Semiconductors

Semiconductors are material in between conductors and insulators in terms of electric current conductivity. 

• The most common semiconductor is Silicon
• The second most common semiconductor is Gallium Arsenide
• There must be electrons not in covalent bonds that can move around in order to conduct electricity

Current/Charge Carriers

• Free electrons: material receives thermal energy → electrons in the valence band jumps to the conduction band → electrons in the conduction band are free electrons
• Holes: material receives thermal energy → electrons in the valence band jumps to the conduction band → empty spots in the valence band are holes
• The two quantities are equal in the intrinsic state (where there are no impurities)

Doping

• Problem: semiconductors in their intrinsic states don’t conduct currents very well
  • This is because there aren’t enough free electrons and holes in the intrinsic state
• Solution: doping, or adding impurities to an intrinsic semiconductor
  • Increases the number of current carriers by adding free electrons or holes
  • This results in higher conductivity of electrical currents

N-Type Semiconductors (N for Negatively Charged)

• Dope with pentavalent atoms (atoms with five valence electrons) to increase free electrons (Phosphorous, Arsenic, Bismuth, Antimony, etc.)
• Result in more free electrons than holes

P-Type Semiconductors (P for Positively Charged)

• Dope with trivalent atoms (atoms with three valence electrons) to increase holes (Boron, Indium, Gallium, etc.)
• Result in more holes than free electrons

Extrinsic Semiconductors

• More conductive than intrinsic semiconductors because impurities have been added
• Combining N-Type and P-Type Semiconductors result in a boundary with interesting properties called N-P Junction, the basis for semiconductors used today

N-P Junction (or PN Junction)

• Free electrons on the N side jumps across the N-P junction to fill in holes on the P side
• Phosphorous atoms on the N side will become positively charged since free-electron went across the junction, resulting in extra proton
• Boron atoms on the P side will become negatively charged since holes are filled with new electrons from the other side of the junction
• Depletion Region forms as the N-P Junction creates a zone with no Current Carriers

Depletion Region

• If a battery is hooked up to a semiconductor with an N-P Junction such that the negative side is connected to the N side and the positive side is connected to the P side, there will be more free electrons on the N side and more holes on the P side, collapsing the Depletion Region once the voltage is above 0.6v
• If a battery is hooked up to a semiconductor with an N-P Junction such that the positive side is connected to the N side and the negative side is connected to the P side, there will be fewer free electrons on the N side and fewer hols on the P side, expanding the Depletion Region
• This creates a Diode: an electrical component that conducts electric currents only in one direction

How a transistor works