Semiconductors, Insulators & Conductors, Basic Introduction, N type vs P type Semiconductor

Introduction to Semiconductors

  • Semiconductors possess properties between those of insulators and conductors.

Conductors and Insulators

  • Insulators:

    • High resistance to electric flow.

    • Do not conduct electricity (e.g., sulfur, iodine).

  • Conductors:

    • Excellent electrical conductors (e.g., zinc, copper, aluminum).

Characteristics of Semiconductors

  • Semiconductors are primarily metalloids.

  • Conduct less electricity than conductors but more than insulators.

  • Common naturally occurring semiconductors include silicon and germanium.

  • Germanium conducts better than silicon.

Temperature and Conductivity

  • Increasing Temperature:

    • For semiconductors (like silicon), conductivity increases as temperature rises.

    • For metals (like copper), conductivity decreases with rising temperature.

Doping Semiconductors

Introduction to Doping

  • Doping involves adding impurities to increase conductivity.

Silicon Crystal Structure

  • Silicon has 4 valence electrons (atomic number 14).

  • Forms covalent bonds with four neighboring silicon atoms, creating a tetrahedral structure similar to diamond.

N-Type Semiconductors

  • Doping with Phosphorus (5 valence electrons):

    • Phosphorus uses 4 of its 5 valence electrons to bond with silicon, leaving 1 free electron.

    • This free electron increases conductivity, resulting in an n-type semiconductor.

    • Charge carriers in n-type: electrons.

P-Type Semiconductors

  • Doping with Aluminum (3 valence electrons):

    • Aluminum uses all 3 valence electrons to bond leaving one unfilled bond (a hole).

    • This creates a p-type semiconductor where conductivity is enhanced by the movement of holes.

    • Charge carriers in p-type: holes (electron vacancies).

PN Junctions

  • Combining p-type and n-type materials creates a PN Junction.

  • Electrons from the n-type drift to fill holes in the p-type, leading to:

    • Negative charge buildup on the p-type side.

    • Positive charge buildup on the n-type side.

  • Formation of an electric field across the junction.

Charge Calculations and Equilibrium

Formal Charge Calculation

  • Formal Charge Formula:

    • Formal Charge = Valence electrons - (Bonds + Dots)

  • Before electron transfer:

    • Phosphorus: neutral (5-4=0).

    • Aluminum: neutral (3-3=0).

  • After electron transfer:

    • Phosphorus becomes positively charged.

    • Aluminum becomes negatively charged.

Effects of the PN Junction

  • The movement of electrons creates a junction potential across the PN junction.

  • Results in a diode which allows current to flow in one direction only:

    • Current flows if the diode is connected correctly to a voltage source.

    • No current flow if the diode is connected in reverse.

Conclusion

  • Overview of semiconductor fundamentals:

    • Effects of temperature on conductivity.

    • The effects of n-type and p-type doping.

    • Formation and significance of the PN junction.