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Module 1 Semiconductors (1).pptx

  1. SEMICONDUCTOR FUNDAMENTALS Free electron model that works well for metals do not apply for crystals. Free electrical model gives us good insight into heat capacity, thermal conductivity, electron conductivity & electrodynamics of metal; but it fails to answer other large question: distinction between metals, semimetals, semiconductors & insulators; occurrence of positive values of Hall coefficient & many detailed transport properties. The reason behind it is the periodicity of the potential characterizing the crystal, whose value at any point is the result of contributions from each atom. When atoms form crystals, presence of neighbouring atoms does not affect the energy levels of inner shell electrons. However, the levels of outer shell electrons are changed considerably, as the outer shell electrons are shared by more than one atom in the crystal.
  2. Quantum mechanics helps to find out the new energy levels of the outer electrons. Coupling exists between the outer shell electrons of the atoms and is responsible for the appearance of the band of closely spaced energy states, rather than widely separated energy level as that of the isolated atoms. Regions in energy for which no wave like electron orbital exists are called energy gaps or band gaps & results from the interaction of the conduction electron wave with the ion cores of the crystal.
  3. IVA,Group Elements Atomic Number Electronic configuration C Si Ge 6 14 32 1s2 2s2 2p2 1s2 2s2 2p6 3s2 3p2 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p2
  4. Each of the elements given above contains 2s electrons and 2p electron in the outer two sub shells. If we ignore the inner shell levels, there are 2N electrons completely filling the 2N possible s levels all at same energy. Well known that p atomic sub shell has 6 possible states, our crystal having widely spaced atoms (2N electrons), will fill only one third of the 6N possible p states, all at the same level. Now if we decrease the interatomic spacing of the crystal, each atom will experience an electric force from it’s neighbouring atoms. This coupling between atoms gives rise to overlapping of atomic wave functions and crystal becomes an electronic system which will obey Pauli exclusion principle. If interatomic distance is decreased sufficiently, the total spread between the minimum and maximum energy may be several electron volts, since N is very large (⋍1023cm-3), although separation between levels is small.
  5. These large numbers of discrete and closely spaced energy levels are called an energy band.
  6. Now each atom has given up 4 electrons to the band at this particular spacing; which no longer belongs to orbit in s or p sub shell of an isolated atom, but rather they belong to the crystal as a whole. The band occupied by these electrons is called valence band. Valence band filed with 4N electrons is separated by a forbidden band (EG) from an empty band consisting of 4N additional states. This upper vacant band is called conduction band. METALS, INSULATORS & SEMICONDUCTORS A very poor conductor of electricity is called insulator, whereas good conductors of electricity is called metal. Semiconductors are those substance whose conductivity lies between these two extremes.
  7. Energy band diagram give explanation for above three categories:- Metal Insulator Semiconductor
  8. Substance for which forbidden energy region is small enough i.e E𝐺 ⋍ 1 𝑒 𝑉, the externally applied field may carry the particle from valence band to conduction band. These free or conduction electrons participate in conductivity & substance now become slightly conducting; it is a semiconductor. The most widely used semiconductor materials are germanium & silicon having band gaps 0.785 and 1.21 eV respectively. Si atom has 14 protons and 14 electrons. 4valence electrons in outer most orbit indicates Si to be an semiconductor. These Si atoms combine to form a crystal. Each Si atom shares its 4 valence electrons with the 4 neighbouring atoms in order to have 8 valence electrons in its valence orbit.
  9. CLASSIFICATION OF SEMICONDUCTORS: Conductivity of semiconductors lies in between 105 to 10-4 Siemens per meter. Since semiconductors have negative coefficient of resistance; their resistance decreases with increases in temperature. Other important property is that conductivity changes considerably with addition of even small amount of substances called impurities are added to them. Semiconductors are classified in following two ways – (a) Intrinsic semiconductor (b) Extrinsic semiconductors.
  11. THE HOLE:
  12. However, at room temperature some of the covalent bonds are broken due to the thermal energy of the crystal causing atoms to vibrate. As a result, conduction is possible due to the availability of charge carriers. The energy EG required to dislodge a valence electron of a covalent bond & to make it free to participate in conduction is about 0.72eV for germanium & 1.1eV for Silicon at room temperature. The absence of electron in covalent bond is represented by a small circle & known as hole. Hole serves as a positive charge carrier of electricity as effective as a free electron (negative charge).
  13. FLOW OF HOLES: Free electrons & holes move in opposite direction i.e. free electrons move towards left along path D,C,B,A. and hole towards right along A,B,C,D,E,F,G acting the same as positive charge.
  14. EXTRINSIC SEMICONDUCTOR: Intrinsic semiconductor have small conductivity at room temperature. In order to increase the conductivity of intrinsic semiconductor a trivalent or pentavalent impurity is added to the semiconductor. This process of adding trivalent or pentavalent impurity to the pure semiconductor in order to increase the conductivity is called doping. Doping is done at the rate such that only one atom of impurity is added per 106 to 1010 semiconductor atoms. Charge carriers i.e. free electrons or holes can be increased, depending on the type of impurity (pentavalent or trivalent) added.
  15. DONOR IMPURITIES: (PENTA VALENT IMPURITIES): 0.05 eV for Si & 0.01 eV for Ge
  17. Questions: Q1. Explain with energy band diagram the difference between insulator, semiconductor and metals.. Q2. Discuss and explain with diagram the difference between intrinsic and extrinsic semiconductor. Q3. Draw and explain the energy band diagram for n type and p type semiconductors.