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ASTR 3130, Majewski [SPRING 2015]. Lecture Notes

ASTR 3130 (Majewski) Lecture Notes


DETECTORS: SEMICONDUCTORS

  • Virtually every astronomical detector makes use of the properties of semiconductors.
  • So-called "elemental semiconductors" from column IVa of the Periodic Table - Silicon and Germanium -- most commonly used.
    • Outermost shell contains 4 electrons, half needed to complete shell.
    • Wants to form covalent bond, sharing one electron with each of four similar atoms in large matrices or lattices.

    • Unlike ionic bonds, where electrons are donated completely, creating attracting ions, e.g., Na+ Cl-
    • In crystal form, column IVa elements form covalent bonded, diamond-like structures. Electrons very strongly held in their bonds.



  • Other elements in the periodic table can be used in semiconductors. Two types:
    • Elemental Semiconductors: From any element in column IVa of the Periodic Table: C, Si, Ge, Sn, and Pb.
    • Compound Semiconductors, usually Diatomic or Triatomic: Elements in columns Ib, IIb, IIIa, Va, VIa, and VIIa of the Periodic Table, especially Ag, Zn, Cd, Hg, Al, Ga, In, P, As, Sb, S, Se, Te, Cl, Br, and I.
    • Compound semiconductors are made from diatomic molecules symmetrically spanning column IVa in the Periodic Table, and acting similarly to collections of group IVa atoms.

      Common examples:

      • AgBr, AgCl - Photographic plates
      • GaAs - Optical phototubes
      • InSb, HgCdTe2 - Infrared detectors

DETECTORS: ELECTRON TRANSITIONS

  • Just as electrons in individual atoms can be in the ground state or various excited states, by which transfer between states (energy levels) takes place by emission or absorption of photons of specific energies / frequencies given by E = hf, .....
    • EXAMPLE: Distinct, allowed transitions for the hydrogen atom.

      Note that the:

      • Balmer series transitions correspond to photons with wavelengths in the optical region of the spectrum and involving the n = 2 level as the lower level.

      • Lyman series transitions correspond to photons in the ultraviolet region of the spectrum (n = 1).

      • Paschen (n = 3), Brackett (n = 4), and Pfund (n = 5) series transitions correspond to photons in the infrared.


      Hydrogen Series Lower Energy Level of Electron Transition Wavelength of Photon Emitted
      Lyman1Ultraviolet
      Balmer2Optical
      Paschen3Near-Infrared
      Brackett4Near/Mid-Infrared
      Pfund 5Mid-Infrared
      Humphreys 6Mid-Infrared
      Spectral series of the hydrogen atom shown on a logrithmic scale. The first transition (meaning from between the lowest level of the series and one level up) in each series (called &alpha) is indicated. The thick colored bands correspond to the most extreme transition in each series, from n = ∞ to the lowest level of the series. Note the overlap of the series after the Balmer series. From Wikipedia.

  • ... so too can electrons in a crystal lattice of multiple atoms exchange between ground states in their covalent bonds (energies in valence band) to excited states (energies in a so-called conduction band) via photon emission / absorption.

  • However, in a crystal lattice, the allowed states occupy bands of very closely packed energy levels:



    • valence band = "ground states" that are normally completely filled

    • conduction band = "excited states" that are normally completely unfilled

    • Just as in the hydrogen atom, where the allowed energy levels are quantized and distinct, the energy levels between the bands in a solid (more specifically semiconductors or insulator type solids) are forbidden.

    • Thus, the minimum distance between the allowed states is represented by an energy bandgap, which is the energy between the top of the valence band and the bottom of the conduction band.

    • An electron must absorb (e.g., in the form of photons) at least the bandgap energy in order for an electron in the ground state to become excited to the conduction band.


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Si atoms image from http://hyperphysics.phy-astr.gsu.edu/hbase/solids/sili.html. Periodic table image from www.mse.cornell.edu/courses/engri111/semicon.htm. H atom transitions image from http://www.chem.vt.edu/RVGS/ACT/graphics/h-atom.gif. Bands image from mrsec.wisc.edu/edetc/ LED/band.htm. Diamond structure image from http://www.ornl.gov/ORNLReview/v30n3-4/edge.htm. Other material copyright © 2002, 2006, 2008, 2012, 2015 Steven R. Majewski. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 313 and Astronomy 3130 at the University of Virginia.