lecture4 - Introduction to Electronics ECED3201-...

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Page 1 ECED3201- Introduction to Electronics Prof. Kamal El-Sankary, Ph.D Kamal.El-Sankary@Dal.ca Introduction to Electronics
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Page 2 Part I: Diodes (cnt’d) ECED3201- Introduction to Electronics
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VII. Physical Operation of Diodes VIII. Special Diode Types Page 3 Lecture Outline ECED3201- Introduction to Electronics
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VII. Physical Operation of Diodes VIII. Special Diode Types Page 4 Lecture Outline ECED3201- Introduction to Electronics
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Page 5 VII.1 Energy Band Diagram The gap between the conduction and valence band determines the conductive properties of the material Metal: negligible band gap or overlap Insulator: large band gap, ~ 8 eV Semiconductor: medium sized gap, ~ 1 eV Electrons can gain energy from lattice to become “free”
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Page 6 VII.2 Model for Good Conductor The atoms are all ionized and a “sea” of electrons can wander about crystal Since they are “free”, they respond to applied fields and give rise to conductions
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Page 7 VII.3 Silicon Intrinsic Silicon: o Although either silicon or germanium can be used to manufacture semiconductor device integrated-circuit technology is based almost entirely on silicon. o Si is a group-IV elements it has four valence electrons. Therefore, it can form covalent bonds with four of its neighbors. o At low temperatures, all covalent bonds are intact and no free electrons are available to conduct electric current. o When temperature goes up, electrons in the covalent bond can become free o When a covalent bond is broken, an electron leaves its parent atom; and a hole is left with the parent atom.
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Page 8 VII.4 Electron-Hole Pair Interaction o With free electrons breaking off covalent bonds, holes are generated. o Holes can be filled by absorbing other free electrons, so effectively there is a flow of positive charge carriers. Holes current happens at the valence band level Electrons current happens at the conduction band level
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Page 9 VII.5 Free Electron Density as a Function of Temperature o In intrinsic semiconductor, the electron density, n, is equal to the hole density, p. o At an absolute temperature T (in kelvins), the intrinsic concentration n i , (i.e., the number of free electrons and holes per unit volume) can be found from where B is a material-dependent parameter= 5.4 x 10 31 for silicon, E G is a parameter known, as the bandgap energy= 1.12 electron volts (eV) for silicon, k is Boltzmann’s constant = 8.62 x 10 -5 eV/K o at room temperature( T 300 K), n i 1.5 x 10 10 carriers/cm 3 . The silicon crystal has about 5 x 10 22 atoms/cm 3 . Thus, at room temperature, only one of every billion atoms is ionized. o The reason that silicon is called a semiconductor is that its conductivity, which is determined by the number of charge carriers available to conduct electric current, is between that of conductors (e.g., metals) and that of insulators (e.g, glass).
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Page 10 VII.6 Doped Semiconductors The intrinsic silicon crystal described above has equal concentrations, denoted n i , of free
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lecture4 - Introduction to Electronics ECED3201-...

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