how to form a PN Junction by using semiconductor

Electrical conductivity:

      In order of conductivity: superconductors, conductors, semiconductors, insulators

1.      Conductors: material capable of carrying electric current, i.e. material which has “mobile charge carriers” (e.g. electrons, ions,..) e.g. metals, liquids with ions (water, molten ionic compounds), plasma.

2.      Insulators: materials with no or very few free charge carriers; e.g. quartz, most covalent and ionic solids, plastics.

3.      Semiconductors: materials with conductivity between that of conductors and insulators; e.g. germanium Ge, silicon Si, GaAs, GaP, InP.

4.      Superconductors: certain materials have zero resistivity at very low temperature.

Semiconductors:

1.      Intrinsic semiconductors,

2.      Doped semiconductors(It is the most common semiconductor used in all the Electronic component),

Ø  n-type materials

Ø  p-type materials

Intrinsic semiconductors:

Semiconductor - material for which gap betweenvalence band and conduction band is small(gap width in Si is 1.1 eV, in Ge 0.7 eV).at T = 0, there are no electrons in the conduction band, and the semiconductor does not conduct (lack of free charge carriers), at T > 0, some fraction of electrons have sufficient thermal kinetic energy to overcome the gap and jump

to the conduction band, fraction rises with temperature. e.g. at 20o C (293 K), Si has 0.9x1010 conduction electrons per cubic centimeter; at 50o C (323 K) there are 7.4x1010.

Electrons moving to conduction band leave “hole” (covalent bond with missing electron) behind, under influence of applied electric field, neighboring electrons can jump into the hole, thus creating a new hole, etc. ⇒ holes can move under the influence of an applied electric field, just like electrons. Both contribute to conduction. In pure Si and Ge, there are equally many holes (“p-type charge carriers”) as there are conduction electrons (“n-type charge carriers”), pure semiconductors also called “intrinsic semiconductors”.

Intrinsic silicon:

Doped semiconductors:

Doped semiconductor (also “impure”, “extrinsic”) -semiconductor with small admixture of trivalent or pentavalent atoms. It is the most favorable and most commonly used semiconductor in all the Electronic components.

N-type material (donor (n-type) impurities):Dopant with 5 valence electrons (e.g. P, As, Sb).4 electrons used for covalent bonds with surrounding Si atoms, one electron “left over”,left over electron is only loosely bound⇒ only smallamount of energy needed to lift it into conductionband (0.05 eV in Si). “n-t
ype semiconductor”, has conductionelectrons, no holes (apart from the few intrinsicholes).Example: doping fraction of 10-8 Sb in Si yields about 5x1016 conduction electrons per cubic centimeter at room temperature, i.e. gain of 5x106 over intrinsic Si. Semiconductor material doped with donors. Material has high concentration of free electrons. Concentration of holes in n-type material is very low. Contains POSITIVELY charged donors (immovable) and NEGATIVELY charged free electrons. Total charge = 0. Semiconductor material doped with donors. Material has high concentration of free electrons. Concentration of holes in n-type material is very low. Contains POSITIVELY charged donors (immovable) and NEGATIVELY charged free electrons. Total charge = 0.

P-type material (acceptor (p-type) impurities): Dopant with 3 valence electrons (e.g. B, Al, Ga,In) ⇒ only 3 of the 4 covalent bonds filled ⇒vacancy in the fourth covalent bond ⇒ hole. “p-type semiconductor”, has mobile holes, very few mobile electrons (only the intrinsic ones). Semiconductor material doped with acceptors. Material has high hole Concentration of free electrons in p-type material is very low. Contains NEGATIVELY charged acceptors (immovable) andPOSITIVELY charged holes. Total charge = 0.

Advantages of doped semiconductors:

o   Can “tune” conductivity by choice of dopingfraction.

o   Can choose “majority carrier” (electron or hole).

o   Can vary doping fraction and/or majority carrierwithin piece of semiconductor.

o   Can make “p-n junctions” (diodes) and “transistors”. 

What happens if n- and p-type materials are in close contact?

Being free particles, electronsstart diffusing from n-type material into p-material Being free particles, holes, too, start diffusing from p-type material into n-material Have they been NEUTRAL particles, eventually all the free electrons and holes had uniformly distributed over the entire compound crystal. However, every electrons transfers a negative charge (-q) onto the p-side and also leaves an uncompensated (+q) charge of the donor on the n-side. Every hole creates one positive charge (q) on the n-side and (-q) on the p-side. Electrons and holes remain staying close to the p-n junction because negative and positive charges attract each other. Negative charge stops electrons from further diffusion Positive charge stops holes from further diffusion. The diffusion forms a dipole charge layer at the p-n junction interface. There is a “built-in” VOLTAGE at the p-n junction interface that preventspenetration of electrons into the p-side and holes into the n-side.

Hence,PN junction is made up on P-type (contains highly doped holes and minority electrons) and N-type (contains highly doped electrons and minority holes) materials. The P & N type materials are combines to form a PN junction is called as Depletion region. In N-type side the concentration of electrons are high, so they move towards the junction is called depletion. In P-type side the concentration of holes are high, so they are move towards the junction.


Also read :

• How does pn junction works in diode
• 5 things you need to know about analysis and overview of pn junction diode
• Get awesome info about basic electronics here