify AC current. They are still used in high power rectifiers, but fragility, size, and heat creation (power wasted) have made them all but disappear to specialty applications.
Today semiconductors form the basis of electronics for all the opposite reasons.
Periodic Table
Modern electronic devices are primarily made from semiconductor elements which have as their basis Silicon (Si) and dopant materials added to silicon such as Phosphorous (P) and Boron (B) as well as others.
Semiconductor action takes advantage of the quantum properties at the thin boundary layer between to types of doped silicon P type (positive) and N type (negative) called the junction.

This was discovered at Bell labs by Dr. Shockley by accident, but the discovery was quickly put to use immediately outdating tube technology and rapidly becoming increasingly miniaturized with pressure from the space race and then computer chip technology all of which led to the computer in your pocket.
(the smartphoine was invented in Waterloo Ontario by Blackberry)
 

Atoms have a nucleus composed of positively charged protons and neutral charged neutrons. The # of protons Z (atomic number defines the atom) is usually equal to the orbiting electrons which are mass less and have a negative charge. This keeps the atom neutrally charged.
In the above example of the simple element helium the atomic number is 2 ( protons ) and the atomic mass is 4 because there are also 2 neutrons and the number of electrons is 2 to keep the charge neutral. The valence shell or orbit of electrons is 2. The inner or 1st orbital always has 2 and the next orbitals try to fill with 8 electrons(stable octet) Remember why conductors conduct? The outer valence electrons are loose and free to move when a voltage is applied. Semi conductors have outer valence electron that are free to move under certain conditions thus the term SEMICONDUCTORS

MAKING SILICON INTO A SEMICONDUCTOR
start with silicon then add an N or P dopant

N (negative) type silicone crystal is made by high temperature diffusion of the dopant impurity Phosphorous a pentavalent atom.

The extra valence electron is free to move under the force of a voltage potential when holes are available.

P type crystal is created when the dopant Boron is diffused into the silicone

crystal lattice .   

Semiconductor components are made when the N and P type materials are joined. Interesting things happen at the junction when a forward voltage is applied.

Semiconductor action

1) when the N (phosphorous) and P (boron)are joined at the junction some of the
free electrons from the N side gravitate into the holes on the P side which creates neutral silicon at the junction called a potential barrier (effectively an insulator).

2) if a negative potential is applied at the N side (cathode) the free electrons will repel across the junction and if a positive potential is applied to the P (anode) the positive charge spaces (holes) will move towards the electrons and if a voltage of about .7 volts is applied then current will begin to flow as electrons are pushed towards the holes.

3) this is called forward biasing of the N - P diode junction and .7 volts is now called the forward voltage the energy required to cross the junction.

-when electrons cross the junction their energy potential is raised to .7 V . when the electrons drop back to ground state they release a photon of infrared light which is not visible to humans

- LEDs are diodes that have various dopants that raise the forward voltage of the electrons that release photons in the visible range of vision. For example a forward voltage of 1.8 volts will create photons that appear as Red LED light.

Electromagnetic spectrum colour and energy vs wavelength

4) if the polarity is reversed then free electrons are pulled to the cathode and holes are pulled towards the anode increasing the potential barrier and blocking current flow

5) this is called reverse biasing the junction and almost no current flows

6) the junction can prevent reverse flow of current until the reverse breakdown voltage is exceded and then massive current flows and usually burns out the junction ruining the diode (straw that breaks the camel's back)

7) this is called reverse breakdown voltage , avalanche voltage or Zener voltage
  

                       P    -      N
                     

Diode Symbols

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