Diodes

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Diodes
Why would you want the equivalent of an
electronic check-valve?
Homework Barnaal 1 (pg.138), 2 (pg.138), for
extra credit 13 (pg.141) Midterm next Thursday,
No lab next week next lab Diodes
Read Barnaal 123-136 (Rectification, Filtering,
Regulation)
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Diode-electronic check-valve
Real diodes and the symbol are shown. The arrow
points in the direction of forward current flow.
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Diode Physics
How does a diode work? A diode can be thought
of as a variable resistance that depends on an
applied voltage. It will conduct in one
direction but not the other. Modern diodes are
constructed of semiconducting materials. Modern
physics has determined that solid materials have
optical and electronic properties that are
determined by both the arrangement of atoms (the
crystal lattice) and the properties of the
associated electronics (the band structure). In
particular there is a gap in allowed electron
energies that makes some materials optically
transparent and insulating (diamond has a gap of
5.0 eV), others transparent to infrared radiation
and semiconducting (silicon has a gap of 1.1 eV)
and others opaque to all radiation and conducting
(i.e.. all metals have a gap of zero).
A detailed description of how a diode works is
provided via java applets at http//jas.eng.buffal
o.edu/. For the purpose of this class it is
enough to know that there is space-charge or
depletion region that limits current flow and its
width is determined by the applied external
voltage.
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Doping a semiconductor
Pure semiconducting materials like Silicon have
very high resistivity at room temperature.
However it can be made conductive by adding extra
electrons. In n-type material this is done via
atoms with one more electron than silicon
(phosphorus). It can also be more conductive if
one electron is removed, allowing the resulting
holes to move about (they are really absences
of valence or bonding electrons that hop from one
site to the next). This is done in p-type
material by adding elements with one less
electron (aluminum). This is known as doping.
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P- and N-type semiconductors
An interesting thing happens if p- and n-type
material are in contact. A region that has no
holes on the p-side and no electrons on the
n-side forms, called the depletion region. This
area is like the original pure semiconductor and
does not conduct, making the junction a very poor
conductor again.
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Reverse and Forward Biased PN-junctions
If the PN junction is reverse biased, the
depletion region widens building a large electric
field barrier across the junction However if the
depletion zone is forward biased (meaning a
voltage is applied) with the p-type positive
relative to the n-type, the depletion region
shrinks and carriers pass through the junction.
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Semiconductor Diode Characteristics
http//jas.eng.buffalo.edu/
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Diode I-V relationship for a real device
The characteristics of all silicon based diodes
are effectively the same, showing a sharp
exponential increase in current at forward bias
and only a few hundred mA at reverse bias. Note
that this knee is temperature-dependent . The
non-ideal behavior shows a slower than
exponential increase due to a series resistance,
like the inductor. At large reverse-bias (not
shown) current starts to flow. We will discuss
this region later for Zener diodes.
1N4148 characteristics
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Diode models
A diode is often approximated as a 2-terminal
device with 0.7 V drop as long as 1-100 mA are
flowing in the forward direction. The curves for
a typical switching signal diode show why this is
OK (note the log scale for current). When it is
either reverse or weakly forward biased it is
modeled as an open circuit.
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Types of diodes
Power diodes (or rectifiers) - designed to handle
large currents, typically used on power supplies
or for switches. Typical values (for 1N4002) Max
Average current 1 A Peak current 30 A Reverse
voltage 100 V Signal diodes - low powered,
often faster switching (smaller depletion
region). Typical values (for 1N4148) Max Average
current 100 mA Peak current 450 mA Reverse
voltage 75 V Recovery time 4 ns Zener diodes
- voltage regulation, similar to power diodes but
has any important specification the zener
voltage, Vz (more later). Designed to breakdown
at precise voltage (range 1-400 V).
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Diode Application Clamp (or Clipper)
What does this circuit do? It clamps or
clips the output voltage. If the magnitude of
the input exceeds Vbatt0.7V, either negative or
positive, the diode starts conducting. This
limits the maximum voltage (this is similar to
surge protectors).
R
Vin
Vout
One limitation of this is that by design it will
distort a time-varying voltage input, cutting it
off at a max and a min value. More clamp circuits
in Horowitz/Hill pg. 49
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Zener diode - characteristics
All diodes show a breakdown under reverse bias
when the internal electric field gets very high,
ripping electrons from atoms. This avalanche
breakdown occurs typically at -50 to -1000 V. If
the depleted region that acts like a barrier to
current is thin there is another possibility.
The electrons can quantum mechanically tunnel
from the n- to the p-side, resulting in a reverse
current. This can be engineered to occur at a
precise voltage called the Zener voltage, VZ.
I
I
II
III
VZ
V
The graph shows three distinct regions, as
labeled. Region III is where Zener diodes
normally operate.
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Zener diode - voltage regulator (1)
How can a Zener regulator act to control the
output voltage? In the circuit shown (note that
the Zener is connected so that it is reverse
biased) current flows through the zener and it is
in region III, so the voltage drop across it is
VZ. A Zener only works as a regulator because it
ensures that enough current is drawn through the
series resistor so that the voltage drop results
in Vout VZ. If no current flows through the
Zener (actually less than a few mA) it no longer
regulates because it is in region II.
Unregulated Vin
Vout
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Zener diode - voltage regulator (2)
R
Vout ? VZ
Vout VZ
The voltage on the load is Rload/(RRload)Vin
because there is no current through the Zener.
The voltage across the load is VZ since there is
current through the Zener (Region III)
(Vin-Vout)/R gt Iout(max).
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Let there be light - Light Emitting Diodes LEDs
An ordinary forward-biased diode dissipates
energy (0.7 V)(10 mA) 7mW, mostly as heat.
However a small fraction of this is released as
radiation at wavelength beyond what we can see,
in the infrared region. There is a
characteristic of silicon (indirect gap) that
makes it less likely to emit photons. It is
possible to select materials that will produce
light instead when an electron injected from the
P-type region recombines with a hole that starts
in the N-type region. This process is called
electroluminescence. A material with a bandgap
of Eg will have a peak emission at EgkBT/2.
http//ledmuseum.home.att.net/museum.htm
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Let there be light LEDs (2)
Material Color l(nm) Vforward (V) AlInGaP
Red 645 2.1 AlInGaP
Yellow 590 2.1 GaPN Green
570 2.1 GaN/SiC Blue 430
3.8
LEDs operate with large forward currents,
meaning that they are forward biased. The
forward voltage drop is large ( 2 V for red
LEDs). An LED is characterized by maximum
values of current and reverse voltage that can be
used. Typical values are 25 mA and 5 V. The only
significant difference between an LED and a diode
laser (all the laser pointers) is a resonant
cavity.
http//www2.whidbey.net/opto/LEDFAQ/The20LED20FA
Q20Pages.html
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Photodiodes
If a diode can emit light, can it detect light?
Sure, in fact every LED is also a detector in the
correct circuit. Indeed solar cells are large
planar diodes that are optimized for absorbing
visible light. A diode made of silicon will
start converting photons into current at
approximately Eg, which is at about 1.1eV.
Converting this to wavelength lnm 1240/EeV
1100 nm. This is far into the infrared, visible
to dogs and bees, but outside of what we can see.
That is why TV remote controls cannot be seen.
http//www.newfocus.com/
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Photodiodes (2)
The N-type cathode is the part that is attached
to the larger internal connector, since it is
often the bulk of the material, as shown below.
The anode is indicated by the longer lead. This
device is most effective when the internal
electric field is large and spread over a large
depletion width. As a result it is normally
reverse biased (cathode positive).
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Photodiodes (3)
The log-log plot of diode current IL vs.
Illuminance (flux per unit area) in lux (there
are about 105 lux in bright sunlight, 103 lux in
an office) shows that we dont expect much
current, so the voltage measured across the
resistor RL is small. However we cant make RL
too large, because then the response time
increases, as seen from the second plot.