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Semiconductor Devices
Physics 355
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.
Optical Absorption
Light absorption, with photon energy    g
can be depicted as a valence band electron making a
transition to a conduction band state.
Light with photon energy    g is not absorbed
and passes through the material.
p-n junctions
p type
junction
n type
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.
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.
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.
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!
p-n junctions
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.
p-n junctions
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.
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.
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
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.
Gunn Effect
JB (Ian) Gunn discovered the Gunn-effect in February 1962.
He observed random noise-like oscillations when biasing ntype 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.
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.
ε
ε
ε
k
k
ε < εth
εth < ε < εsat
k
ε > εsat
Gunn Effect
Gunn Effect
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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.
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.