Semiconductor Devices

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Semiconductor Devices

• Physics 355

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Semiconductor Devices

• The control of semiconductor electrical and
  optical properties make these materials useful
  for electronic and photonic devices.
• The properties include, for example, electrical
  resistivity and optical absorption, which are
  related to one another by the semiconductor
  electronic structure.

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Optical Absorption
Light absorption, with photon energy can be
depicted as a valence band electron making a
transition to a conduction band state. Light with
photon energy is not absorbed and
passes through the material.
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p-n junctions
p type
n type
junction
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p-n junctions
Free electrons on the n-side and free holes on
the p-side can initially wander across the
junction. When a free electron meets a free hole
it can 'drop into it'. So far as charge movements
are concerned this means the hole and electron
cancel each other and vanish.
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p-n junctions
As a result, the free electrons near the junction
tend to eat each other, producing a region
depleted of any moving charges. This creates what
is called the depletion zone.
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p-n junctions
Now, any free charge which wanders into the
depletion zone finds itself in a region with no
other free charges. Locally it sees a lot of
positive charges (the donor atoms) on the n-type
side and a lot of negative charges (the acceptor
atoms) on the p-type side. These exert a force on
the free charge, driving it back to its 'own
side' of the junction away from the depletion
zone.
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p-n junctions

Usually, we represent this barrier by 'bending'
the conduction and valence bands as they cross
the depletion zone. Now we can imagine the
electrons having to 'get uphill' to move from the
n-type side to the p-type side. For simplicity we
tend to not bother with drawing the actual donor
and acceptor atoms which are causing this effect!
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p-n junctions
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p-n junctions
On the basis of the explanation given above we
might expect no current to flow when the diode is
reverse biased. In reality, the energies of the
electrons holes in the diode aren't all the
same. A small number will have enough energy to
overcome the barrier. As a result, there will be
a tiny current through the diode when we apply
reverse bias. However, this current is usually so
small we can forget about it.
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p-n junctions
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Light-Sensitive Diodes

• If light of the proper wavelength is incident on
  the depletion region of a diode while a reverse
  voltage is applied, the absorbed photons can
  produce additional electron-hole pairs. This is
  photoconduction and many photocells are based on
  this property.

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Light Emitting Diodes (LEDs)
As electrons fall into holes, photons are emitted
with energies corresponding to the band
gap. LEDs emit light in proportion to the
forward current through the diode. LEDs and
photodiodes are often used in optical
communication for both receiver and transmitter.
While all diodes release light, most don't do it
very effectively. In an ordinary diode, the
semiconductor material itself ends up absorbing a
lot of the light energy. LEDs are specially
constructed to release a large number of photons
outward. They are housed in a plastic bulb that
concentrates the light in a particular direction.
As you can see in the diagram, most of the light
from the diode bounces off the sides of the bulb,
traveling on through the rounded end.
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Light Emitting Diodes (LEDs)
Conventional LEDs are made from a variety of
inorganic semiconductor materials, producing the
following colors aluminium gallium
arsenide (AlGaAs) - red and infrared
aluminium gallium phosphide (AlGaP) - green
aluminium gallium indium phosphide (AlGaInP) -
high-brightness orange-red, orange, yellow, and
green gallium arsenide phosphide (GaAsP) -
red, orange-red, orange, and yellow gallium
phosphide (GaP) - red, yellow and green
gallium nitride (GaN) - green, pure green (or
emerald green), and blue also white (if it has an
AlGaN Quantum Barrier) indium gallium
nitride (InGaN) - near ultraviolet, bluish-green
and blue silicon carbide (SiC) as substrate
blue silicon (Si) as substrate blue
(under development) sapphire (Al2O3) as
substrate blue zinc selenide (ZnSe) -
blue diamond (C) - ultraviolet
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Light Emitting Diodes (LEDs)
In September 2003 a new type of blue LED was
demonstrated by the company Cree, Inc. to give
240 lm/W at 20 mA. This produced a commercially
packaged white light giving 65 lumens per watt at
20 mA, becoming the brightest white LED
commercially available at the time, and over four
times more efficient than standard incandescents.
In 2006 they demonstrated a prototype with a
record white LED efficacy of 131 lm/W at 20 mA.
Also Seoul Semiconductor has plans for 135 lm/W
by 2007 and 145 lm/W by 2008, which would be
approaching an order of magnitude improvement
over standard incandescents. Nichia Corp. has
developed a white light LED with efficacy of 150
lm/W at a forward current of 20 mA.
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Gunn Effect
JB (Ian) Gunn discovered the Gunn-effect in
February 1962. He observed random noise-like
oscillations when biasing n-type GaAs samples
above a certain threshold. He also found that the
resistance of the samples dropped at even higher
biasing conditions, indicating a region of
negative differential resistance.
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Gunn Effect
The semiconductor materials that exhibit the Gunn
Effect, such as GaAs, InP, GaN, must be direct
bandgap materials that have more than one valley
in the conduction band and the effective mass and
the density of states in the upper valley(s) must
be higher than in the main valley.
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Gunn Effect
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Gunn Effect

• It is important to note that the sample had to be
  biased in the NDR region to produce a
  Gunn-domain.
• Once a domain has formed, the electric field in
  the rest of the sample falls below the NDR region
  and will therefore inhibit the formation of a
  second Gunn-domain.
• As soon as the domain is absorbed by the anode
  contact region, the average electric field in the
  sample rises and domain formation can again take
  place.
• The successive formation and drift of
  Gunn-domains through the sample leads to ac
  current oscillations observed at the contacts. In
  this mode of operation, called the Gunn-mode, the
  frequency of the oscillations is dictated
  primarily by the distance the domains have to
  travel before being annihilated at the anode.
  This is roughly the length of the active region
  of the sample, L. The value of the dc bias will
  also affect the drift velocity of the domain, and
  consequently the frequency.

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Gunn Diodes
Gunn diodes are semiconductor diodes that form a
cheap and easy method of producing relatively low
power radio signals at microwave frequencies.
Gunn diodes are a form of semiconductor component
able to operate at frequencies from a few
Gigahertz up to frequencies in the THz region. As
such they are used in a wide variety of units
requiring low power RF signals.
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