N-type Semiconductor

• Extrinsic semiconductors are of two types, based on the element of doping: n-type and p-type.

N-type Semiconductor

• Formed by doping an intrinsic semiconductor (like Si or Ge) with atoms having five valence electrons.
• Consider silicon (Si), which has four valence electrons, each covalently bonded with one of the four adjacent Si atoms.
• If an atom with five valence electrons, like phosphorus (P) or arsenic (As), is incorporated into the crystal, the dopant atom's electrons participate in covalent bond formation, leaving an extra electron in the unbounded state.
• This extra electron is weakly bound to the atom and enters an energy level in a donor state just below the conduction band.
• At room temperature, these electrons can get excited to the conduction band even with a small increase in external energy, leaving the parent atom positively ionized.
• Pentavalent atoms donate electrons to the conduction band to obtain n-type semiconductors, and are called donor atoms.
• Donor energy (Ed) is the minimum energy required for an electron to enter the conduction band.
• Excitation of weakly bound electrons doesn't result in hole formation, so the number of electrons in the material exceeds the number of thermally generated holes.
• Electrons are the majority carriers and holes are the minority carriers

P-type Semiconductor

• Created by doping an intrinsic semiconductor (like Si or Ge) with atoms having three valence electrons.
• Trivalent elements like gallium (Ga), indium (In), or boron (B) can be added as a dopant to an intrinsic semiconductor.
• If a trivalent element, such as Ga, is added to intrinsic Si, the three valence electrons of Ga form three covalent bonds with three neighboring Si atoms
• The dopant lacks an electron for completing the fourth covalent bond, so it needs an extra electron.
• The extra electron can be supplied by Si, which creates an electron hole in the valence band. This hole can be filled by electrons from other locations in the band, creating another vacant site.
• These holes act as acceptors of electrons.
• Hole sites have an energy slightly higher than the normal energy, creating an acceptor energy level that lies just above the valence band.
• Dopant atoms like Ga accept electrons and are called acceptors.
• An electron gains energy (Ea) to create a hole in the valence band. Acceptor atoms become negatively ionized after accepting electrons from the valence band. Holes are created at room temperature and are ready for conduction.
• When many acceptor atoms are added, the number of holes greatly exceeds the number of thermally excited electrons. Holes become the majority carriers, while electrons are the minority carriers in p-type semiconductors.
• At sufficiently high temperatures, additional electron-hole pairs are generated due to the breaking of covalent bonds.