OUTLINE

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Lecture 14

• OUTLINE
• pn Junction Diodes (contd)
• Transient response turn-on
• Summary of important concepts
• Diode applications
• Varactor diodes
• Tunnel diodes
• Optoelectronic diodes
• Reading Pierret 9 Hu 4.12-4.15

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Turn-On Transient

• Consider a pn diode (Qp gtgt Qn)

i(t)
Dpn(x)
t
vA(t)
x
xn
t
For t gt 0
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• By separation of variables and integration, we
  have
• If we assume that the build-up of stored charge
  occurs quasi-statically so that
• then

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• If tp is large, then the time required to turn on
  the diode is approximately DQ/IF

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Summary of Important Concepts

• Under forward bias, minority carriers are
  injected into the quasi-neutral regions of the
  diode.
• The current flowing across the junction is
  comprised of hole and electron components.
• If the junction is asymmetrically doped (i.e. it
  is one-sided) then one of these components will
  be dominant.
• In a long-base diode, the injected minority
  carriers recombine with majority carriers within
  the quasi-neutral regions.

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• The ideal diode equation stipulates the
  relationship between JN(-xp) and JP(xn)
• For example, if holes are forced to flow across a
  forward-biased junction, then electrons must also
  be injected across the junction.

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• Under reverse bias, minority carriers are
  collected into the quasi-neutral regions of the
  diode.
• Minority carriers generated within a diffusion
  length of the depletion region diffuse into the
  depletion region and then are swept across the
  junction by the electric field.
• The negative current flowing in a reverse-biased
  diode depends on the rate at which minority
  carriers are supplied from the quasi-neutral
  regions.
• Electron-hole pair generation within the
  depletion region also contributes negative diode
  current.

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pn Junction as a Temperature Sensor
C. C. Hu, Modern Semiconductor Devices for ICs,
Figure 4-21
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Varactor Diode

• Voltage-controlled capacitance
• Used in oscillators and detectors
• (e.g. FM demodulation circuits in your radios)
• Response changes by tailoring doping profile

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Optoelectronic Diodes
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R.F. Pierret, Semiconductor Fundamentals, Figure
9.2
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Open Circuit Voltage, VOC
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C. C. Hu, Modern Semiconductor Devices for ICs,
Figure 4-25(b)
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Solar Cell StructureCyferz at en.wikipedia
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Textured Si surface for reduced reflectance

• Achieved by anisotropic wet etching (e.g. in KOH)

M. A. Green et al., IEEE Trans. Electron Devices,
Vol. 37, pp. 331-336, 1990
P. Papet et al., Solar Energy Materials and Solar
Cells, Vol. 90, p. 2319, 2006
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p-i-n Photodiodes

• W ? Wi-region, so most carriers are generated in
  the depletion region
• ? faster response time (10 GHz operation)
• Operate near avalanche to amplify signal

R.F. Pierret, Semiconductor Fundamentals, Figure
9.5
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Light Emitting Diodes (LEDs)

• LEDs are made with compound semiconductors
  (direct bandgap)

R.F. Pierret, Semiconductor Fundamentals, Figure
9.13
R.F. Pierret, Semiconductor Fundamentals, Figure
9.15
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• Question 1 (re Slide 12) Why are the contacts
  to the back (non-illuminated) side of a solar
  cell made only at certain points (rather than
  across the entire back surface)?
• Answer To increase energy conversion efficiency
• The absorption depth (average distance a photon
  travels before transferring its energy to an
  electron) for long-wavelength photons is greater
  than the Si thickness.
• ? The bottom surface oxide and metal layer
  effectively form a mirror that reflects light
  back into the silicon.
• There is more recombination in heavily doped
  contacts than at a good Si/SiO2 interface most
  of the back surface should be covered by SiO2 so
  that generated carriers have a high probability
  of diffusing to the depletion region before they
  recombine.

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• Question 2 (re Slide 15) What limits the
  lifetime of an LED?
• Answer
• LED lifetime is defined to be the duration of
  operation after which the light output falls to
  only 70 of original.
• (Even afterwards, the LED will continue to
  function.)
• The power density of an LED can be high (up to 10
  W/cm2, comparable to an electric stove top),
  causing significant heating which can degrade the
  light output through various mechanisms
• Degradation of epoxy package causing partial
  absorption of light
• Mechanical stress weakening the wire bond
  (electrical connection)
• Formation/growth of crystalline defects, or
  diffusion of metal into the semiconductor,
  resulting in increased recombination via mid-gap
  states

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