Semiconductors Knowledge-Based Study

Study notes

1. What Are Semiconductors?

   Semiconductors are materials whose electrical conductivity lies between that of conductors (such as copper and aluminum) and insulators (such as glass and diamond). In materials chemistry, the electrical behavior of solids is explained using band theory.

   When large numbers of atoms bond in a solid, their discrete atomic orbitals overlap to form continuous energy bands:

   • Valence Band: The highest occupied energy band that is largely filled with valence electrons at absolute zero.
   • Conduction Band: The lowest unoccupied or partially filled energy band where electrons can move freely and conduct electric current.
   • Band Gap: The forbidden energy range separating the valence band from the conduction band.

   In conductors, the valence and conduction bands overlap, requiring virtually no energy for electrons to move into conducting states. In insulators, the band gap is large (typically greater than 4 eV), making electron promotion under normal conditions negligible. In semiconductors, the band gap is moderate (typically 0.5 to 3.0 eV), allowing electrons to be thermally or photochemically excited from the valence band into the conduction band.

   Pure Semiconductors

   An intrinsic semiconductor is an undoped, chemically pure crystal consisting of a single element (such as silicon, Si, or germanium, Ge) or a stoichiometric compound (such as gallium arsenide, GaAs).

   In a crystalline silicon lattice, each silicon atom shares four covalent bonds with adjacent atoms. At absolute zero (0 K), all valence electrons are locked within these covalent bonds, making the material an insulator.

   Charge Carriers: Electrons and Holes

   When thermal energy increases at room temperature:

   • Thermal vibrations break a small fraction of covalent bonds, promoting electrons into the conduction band.
   • Each promoted electron leaves behind an empty electronic state in the valence band known as a hole.
   • A hole possesses an effective positive charge and can be filled by an electron from an adjacent bond, causing the hole to propagate through the lattice.
   • In intrinsic semiconductors, the number of free conduction electrons is always strictly equal to the number of holes.

   Unlike metallic conductors whose conductivity decreases with increasing temperature due to increased lattice scattering, intrinsic semiconductors exhibit an increase in electrical conductivity as temperature rises because the generation of charge carriers increases exponentially.

   Modifying Conductivity via Doping

   Because intrinsic semiconductors have low charge carrier densities at ambient temperatures, practical electronic applications require doping: the deliberate introduction of trace impurity atoms into an intrinsic semiconductor lattice to drastically enhance conductivity.

   n-type Semiconductors

   When a tetravalent silicon crystal is doped with a pentavalent element from Group 15 (such as phosphorus, P, or arsenic, As):

   • Four of the impurity valence electrons form covalent bonds with neighboring silicon atoms.
   • The fifth electron is weakly bound and easily excited into the conduction band at room temperature.
   • These impurities are called donor impurities because they donate negative conduction electrons.
   • In n-type semiconductors, electrons are the majority carriers and holes are the minority carriers.

   p-type Semiconductors

   When silicon is doped with a trivalent element from Group 13 (such as boron, B, or gallium, Ga):

   • The dopant atom has only three valence electrons, leaving an incomplete bond in the lattice.
   • This missing electron acts as a localized hole that can readily accept an electron from neighboring bonds.
   • These impurities are called acceptor impurities.
   • In p-type semiconductors, holes are the majority carriers and electrons are the minority carriers.

   The Fundamental Building Block of Electronics

   When an n-type semiconductor and a p-type semiconductor are joined within a single continuous crystal, a p-n junction is established. This interface forms the operational foundation of modern solid-state electronics.

   Formation of the Depletion Region

   At the metallurgical interface:

   • Free electrons from the n-type region diffuse across into the p-type region, while holes from the p-type region diffuse into the n-type region.
   • Recombination occurs near the interface, neutralizing mobile charge carriers.
   • Fixed, ionized donor ions (positively charged) remain on the n-side, and fixed, ionized acceptor ions (negatively charged) remain on the p-side.
   • This uncompensated zone is called the depletion region, which establishes a built-in electric field that prevents further carrier diffusion.

   Primary Applications

   The asymmetric charge transport of the p-n junction enables diverse technological applications:

   • Rectifier Diodes: Allow electric current to flow predominantly in one direction (forward bias) while blocking it in the reverse direction.
   • Photovoltaic Cells: Absorb incident photon energy greater than the band gap, separating electron-hole pairs via the built-in electric field to produce electric power.
   • Light-Emitting Diodes (LEDs): Recombine electrons and holes under forward bias to release energy as photons, where the photon wavelength depends directly on the material band gap.