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PowerPoint Presentation - Nitride-based Semiconductors and their Applications

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Day 3-4: Research on the ... Nitride semiconductors and their applications Part I: ... Laser Diode (LD) Is basically a pn-junction Same principle as LEDs, ... – PowerPoint PPT presentation

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Title: PowerPoint Presentation - Nitride-based Semiconductors and their Applications


1
Outline of lectures Day 1-2 Research on the
physics of nitride semiconductors Fundamentals
of semiconductor physics Research on
nitrides Day 3-4 Research on the teaching and
learning of physics Research in cognitive
science Research in physics education
2
Nitride semiconductors and their
applicationsPart I Basic Semiconductor Physics
  • One should not work on semiconductors, that is a
    filthy mess who knows whether they really
    exist.
  • Attributed to Wolfgang Pauli (1931)

3
What are semiconductors?
  • Metals, semimetals, semiconductors, insulators
  • Characteristics
  • Conductivity increases dramatically with
    temperature (conductivity at T 0 K is zero)
  • Conductivity changes dramatically with addition
    of small amounts of impurities
  • Applications
  • Anything in which you want to control the flow of
    current (transistors, amplifiers,
    microprocessors, etc.)
  • Devices for producing light
  • Radiation detectors

4
History of semiconductors
  • 1833 Michael Faraday discovers temperature-depende
    nt conductivity of silver sulfide
  • 1873 Willoughby Smith discovers photoconductivity
    of selenium
  • 1874 Ferdinand Braun discovers that point
    contacts on some metal sulfides are rectifying
  • 1947 John Bardeen, Walter Brattain, and William
    Shockley invent the transistor

5
Semiconductor materials
6
Semiconductor materials
  • Examples
  • IV C, Si, Ge
  • III-V GaAs, GaN, InP, AlSb, GaAlAs, GaInN
  • II-VI ZnSe, CdTe

7
Physical Structure
Basic lattice Face-centered cubic (fcc)
Diamond structure Si, Ge
Zincblende GaAs, InP, ZnS,...
Zincblende ABCABC Wurtzite ABABAB
  • About 1022 atoms in each cm3.

8
Electronic Structure
  • Bands analogous to electronic energy levels of
    single atoms
  • Band gap between 0 and 5 eV (1 eV 3.83 x 10-23
    Cal)
  • Electrons in valence band are involved in atomic
    bonding
  • Electrons in conduction band are free to wander
    the crystal
  • Temperature dependence of resistance is due to
    thermal excitation of electrons across bandgap

9
Band structure of Si
Chelikowski and Cohen, Phys. Rev. B 14, 556 (1976)
10
Growth (bulk)
  • Czochralski growth (1918)
  • Crystals grown near melting point of material (gt
    1410 C for silicon)
  • Boules up to 12 diameter and 6 feet long
  • Growth rate few mm/min
  • Used for Si, Ge, GaAs, InP

From http//kottan-labs.bgsu.edu/teaching/workshop
2001/chapter5.htm
11
Growth (layers)
  • MOCVD (Metal-Organic Chemical Vapor Deposition)
  • Also known as MOVPE, etc.
  • Growth temperatures near melting point
  • Growth rate 1 µm/min.

From http//kottan-labs.bgsu.edu/teaching/workshop
2001/chapter5.htm
12
Fun facts about AsH3
  • OSHA Permissible Exposure Limit 0.05 ppm
    (averaged over 8 hour work shift)
  • Detection Garlic-like or fishy odor at 0.5 ppm
  • IDLH (Immediately Dangerous to Life or Health) at
    6 ppm. (IDLH for other toxic gases such as
    Chlorine or Phosphine are gt1000 ppm.)

13
Growth (layers)
  • MBE (Molecular-Beam Epitaxy)
  • Low growth temperature
  • Growth rate few µm/hr.
  • Can grow atomically flat surfaces and monolayers

From http//kottan-labs.bgsu.edu/teaching/workshop
2001/chapter5.htm
14
Doping
  • Adding impurities to alter the electrical
    properties
  • n-type (donors) or p-type (acceptors)
  • Deep or shallow
  • Single/double/triple

n-type
p-type
Si Doped with Group V
Si Doped with Group III
15
Doping
  • Shallow donors can be modeled as hydrogen atoms
    in a dielectric medium.
  • The donor electron level is only a few (6-50) meV
    below conduction band.
  • Hydrogen-like and helium-like levels are observed.

16
Doping
  • Grown in
  • Diffusion
  • Neutron transmutation(30Si(n,g)31Si --gt 31P
    b-, T1/22.6 hr.)
  • Ion implantation

17
Characterization (electrical)
  • Hall effect enables determination of
  • charge of carriers
  • density of carriers
  • binding energy of carriers (temperature dependent)

18
Characterization (optical)
  • Infrared (IR) spectroscopy allows determination
    of
  • impurity species
  • electronic and vibrational energies of impurities

Agarwal et al., Phys. Rev. 138, A882 (1965).
19
Applications
  • The pn-junction is the basis of many
    semiconductor devices.
  • Three semiconductor devices
  • Field effect transistor
  • Light-emitting diode
  • Laser diode

20
pn-junction
  • Consists of p-type material next to n-type
    material.
  • Electrons from the n-type material fill in the
    acceptors on the p-type side near the junction
    and vice versa.
  • Process stops when the layer of negatively
    charged acceptors becomes too think for the
    remaining electrons to get through.










Negatively charged acceptors
Positively charged donors
21
pn-junction
  • Current will flow if a battery is hooked up as
    shown. The positive terminal of the battery
    attracts electrons, pulling them through the
    depletion region.
  • A certain minimum voltage is required to overcome
    the repulsion of the depletion region.









22
pn-junction
  • If the battery is hooked up in the opposite
    direction, then no current flows. (The depletion
    region actually gets bigger.)
  • If too much voltage is applied in this direction,
    current flows, but your junction is unhappy.









23
Another view of the pn-junction
  • No bias
  • Reverse bias
  • (no current)
  • Forward bias
  • (current)





24
Field Effect Transistor (FET)
25
Light Emitting Diode (LED)
  • Is basically a pn-junction
  • When an electron and a hole collide, a photon
    (light) is emitted. The energy of the light is
    equal to the bandgap energy.Si bandgap 1.2
    eV (infrared)GaAs bandgap 1.5 eV (red)
  • Defects in crystal can cause electron-hole
    collisions to occur without emission of light
    (non-radiative recombination).

26
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27
Laser Diode (LD)
  • Is basically a pn-junction
  • Same principle as LEDs, however, waveguides are
    added to the structure to enable the light to
    reach lasing intensities. Some surfaces are
    polished mirror-flat to allow light to reflect
    back and forth inside the active region.
  • Much better material quality (smaller density of
    defects) is required for LDs than LEDs.

28
Other applications
  • Radiation detectorsRadiation hitting the
    material knocks an electron from the valence to
    the conduction band, creating a free carrier. An
    applied voltage sweeps the carrier out of the
    material where it is detected as current.
  • Solar cellsAgain, a pn-junction. Light creates
    an electron-hole pair which is forced out of the
    material as electric current by the electric
    field in the depletion region.
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