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Semiconductor fundamentals

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Semiconductor fundamentals

  1. 1. Semiconductor Fundamentals Dr. M. Yousuf Soomro Semiconductors Fundamentals
  2. 2. Course Outline To understand the semiconductor materials that are suitable for electronic devices To study the properties of materials for electronic devices Semiconductor devices are fabricated using specific materials has the desired physical properties
  3. 3. Solid State Materials Metals (conductor) Insulators Semiconductors Superconductors
  4. 4. Metals Materials with zero bandgap are metals A metal has a partially filled conduction band, so there is no energy gap between filled and unfilled regions. A significant number of electrons can be excited by heat into empty energy levels and move easily throughout the material, allowing the material to conduct electricity
  5. 5. Insulators Materials with an energy gap larger than 3 eV An insulator possesses a considerable energy gap between the valence band and the conduction band It is difficult to excite electrons from the valence band to the conduction band. As a result an insulator does not conduct electricity
  6. 6. Semiconductor Special class of materials having conductivity b/w that of a good conductor and that of an insulator A material with electrical resistivity lying in the range of 10-2 – 109 Ω.cm Material whose energy gap for electronic excitations lies between zero and about 4 electron volts (eV). a small number of electrons from the valence band can be promoted to the conduction band by an energy input (e.g. thermal energy from heat)
  7. 7. Semiconductor Classification of Semiconductor materials oElemental semiconductor oCompound semiconductor oNarrow band-gap semiconductor oWide band-gap semiconductor oOxide semiconductor oMagnetic semiconductor oOrganic semiconductor oLow dimension semiconductor
  8. 8. Semiconductor Elemental semiconductor Silicon (Si) Germanium (Ge) o These are important group IV elemental semiconductors o All of them have diamond crystal structure Boron (B) o It belongs to group III o It has rhombohedral crystal structure Phosphorus (P) o It belongs to group V Sulphur (S) Selenium (Se) Tellurium (Te) o These belong to group VI
  9. 9. Semiconductor  Elemental semiconductor Currently silicon is the most important semiconductor material used in electronic devices Advantages of Si over other semiconductors are: A relative ease of passivating the surface by oxidizing in a controlled manner forming a layer of stable native oxide that substantially reduces the surface recombination velocity Its hardness that large wafers to be handled safely without damaging it It is thermally stable up to 11000C that allows high- temperature processes like diffusion, oxidation, and annealing It is relatively low cost due to established processes
  10. 10. Semiconductor  Elemental semiconductor  limitations of silicon Its energy band-gap is 1.12eV It is a direct semiconductor that limits the application in optoelectronics It has relatively low carrier mobility as compared to other semiconductor such as gallium arsenide GaAs
  11. 11. Semiconductor Compound Semiconductors They are usually formed from o III-V group o II-VI o IV-VI III-V group semiconductors are GaAs, GaP, GaN, A1As, InSb, InAs, InP etc In general, these crystallized materials
  12. 12. Semiconductor Compound Semiconductors GaAs, InAs, InP, InSb have direct energy band-gaps and high carrier mobilities Common applications of these semiconductors:  used to design a variety of optoelectronic devices for 1. detection and generation of electromagnetic radiation 2. in high-speed electronic devices The energy band-gaps of these compounds are useful for optoelectronic applications The energy bandgap ranges from 0.17eV for InSb to 3.44eV for GaN
  13. 13. Semiconductor Compound Semiconductors II-VI compound semiconductor II-VI compound semiconductor such as Zn and compounds with oxygen O, S, Se These cover a wide range of electronic and optical properties due to the wide variations in their energy bandgap These are typically n-type as grown, except ZnTe, which is p- type All the II-VI compound semiconductors have direct energy bandgaps
  14. 14. Semiconductor Compound Semiconductors IV-VI compound semiconductor PbS, PbSe, and PbTe characterized by narrow energy gaps, high carrier mobilities, and high dielectric constants The unique feature of the direct energy gap in these compounds is that its energy band-gap increases with increasing temperature, which means the energy gap has a positive temperature coefficient Main applications of these compounds are in light emitting devices and detectors in the infrared spectral region
  15. 15. Semiconductor Narrow Band-gap Semiconductor  InSb  InAs  PbSe  PbTe  PbS They have the energy band-gap below about 0.5eV they are direct semiconductor materials extensively employed in: infrared optoelectronic device applications as detectors and diode lasers
  16. 16. Semiconductor Wide Band-gap Semiconductor  SiC  II-V nitrides high thermal conductivity high saturation electron drift velocity high breakdown electric field superior chemical stability physical stability It has wide band-gap that enables detection and emission of light in short-wavelength region likes blue and ultraviolet
  17. 17. Semiconductor Oxide Semiconductor  Cu2O  Bi2O  ZnO These are also referred as semiconductor ceramics They are used in electronic devices and sensors
  18. 18. Semiconductor Magnetic Semiconductor Semiconductor compound that contains magnetic ions such as Cr, Mn, Fe, Co, Ni, may exhibit magnetic properties Some oxides such as FeO and NiO exhibit antiferromagnetic properties and oxide such as europium oxide EuO is ferromagnetic properties The semiconductor exhibits large magneto-optical effect that can be used to design optical modulators
  19. 19. Semiconductor Organic Semiconductor  Anthracene CH14H10  Polyacetylene (CH)n  advantages of organic semiconductors 1. Diversity 2. relative ease of changing their properties to specific application
  20. 20. Semiconductor Organic Semiconductor One of the promising applications of organi semiconductors is in less iexpensive light emitting diode, covering whole the spectrum of colors The main advantages of organic materials in such applications include I. low operating voltages II. color tunability III. relative simplicity of device fabrication