Physics

• Vacuum Tubes


  • Has two electrodes; cathode (plate) and anode
  • Electrons are supplied by heated cathode
  • Vacuum is required in the inter-electrode space so that the moving electrons don't lose their energy upon collision with the air molecules
  • Electrons only flow in one direction i.e.; from cathode to anode. Therefore, referred to as valves
  • Bulky, consume high power, operate generally at high voltages (~100 V) and have limited life and low reliability
• Vacuum Tube Types


  • 2 electrodes
  • Triode-3 electrodes (cathode plate and grid)
  • Tetrode-4 electrodes
  • Pentode-5 electrodes
• Solid-state Semiconductors


  • Supply and flow of charge carriers in the semiconductor devices are within the solid itself
  • Are small in size, consume low power, operate at low voltages and have long life and high reliability
• Classification of Solids


  • Metals
  • Semiconductors
  • Insulators
• Metals


  Possess very low resistivity (or high conductivity)
• Semiconductors


  Have resistivity or conductivity intermediate to metals and insulators
• Insulators


  Have high resistivity (or low conductivity)
• Types of Semiconductors


  • Elemental semiconductors- Si and Ge
  • Inorganic compound semiconductors-CdS, GaAs, CdSe, InP, etc.
  • Organic semiconductors-anthracene, doped phthalocyanines, etc.
  • Organic polymer semiconductors- polypyrrole, polyaniline, polythiophene, etc.
• According to the Bohr atomic model, in an isolated atom the energy of any of its electrons is decided by the orbit in which it revolves
• When the atoms come together to form a solid, the outer orbits of electrons from neighboring atoms would come very close or could even overlap
• Inside the crystal each electron has a unique position and no two electrons see exactly the same pattern of surrounding charges. Because of this, each electron will have a different energy level
• Energy bands


  These different energy levels of electrons (when present in bulk/solid) with continuous energy variation form energy bands
• Valence band


  The energy band which includes the energy levels of the valence electrons
• Conduction band


  The energy band above the valence band
• With no external energy, all the valence electrons will reside in the valence band
• If the lowest level in the conduction band happens to be lower than the highest level of the valence band, the electrons from the valence band can easily move into the conduction band
• If there is some gap between the conduction band and the valence band, electrons in the valence band all remain bound and no free electrons are available in the conduction band
• For Si, the outermost orbit is the third orbit (n = 3), while for Ge it is the fourth orbit (n = 4). The number of electrons in the outermost orbit is 4 (2s and 2p electrons). Hence, the total number of outer electrons in the crystal is 4N. The maximum possible number of electrons in the outer orbit is 8 (2s + 6p electrons). So, for the 4N valence electrons there are 8N available energy states
• At the distance between the atoms in the crystal lattices of Si and Ge, the energy band of these 8N states is split apart into two which are separated by an energy gap Eg
• Valence band


  The lower band which is completely occupied by the 4N valence electrons at temperature of absolute zero
• Conduction band


  The other band consisting of 4N energy states, called the conduction band, is completely empty at absolute zero
• Energy band gap (Eg)


  The gap between the top of the valence band and bottom of the conduction band
• Conductor


  • When the conduction band is partially filled and the balanced band is partially empty or when the conduction and valance bands overlap
• Insulator


  • When a large band gap Eg exists (Eg > 3 eV). There are no electrons in the conduction band, and therefore no electrical conduction is possible
• Semiconductor


  • When a finite but small band gap (Eg <3 eV) exists, at room temperature some electrons from valence band can acquire enough energy to cross the energy gap and enter the conduction band
• Si and Ge have four valence electrons. In its crystalline structure, every Si or Ge atom tends to share one of its four valence electrons with each of its four nearest neighbor atoms, and also to take share of one electron from each such neighbor. These shared electron pairs are referred to as forming a covalent bond/valence bond
• At low temperatures, all bonds are intact and no bond is broken. As the temperature increases, more thermal energy becomes available to these electrons and some of these electrons may break-away (becoming free electrons contributing to conduction)
• Hole


  The vacancy with the effective positive electronic charge left behind when an electron breaks away
• In intrinsic semiconductors, the number of free electrons (ne) is equal to the number of holes (nn)
• In semiconductors, apart from the electrons, the holes also move. Holes move in the direction opposite from that of motion of electrons. But, the motion of hole is independent of the motion of free electrons and is a way of describing the motion of bound electrons
• The total current is the sum of conduction current and hole current
• Apart from the process of generation of conduction electrons and holes, a simultaneous process of recombination occurs in which the electrons recombine with the holes. At equilibrium, the rate of generation is equal to the rate of recombination of charge carriers
• An intrinsic semiconductor will behave like an insulator at T = 0 K. It is the thermal energy at higher temperatures (T > OK), which excites some electrons from the valence band to the conduction band
• Extrinsic Semiconductor


  When a few parts per million (ppm) of a suitable impurity is added to the pure semiconductor, the conductivity of the semiconductor is increased manifold
• Doping


  The deliberate addition of a desirable impurity
• Dopant Types


  • Pentavalent (valency 5)- Arsenic (As), Antimony (Sb), Phosphorous(P)
  • Trivalent (valency 3)- Indium (In), Boron (B), Aluminum (AI)
• 1. type Semiconductor


  When an atom of +5 valency element occupies the position of an atom in the crystal lattice of Si, four of its electrons bond with the four silicon neighbors while the fifth remains very weakly bound to its parent atom. This fifth electron is available for conduction
• 1. type Semiconductor


  When Si or Ge is doped with a trivalent impurity like Al, B, In, etc. the dopant has one valence electron less than Si or Ge and, therefore, this atom can form covalent bonds with neighboring three Si atoms but does not have any electron to offer to the fourth Si atom, creating a hole available for conduction
• 1. type semiconductors


  Semiconductors where the majority charge carriers are electrons
• For n-type semiconductors, ne >> nn