Question

Describe the two main types of semiconductors and contrast their conduction mechanism.

Answer 1

The two main types of semiconductors are:

1. n-type semiconductor
2. p-type semiconductor

(1) n-type semiconductor:

• The semiconductor whose increased conductivity is a result of negatively charged electrons is called an n-type semiconductor. 
• When  the crystal of a group 14 element like Si (Silicon) or Ge (Germanium) is doped with a group 15 element like P (Phosphorus) or As (Arsenic), an n-type semiconductor is formed.
• Si and Ge each have four valence electrons. 
• In their crystals, each Si or Ge atom forms four covalent bonds.
• P and As contain five valence electrons each.
• When Si or Ge is doped with P or As, these elements take up some of the crystal’s lattice sites.
• Four out of five electrons from P, or As, are used in the formation of covalent bonds with four neighbouring Si or Ge atoms.
• The remaining fifth electron is delocalised, which increases the conductivity of the doped Si or Ge.

(2) p-type semiconductor:

• The semiconductor whose increase in conductivity is a result of an electron hole is called a p-type semiconductor.
• When a crystal of group 14 elements such as Si (Silicon) or Ge (Germanium) is doped with a group 13 element such as B (Boron), Al (Aluminum), or Ga (Gallium) (which contain only three valence electrons), a p-type semiconductor is formed.
• When a Si crystal is doped with B, the three valence electrons are used to form three covalent bonds with neighbouring Si atoms.
• The fourth bond needed to complete the Si’s covalent bonding is missing, creating an electron hole in the lattice.
• An electron from a neighbouring atom can move and fill the electron hole. However, in doing so, it creates an electron hole at its original position.
•  This process makes it seem like the electron hole is moving opposite the electron that filled it.
• When an electric field is applied, electrons move toward the positively charged plate through the electron holes.
• It will appear as if the electron holes are positively charged and are moving toward the negatively charged plate.