Atomic Structure and Electrical Properties

• Electrical characteristics of elements are determined by the structure of their atoms

The Atom

• Atoms contain a positively charged nucleus, protons, and neutrons at the center
• Negatively charged electrons revolve around the nucleus
• The atomic number indicates the number of protons and electrons in an atom

Orbital Ring

• Planetary electrons are located in successive shells (K, L, M, N, O, P, and Q) around the nucleus
• Shells of orbital electrons hold a maximum number of electrons
• K holds 2
• L holds 8
• M holds 18
• N holds 32
• O holds 50
• The formula to calculate the number of electrons in each orbit per ring is 2n²
• Valence electrons are the number of electrons in the incomplete outermost shell or the highest energy level of an atom
• Core electrons occupy the innermost shell or have the lowest energy levels

Valence Electrons

• Given the example:
• Oxygen (atomic number 8) has 2 electrons in shell K and 6 electrons in shell L
• VE is 6 electrons
• CE is 2 electrons
• Sodium (Na) (atomic number 11) has
• 2 electrons in shell K
• 8 electrons in shell L
• 1 electron in shell M
• VE is 1 electron
• CE is 10 electrons

Materials Used in Electronics

• Electronics is a branch of physics and electrical engineering, dealing with the emission, behavior, and effects of electrons in electronic devices
• Matter is classified into conductors, semiconductors, and insulators based on their electrical properties

Insulators

• Insulators do not conduct electrical current under normal conditions.
• Valence electrons are tightly bound; insulators have very few free electrons
• Examples are rubber, plastics, glass, and quartz

Conductors

• Conductors easily conduct electrical current
• Conductors contain a large number of free electrons
• The best conductors are single-element materials with atoms that have only one loosely bound valence electron
• Examples are copper (Cu), silver (Ag), gold (Au), and aluminum (Al)

Semiconductors

• Semiconductors have conductivity between insulators and conductors
• Semiconductors can be altered to function as either
• Single-element semiconductors include antimony (Sb), arsenic (As), boron (B), silicon (Si), and germanium (Ge)
• Semiconductors are characterized by atoms with four valence electrons
• Silicon is the most commonly used semiconductor
• Insulators, conductors, and semiconductors have +/- 4 valence electrons

Electrons and Holes

• In a pure silicon crystal at room temperature, thermal energy allows valence electrons to jump to the conduction band, becoming free electrons also known as conduction electrons
• A vacancy left in the valence band when an electron jumps to the conduction band is called a hole
• For every electron raised to the conduction band, one hole remains in the valence band which creates an electron-hole pair
• Recombination occurs when a conduction-band electron loses energy and falls back into a hole in the valence band

Electron and Hole Current

• Intrinsic silicon contains free electrons drifting randomly and an equal number of holes
• When voltage is applied to intrinsic silicon, free electrons move toward the positive end, which creates electron current
• Valence electrons move to nearby holes creating hole current
• Hole current differs from electron current; the movement of valence electrons shifts the position of holes

Conduction in Semiconductors

• Conduction in semiconductors results from free electron flow in the conduction band and hole movement in the valence band
• Copper has free-moving electrons due to metallic bonding with valence electrons not tied to individual atoms
• Only free electrons contribute to current; there are no holes in metallic structures