Electronics 245 - Week 1 - Lecture 2

Define an insulator

• Materials in which electricity does not flow

• glass, wood, plastic

Define a conductor

• Material in which electricity flows due to low resistiance

• aluminium, copper, etc

Define a semiconductor

• Materials with a electrical conductivity between conductors and insulators

Define the conduction band

• energy levels where an electron is allowed to freely move through a material

Define the valence band

• The energy levels that electrons possess when in the outer shell of atoms of a material

Define band gap

• The difference in energy between the conduction band and the valence band

• Represents the amount of energy required to break covalent bonds

Describe Valency

• Uses the Bohr atomic model

• valence electrons are in the outer shell of an atom

• valence electrons are responsible for chemical reactivity

Define an element

• substances made up of one type of atom

Define a compound

• substances made of more than type of atom/element

State the 2 most common pure elemental semiconductors

• Silicon

• Germanium

Describe the silicon crystal lattice

• 4 valence electrons

  • can form covalent bonds with 4 other silicon atoms

  • all used in bonding

Describe the breaking of the covalent bonds of a silicon crystal lattice at T = 0 K

• absolute zero

• electrons dont have enough energy to break covalent bonds

• acts as insulator

Describe the breaking of the covalent bonds of a silicon crystal lattice at T > 0 K

• Electrons can obtain enough energy to break its covalent bond

Define bandgap energy

• minimum energy required to break a covalent bond

Define Ev and Ec

• Ev = max energy of valence energy band

• Ec = min energy of conduction band

Describe the conduction, valence and forbidden bandgaps of:

• Insulators

• Semiconductors

• Conductors

• Insulators have a very large forbidden band gap

  • meaning electrons wont move to the conductive band without a significant amount of energy

• Semiconductors have a relatively small forbidden gap

  • therefore, some energy supplied can induce conductivity

  • no energy means no conductivity

• Conductor’s valence gap and conduction gap overlap

  • no energy is required for the material to be conductive

Describe the concept of a “hole”

• when a covalent bond breaks, an empty positive space remains

• adjacent electrons can occupy that “hole” and create a “hole” where they previously were

• thus there is a positive moving charge

• holes are charge carriers

Describe an intrinsic semiconductor

• semiconductor that is made of one element

• concentration of electrons and holes are equal

State the formula for the number of electrons in the conduction band in an intrinsic semiconductor, ni

• ni = BT ^ (3/2) * e ^ (-Eg / 2kT)

  • 𝐵– coefficient for semiconductor material, in cm^(−3) K^(− 3/2)

  • 𝑇– temperature in Kelvin, K

  • 𝐸𝑔– bandgap energy in eV

  • 𝑘– Boltzmann’s constant = 86 x 10-6eV/K

Describe extrinsic semiconductors

• Made of more than one type of element

• when low concentration of free electrons in instrinsic:

  • low currents

Describe the 2 types of extrinsic semiconductors

• n-type

  • group V impurities are added

  • donor impurities are present, which donates an electron

  • number of electrons > number of holes

• p-type

  • group III impurities are added

  • acceptor impurities are present, which accept an electron

  • number of holes > number of electrons

Describe the conductivity in an Extrinsic Semiconductor

• no po = ni²

  • no = conc of free electrons

  • po = conc of holes

  • ni = conc of intrinsic carrier

• if donor conc, Nd > ni

  • no = Nd

  • po = ni² / Nd

• if acceptor conc, Na > ni

  • po = Na

  • no = ni² / Na