Basic Solid State Electronics

Sources

1. BASIC SOLID STATE PRINCIPLES (Lecture Slides)
2. Class Lecture

The Atom’s Structure

• Matter is made up of atoms
• An atom is made up of three types of charges:
  • Electrons
  • Neutrons
  • Protons
• Protons and Neutrons make up nucleus—the atom’s central mass
• Electrons revolve around the nucleus in a fixed orbit
• Electrons distribute themselves in shells
• Valence electrons refer to the electrons at the outermost shell
• The total number of electrons in a neutral atom is equivalent to the number of protons in the atom
• Bohr’s model is the most popular model for representing an atom
• Charges are denoted using the unit Coulomb ()
• Protons are bigger than electrons because electrons are negatively charged. On the other hand, neutrons share a similar size with protons.
• The charges separate when heated
• The number of valence electrons influences the atom’s electrical property.
  • This is because the valence electrons have the highest chances of leaving and finding a pair, generating current—the movement of charge—as a result.
• An electron becomes a free electron once it becomes unbounded to an atom, enabling it to freely move and find a pair
• Electrons move away from their current shell when voltage—the work needed to move a charge—is applied
• 3 electrical states of matter:
  • conductor
  • semiconductor
  • insulator
• An atom is stable only when it has eight valence electrons (the maximum number of valence electrons in an atom), except for the atom with only one shell.
  • This is because the force pulling the electrons away from the nucleus counteracts the force pulling the valence electrons towards the nucleus.

Three Electrical Classifications of Matter

Conductor

• A conductor is a material that easily conducts current because of its atoms’ unstable outermost shell. In other words, their atoms’ valence electrons are weakly attached to the nucleus.
• Their valence electrons will become free electrons after receiving enough energy; as a result, the free electrons will move randomly from one atom to another.
  • If voltage is applied, the free electrons will move in one direction and generates an electric current
  • The free electrons are locating a positive pair
• The energy state determines how good a conductor at conducting current (higher means better)
• has 1 valence electron

Finding the Number of Neutrons

The number of neutrons is equivalent to the difference between the atomic mass and the atomic number (number of protons)

Insulator

• Insulators can hardly conduct current because its atoms’ have a stable outermost shell.
• Current is rarely produced (even with high amounts of voltage applied) in an insulator because valence electrons from the insulator’s atoms tend to stay in their orbit.
• has 8 valence electrons

Semiconductor

• Semiconductors have a conductivity between the extremes of an inductor and conductor. They are a combination of a conductor and an insulator.
• has 4 valence electrons to form a covalent bond (sharing of electrons)
• their electrons are arranged to form a crystal
• in order to become stable, the atoms of semiconductors share their valence electrons with each other. As a result, they neither gain nor lose electrons.

Types of Semiconductor

Intrinsic (Pure) Semiconductor

• A silicon crystal is considered an intrinsic semiconductor when there is no other atoms besides silicon atoms present in the material.
  • In the same token, a germanium crystal is considered an intrinsic semiconductor when there is no other atoms besides germanium.
• Although they share electrons, they can still produce free electrons and current when voltage/heat is applied; there, resulting in a conductor. This happens because the heat breaks the bonds formed.
  • Conversely, low temperature will result in an insulator—due to the lack of free electrons to produce current.
  • Separating an electron from its parent atom will leave a hole in the valence bond. It is represented as a positive sign because it behaves like a proton which attracts electrons from neighboring atoms.
    • At times, an electron merges with the hole and becomes part of the covalent bond. This process is referred to as recombination. The amount of time for the electron to become free and then bounded back to an atom is referred to as a lifetime.
    • the flow of charges from positive to negative is known as the hole flow; however, if it flows from negative to positive, it is known as the electron flow.
      • electron flow - movement of free electrons in one direction
      • hole flow - the movement of free electrons to fill a neighboring hole

Extrinsic Semiconductor

• An extrinsic semiconductor is a semiconductor that was subjected to doping—addition of impure elements to pure silicon/germanium crystal—to alter its conductivity.
• Extrinsic semiconductor either results in excess electrons or excess holes; hence, we classify extrinsic semiconductor into two types: n-type and p-type.

N-Type (Negative Type) Semiconductor

• This type is formed by adding impure atoms with 5 valence electrons (a.k.a. donor atoms)
• There is an excess electron donated to the material to contribute to the conduction
• The majority current carriers are the thermally-generated free electrons.

P-Type (Positive Type) Semiconductor

• This type is formed by adding impure atoms with 3 valence electrons (a.k.a. acceptor atoms)
• There is an excess hole which accepts electron from neighboring atoms to complete a covalent bond
• The minority current carriers are the thermally-generated free electrons.

Energy Level Diagram

• An energy level diagram provides another way for determining electrical characteristic of a material.
  • The energy level diagram of a material indicate the band of energy level associated with each electron of the atom, and the required energy to free the valence electrons.
• valence electrons have a higher energy state than the electrons at the inner shell
• any electrons that have left their parent atom have a higher energy state than those within their parent atom
• three regions of the energy level diagram:
  • the conduction region - range of energy values for free electrons
  • the forbidden region - the energy gap that separates the valence band and the conduction region. It represents the energy levels required to free the valence electrons
  • the valence band - range of energy values for valence electrons
• Energy level diagrams based on the material:
  • for insulators, the forbidden region requires 5 for valence electrons to move to the conduction region. In other words, its energy gap is 5
  • for conductors, the valence and conduction band overlaps
  • for semiconductors, the energy gap for silicon is 1.1 , while it is 0.67 for germanium