semiconductor (boylestad

• The characteristics of an ideal diode are those of a switch that can conduct current in only one direction
• resistivity of semiconductor is less than the resistivity of insulator and greater than resistivity of the conductors
• flow of current in semiconductor, current in insulator is greater than the insulator and less than the conductor
• The ideal diode, therefore, is a short circuit for the region of conduction
• The ideal diode, therefore, is an open circuit in the region of nonconduction
• The term conductor is applied to any material that will support a generous flow of charge when a voltage source of limited magnitude is applied across its terminals.
• An insulator is a material that offers a very low level of conductivity under pressure from an applied voltage source
• semiconductor, therefore, is a material that has a conductivity level somewhere between the extremes of an insulator and a conductor
• resistivity often used when comparing the resistance levels of materials
• typical resistivity of conductor

  10^-6 ohms per centimeter
• typical resistivity of semiconductor

  50 ohms per centimeter for germanium

  50x10^3 ohms per centimeter for silicon
• resistivity for insulator


  10^12 ohms per centimeter for mica
• The ability to change the characteristics of the material significantly through this process, known as “doping,”
• One complete pattern is called a crystal and the periodic arrangement of the atoms a lattice
• Any material composed solely of repeating crystal structures of the same kind is called a single-crystal structure.
• the atom is composed of three basic particles: the electron, the proton, and the neutron.
• In the atomic lattice, the neutrons and protons form the nucleus while the electrons revolve around the nucleus in a fixed orbit
• germanium has 32 electrons
• silicon has 14 electrons
• both germanium and silicon has four valence shell, so they are called as the tetravalent atoms
• bonding atoms, strengthened by the sharing of electrons, is called covalent bonding
• Although the covalent bond will result in a stronger bond between the valence electrons and their parent atom, it is still possible for the valence electrons to absorb sufficient kinetic energy from natural causes to break the covalent bond and assume the “free” state
• The term free reveals that their motion is quite sensitive to applied electric fields such as established by voltage sources or any difference in potential.
• natural causes


  These natural causes include effects such as light energy in the form of photons and thermal energy from the surrounding medium
• Intrinsic materials are those semiconductors that have been carefully refined to reduce the impurities to a very low level—essentially as pure as can be made available through modern technology.
• • An intrinsic semiconductor is a pure semiconductor material (like silicon or germanium) with no intentional impurities added (undoped).
• Conductivity of Intrinsic Semiconductors:

  The conductivity of an intrinsic semiconductor is relatively low because the density of free carriers (electrons and holes) generated by thermal energy is limited
• germanium is a better conductor at room temperature.
• in intrinsic state germanium and silicon are poor conductor
• an increase in temperature of a semiconductor can result in a substantial increase in the number of free electrons in the material.
• as the temperature increases


  As the temperature rises from absolute zero (0 K), an increasing number of valence electrons absorb sufficient thermal energy to break the covalent bond
• as the temperature increases, the conductivity index will increase, and result in low resistance level
• semiconductor materials such as Ge and Si that show a reduction in resistance with increase in temperature are said to have a negative temperature coefficient.
• An increase in temperature therefore results in an increased resistance level and a positive temperature coefficient.
• EGSemiconductor

  Eg = 1.1 eV (Si)

  Eg = 0.67 eV (Ge)

  Eg = 1.41 eV (GaAs)
• insulators


  electron volt is equal to 6 or more than 6
• semiconductor


  material in which the forbidden band gap is nearly equal to 1 electron volt
• conductor


  they overlap
• energy gap (forbidden gap)

  outermost orbit split into two.

  1. conduction band
  2. valence band
• You will note that the energy associated with each electron is measured in electron volts