Introduction to Semiconductor Theory Part 2 Electrical Classifications of Material • The number of valence electron is a common indication that tells us the electrical characteristic of material. Conductor Insulator Semiconductor Conductor - material with less than four valence electrons. • allows electrical current to flow easily because they have more free electrons. • Silver, copper, gold, and aluminum are materials with many free electrons and make good conductors • Silver is the best conductor, followed by copper, gold, and aluminum. • Copper is used more often than silver because of cost. Aluminum is used where weight is a major consideration, such as in high-tension power lines, with long spans between supports. • Gold is used where oxidation or corrosion is a consideration and a good conductivity is concerned Insulator • Material with more than four valence electrons • Insulators will not allow electric current to flow easily because they have very few or even no free electrons • Some examples of these materials are rubber, plastic, enamel, glass, dry wood, and mica. • Just as there is no perfect conductor, neither is there a perfect insulator. Semiconductor • With exactly four valence electrons • Have electrical characteristics in between conductors and insulators • Elementary Semiconductors (group IVA) • Silicon (Si) • Germanium (Ge) • Compound Semiconductor (IIIA-VA) • Gallium Arsenide (GaAs) • Aluminum Arsenide (AlAs) • Gallium Phosphide (GaP) Energy Band Band Gap This is equivalent to the energy required to free an outer shell electron from its orbit about the nucleus to become a mobile charge carrier, able to move freely within the solid material, so the band gap is a major factor determining the electrical conductivity of a solid. Conduction Band electrons in this band are easily removed by the application of external electric fields Forbidden Band electrons never found in this band Valence Band composed of series of energy levels containing valence electrons electrons in this band are more tightly bound to the individual atom than the electrons in the conduction band QUANTA energy required in definite units to move electrons from one shell to next higher shell IONIZATION process of electron exchange where atom losses or gain electrons Negation Ion Anion - an atom having more than its normal amount of electrons acquires a negative charge Positive Ion Cation - the atoms that give up some of its normal electrons and left with fewer negative charges than positive charges Energy Gap Comparison The insulator with a very wide energy gap. The wider this gap, the greater the amount of energy required to move electron from the valence band to the conduction band. An insulator requires a large amount of energy to obtain in a small amount of current. The insulator insulates because of the wide forbidden band or energy gap. Energy gap is greater than 5eV. The semiconductor has a smaller forbidden band and requires less energy to move an electron from the valence band to the conduction band. Silicon 1.1eV Germanium 0.67eV For a certain amount of applied voltage, more current will flow in the semiconductor than in the insulator. • There is no forbidden band or energy gap in a conductor. • The valence band and conduction band overlaps. • It takes a small amount of energy to move electrons into the conduction band. • Conductors pass electrons very easily. Bonding of Atoms • Ionic Bond or Electrovalent or Electrostatic Bond • Metallic Bond • Covalent Bond Ionic Bond or Electrovalent or Electrosatic Bond • Results from attractive forces between positive and negative ions or between pairs of oppositely charge ions • An ionic bond is typically formed between a metal and a non-metal. A bond between two elements caused by a difference in charge created by the donation of an electron by one of the atoms to another. Metallic Bond • Results from attractive forces between a group of positive ions and a sea of electrons that are free to move about among its ions. • It may be described as the sharing of free electrons among a lattice of positively charged ions (cations) Covalent Bonding • Results when atoms share their valence electrons with other atoms. • The shared electrons are attracted simultaneously to two atoms resulting in a force that holds them together. Covalent Bonding in Semiconductors • Crystal • An orderly pattern of arrangement of silicon atoms when they form a solid • Another name for solid whose atoms or molecules are arranged in a three-dimensional geometrical pattern commonly referred to as lattice. • Amorphous solid - any non-crystalline solid in which the atoms and molecules are not organized in a definite lattice pattern. Such solids include glass, plastic, and gel. Conduction Process in Semiconductors • When enough energy is absorbed by the valence electrons, it is possible for them to break some of their covalent bonds. Once the bonds are broken, the electrons move to the conduction band where they are capable of supporting electric current. When a voltage is applied to a crystal containing these conduction band electrons, the electrons move through the crystal toward the applied voltage. This movement of electrons in a semiconductor is referred to as electron current flow.
