Transcript Digital Switching Overview

Electronics Review B
EETS8304/TC715-N
SMU/NTU
Lecture Scheduled Jan. 27, 2004
Electronic Devices
(print slides only, no notes pages)
Page 1
© 1997-2004 R.Levine
Junctions
• When two materials are in contact
– In general, some electrons transfers from one material to the
other
– Materials with a higher atomic number have more positive
charge on the atomic nuclei in the atoms, and thus they
attract negative-charged electrons with greater force.
Electrons move into that material from the other.
• In a mixture of atoms (an alloy or an almost pure material
“doped*” with a small amount of a second material) the average
positive atomic charge is used, based on a large number of
atoms
– Materials are classified in reference books according to their
“electro-negativity” or “contact potential” or “ionization
potential” measured in volts
• This affects other situations when electrons leave or enter
a piece of material
– Electrodes in electric batteries (flashlight, automobile, etc.)
– Photoelectric emission of electrons from metals (“electric eye”)
*“Doping” is alloying using very small amounts of minor materials
Page 2
© 1997-2004, R.Levine
“Static” Electricity Example
• When you rub two dissimilar objects together and
then separate them quickly:
– hard rubbing removes any contamination on
the surface, permitting good contact
– electrons transfer to the material with higher
“average” atomic number, producing negative
net movable electric charge
– The other material is left with a deficit of
electrons and a net positive charge
• Also occurs when you:
– quickly break solid objects (e.g., a sugar cube
or mint candy) into pieces
– pull adhesive tape from a roll
Page 3
© 1997-2004, R.Levine
Safe to Try This at Home!
• Take roll of “Scotch” brand or similar sticky tape:
– Wait in a darkened room until the pupils of
your eyes accommodate to the darkness
– Rapidly pull about 50 cm (20 inches) of tape off
the roll while looking at the point where the
adhesive side separates from the layer below it
– You will see a line of electric sparks...Due to
electrons which cling to one of the separated
materials, and then jump back through the air
– Safe “experiment” to do with/for children!
• don’t bump into anything in the dark!
• don’t waste too much tape!
Page 4
© 1997-2004, R.Levine
Pencil used
as axle
Static Electric Effects
• When you brush your hair, rub your shoes on a carpet, rub
a glass rod with fur, etc. etc., you produce so-called “triboelectricity” (electricity due to rubbing)
– If you separate the two dissimilar touching objects
quickly, each object becomes oppositely electrically
charged (some extra electrons stay with one object).
– Best done in dry, low atmospheric humidity conditions
(winter months, dry climate area, etc.)
• High humidity (water vapor in air) causes surface
condensation, producing an electrically conductive
surface condition, which allows electric charge to
move to other areas and thus neutralize a local
charged area
• Anti-static sprays for clothing, etc., produce an
electrically conductive surface
• Good conductors (like metals) don’t retain charge at one
spot, but spread it over the surface of the entire object
Page 5
© 1997-2004, R.Levine
Semiconductors and
Insulators
• In a good insulator, surface charge stays put for a
very long time
• Semiconductor spot surface charge very slowly
moves (diffuses) away
– slow movement is due to thermal diffusion (random
motion due to thermal energy) of electrons
– electrons are always in some random motion, which we
perceive as motion (kinetic) energy of “heat”
– Somewhat like a “neat” pile of leaves eventually
spreading out over the whole lawn due to random
motions from changing wind directions, etc.
Page 6
© 1997-2004, R.Levine
Controlled Charge Layers
• The operation of “active” semi-conductor devices
depends on producing and controlling layers of
electric charge
• These usually occur at the interface between two
kinds of semi-conductor materials, or between a
semi-conductor and a metal conductor
• “Favorite” semiconductor is silicon (Si), which is
abundantly available (purified from beach sand
SiO2, for example) and which forms an excellent
protective layer of SiO2 on the surface of
integrated circuits, transistors, etc.
• Other semiconductors are germanium (Ge) which
is scarcer, and gallium arsenide (GaAs) 50-50%
alloy
Page 7
© 1997-2004, R.Levine
Purified Semiconductors
•
To produce a controlled result, semiconductors are first highly
purified
– Typically only one “impure” atom in 100,000,000 silicon atoms!
•
A thick silicon rod is “zone refined”
– Melted and then cooled slowly, starting from one end, to form a very pure
solid
•
This “zone refining” process is similar to freezing pure water ice out
of salty ocean water
– Icebergs near the earth’s poles consist of pure water (no salt)
– “silicon ice” (solid) which is slowly frozen from melted silicon is very pure
• The impurities are mostly trapped in the end of the rod which solidifies
last
• That end is cut off and used for other purposes where the silicon does
not need to be so pure
– Example: making “Varistors” for telephone sets (discussed in
EETS8302)
•
Purified silicon (or a Group III-V alloy – described p.9) is then “doped”
to produce a slight (1 part in 106) fraction of nuclei with either higher
or lower electric charge than the average nuclear charge
Page 8
© 1997-2004, R.Levine
Materials Used
• Most semiconductor materials are in Group 4(a) of the
Mendeleyev Periodic Table of the elements
– Doping materials are taken from Groups 3a and 5a
• Similar atomic size and electron bonding
• Fits into the crystal structure of the solid semiconductor
• In some cases a 50-50 mixture (alloy) of materials
from Groups 3a and 5a is the base material
– Called III-V (Roman numerals 3-5) materials
– Gallium Arsenide (GaAs) is used extensively because of
higher electron mobility (electron waves move further-on average --before interacting with nuclei).
Consequently transistors have better high frequency or
fast switching performance
– Doping achieved by using slightly more/less than 50%
of the Group 3 or 5 material
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© 1997-2004, R.Levine
Periodic Table (in part)
Group 3a
Boron
Aluminum
Gallium
Indium
Group 4a
Carbon
Silicon
Germanium
Tin (Sn)
Group 5a
Nitrogen
Phosphorus
Arsenic (As)
Antimony (Sb)
Yellow-highlighted names are elements used in practical
room-temperature semiconductor devices.
• Chemical abbreviation names are underlined.
• C and Sn have multiple crystal structures, only one of
which (diamond structure) is a semiconductor
• Elements in groups 3, 5 are used as dopants
• Germanium is used only rarely for special applications.
Page 10
© 1997-2004, R.Levine
Alloying or Doping
•
When Group 5 material is added, the average atomic number is
higher. This is called N (negative) type material
– The average nuclear positive charge per unit volume is greater than
“intrinsic” (pure) silicon, but there are also more electrons as well
– Of course, a piece of material as a whole is electrically neutral
– When Group 3 material is added, the average atomic number is lower.
This is called P (positive) type material
– The average nuclear positive charge per unit volume is less than
“intrinsic” (pure) silicon
• A semiconductor diode is made by joining two pieces of
silicon: P and N material respectively, and outer electrodes
– By welding two pieces in historically early transistors
– Depositing built-up layers from vapor in a vacuum chamber
– Implanting ions from the surface using an electric “ion gun” in
a vacuum chamber to produce doping in layers
Page 11
© 1997-2004, R.Levine
So-called “depletion layers”
P-type
}
PN Junction Diode
Electrically
neutral
region
N-type
Electrically
neutral
region
Electrode
Region of extra electrons,
represented by green color.
Graphic
Symbol
+
-
Region of missing or
“depleted” electrons,
represented by red color.
Graph shows net electric charge
density vs. distance right or left
of junction
P N
Anode Cathode
Page 12
© 1997-2004, R.Levine
When P and N Pieces First Touch...
• (Touching surfaces must be
microscopically clean in this example...)
• First, electrons spill over from the border
side of the N material into the P material,
because they are attracted by the greater
nuclear positive electric charge of the P
material
• This leaves a layer just inside of the left
surface of the N material (red color) which
has less electrons per unit volume than
the neutral parts of the N piece
Page 13
© 1997-2004, R.Levine
Depletion Layers
• The width of these two layers increases until they reach an
equilibrium condition in which just enough electrons are on
the left side to repel any more electrons spilling over.
• If we could mechanically break the P and N pieces suddenly
apart at this time, we would leave some negative charge
trapped on the P side, and a net positive charge trapped on
the N side. (The charge may jump back creating a spark!)
• Because this is a semiconductor instead of a good
conductor, these layers (called depletion layers, although
only one of them is actually “depleted” below the normal
number of electrons) stay in place at the two sides of the
interface. (In a metal, the extra electrons would move
quickly away from the interface and go all over the surface
of piece of metal.)
• This double layer of two opposite net electric charges (+
and -) is also called a “dipole”
Page 14
© 1997-2004, R.Levine
Current-Voltage Measurement
• The Diode is a “non-linear” electrical device. This setup
(shown schematically) measures current, i, at various
voltage v values
A
Ammeter, measures
current
+
-
V
i
Adjustable or variable voltage source,
can produce both positive and negative
voltages.
Ideal voltmeter measures diode’s voltage, but no current
flows through the voltmeter. Real voltmeters allow very
small current flow. Anode (top) of diode symbol is the
conventional positive voltage terminal.
Page 15
© 1997-2004, R.Levine
Typical Diode i-v Curve
• Several distinct regions of operation
Region
Description
A-B
Approximately linear increase in
current vs. voltage.
B-C
Accurate theoretical formula:
i = Io (e qv/kT -1)
where Io is temperature dependent,
but is typically ~10 µA at room
temperature. Also kT/q  0.2 volts
at room temperature.
Zener or avalanche breakdown
region. Approximately constant
voltage. Vz can be 3 to 600 V,
depending on design of diode.
C-D
Vz
C
© 1997-2004, R.Levine
mA=milliAmperes
of current;
1 mA=0.001 A.
A
B is approx.boundary
between exponential
and linear parts.
1
Note: a section of negative
voltage axis is not shown.
D
Page 16
i
(mA)
B
1
2
Origin of graph,
v=0, i=0
v (volts)
Forward Current Regions
• In region A-B, the voltage across the depletion layer is very
small, and we mainly see the ordinary electrical resistance of
the two neutral parts of the diode, resistance Rf.
– The depletion layer is very thin.
• In region B-C, the depletion layer gets thicker or thinner, adding
or removing electrons at their outer edges, when the voltage
changes.
• When the applied voltage is positive, the depletion layer is very
narrow, and most electrons can go across the junction (right to
left flow of electrons makes a positive current left-to-right, since
positive current flow is opposite negative electron flow*)
• The number of electrons which have enough energy to get across the
depletion layer is dependent on temperature (more about this later)
• The theoretical prediction of this formula (stated without proof), based
on electron thermal energy level, is very accurate in this region
* Blame Benjamin Franklin for using negative numbers for one kind of static electricity. If he knew then
that electric current is mainly from electrons, he would have made the opposite choice, I’m sure!
Before Franklin’s suggestion “positive” electricity was classified as vitreous (from rubbing glass)
and “negative” electricity was classified as resinous (from rubbing amber). Franklin realized that they
were two polarities of the same qualitative type, instead of two qualitatively different things.
Page 17
© 1997-2004, R.Levine
Two-segment Approximation
•
In some situations, a two segment straight line approximation can
be reasonably accurate for some mathematical analysis purposes
i
(mA)
Forward region is described
as a resistance Rf.
mA=milliAmperes
of current;
1 mA= 0.001 A
Special
case is Rf=0 ohms.
Reverse current is described
as zero. No description of
breakdown voltage region
in this example.
Page 18
© 1997-2004, R.Levine
v (volts)
1
2
Origin of graph,
v=0, i=0
Reverse Current
• Reverse Current
– The depletion layer is very wide when reverse voltage is
high. Very few electrons get into the depletion layer from
the neutral parts of the diode. Only electrons “produced”
inside the depletion layer will move through it. These
electrons are “produced” by giving more energy to
valence band electrons so that they become conduction
band electrons (a change of electron wavelength). Two
methods for giving electrons more energy:
• Higher temperature
• Shine light (infra-red, visible, ultra-violet) on the junction
– The reverse “leakage” current (from origin to point C) is
almost constant over most of the negative voltage range.
Reverse current depends on the number of electrons per
second which “appear” in the depletion layer, and not
upon the voltage. Mainly temperature dependent.
Page 19
© 1997-2004, R.Levine
Breakdown Current
• In region C-D, the diode has a sudden increase in current.
This is called the “avalanche breakdown” or “Zener” region
(named for physicist Clarence Zener)
• In this region, the high electric field in the middle of the
depletion layers accelerates electrons produced there (by
action of heat or light) so much that they can “dislodge”
other electrons from the valence band (into the conduction
band)
• When one energetic electron can “dislodge” two or more
such electrons, we start a “chain reaction” in which these
electrons can produce even more conduction electrons.
• This is like a geological avalanche, in which the first
boulders rolling down a hill dislodge other boulders and so
forth...
Page 20
© 1997-2004, R.Levine
Depletion Layer Thickness
• Depletion layer becomes narrow when positive voltage is
applied to the diode
– Then more electrons spill over from N to P part of diode.
• Depletion layer becomes thicker when negative voltage is
applied to the diode
• Thickness of depletion layer is main thing which controls
how many electrons can cross the depletion layer “barrier”
Vertical axis is
net electric charge
density
Negative voltage on
diode (green)
Zero volts on
diode (blue)
Positive voltage
on diode (red)
Page 21
© 1997-2004, R.Levine
distance right or left
of junction
Electron Energy
• The average number of electrons at each level of
internal energy in a solid is given by the Fermi
formula (stated here without proof). Non-integer
values of n(E) indicate average of various
integers.
Very low temperature (blue)
Medium temperature (green)
Very high temperature (red)
1
n(E)
1
n(E) = e ((E-Ef)/kT) +1
0
E
Grey shaded area on graph indicates
energy levels with electrons at medium
temperature. Gap surrounding Ef is due
to the band gap in a semiconductor.
Page 22
Eb, a typical “barrier” energy
Ef (the Fermi energy level)
© 1997-2004, R.Levine
There are Either 1 or 0 Electrons at a
Specific Energy Level (Pauli’s Exclusion
Principle)
• Because of electron “spin” (intrinsic
magnetism and angular momentum) there
are two wave arrangements at almost the
same energy level
• Some documents describe the maximum
number of electrons at each level as 2
• Some documents describe each level with
different spin separately, and give the
maximum number of electrons per level as
1
Page 23
© 1997-2004, R.Levine
How Many Electrons Pass Over the Barrier?
• The depletion layers in the diode act as an adjustable
energy level barrier to control electron flow across the two
parts of the depletion layer
– Positive applied battery voltage lowers the energy barrier, and
negative voltage raises the energy barrier
• The amount of current flow is related to the number of
electrons which have enough (thermal) energy to naturally
get past the barrier
– This is shown on the previous graph by the shaded area under
a curve from the barrier energy, Eb, upward
– Such a typical area is shaded under part of the medium
temperature (green) curve
– You can see that the corresponding area would be greater
under the high temperature curve, although it is not marked
• For a positive voltage, the barrier is lowered so much that
almost all the conduction electrons can pass through
– Only the ordinary “ohmic” resistance of the neutral parts limits
the current when very high positive voltage is used!
Page 24
© 1997-2004, R.Levine
•
Reverse
Current
When the energy barrier is very high (large negative voltage)
almost no electrons have enough thermal energy to pass over
the two parts of the depletion layer
• But the electric field in the junction, between the two parts of the
depletion layer, is very strong:
– electrons in the left depletion layer repel any electron at the junction,
pushing it to the right
– the right (positive) depletion region pulls any electrons at the junction to the
right
• We have all the forces to move electrons an produce a large negative
current…. except that there are almost no conduction electrons located
at the junction!
• If a conduction electron is produced or created in the middle of the
junction, it will immediately be moved by the strong electric field
• A few electrons “appear” in the junction each second because they have
enough thermal energy to change from the valence to the conduction
band just at the junction! (consider case without light on the junction)
• Thus the reverse current is dependent on the number of thermally produced
conduction electrons, and not on the reverse voltage. It changes only due to
temperature, not due to voltage changes.
Page 25
© 1997-2004, R.Levine
Avalanche (Zener) Breakdown
• Zener breakdown occurs due to high electric field in the
junction
• High reverse electric fields are produced by:
– Heavily doped P and N materials to fabricate diode
– High negative voltage
• They produce a larger charge density in the depletion
layer, even at low reverse voltage
• Diodes made specifically to “break down” at low reverse
voltages are called Zener diodes. They also are designed
and made with cooling fins, etc. to keep them from melting
under high voltage and high current (high power)
• Current is then limited only by the ohmic resistance
(usually an external resistor designed to be used with the
diode)
• Zener diodes are mostly used to produce an accurate
reference voltage for measurement devices or analogdigital converters, etc.
Page 26
© 1997-2004, R.Levine
Semiconductor Applications
•
One important use for diodes is to convert alternating current into
direct (unidirectional) current in power supply circuits.
• Diodes are also useful in some logic devices, and we can make
some types of digital logic circuits using only diodes and
resistors
• Transistors are more interesting and have more applications than
diodes.
• Two types of transistors are widely used:
– Bipolar Junction transistors1 (BJTs), which are physically like
two junction diodes back-to-back
– Field Effect Transistors (FETs), consisting of a singlelarge area
junction diode, in which we use the voltage on a control (gate)
electrode to modify the available current flow area outside the
depletion layer for transverse current flow in the other part of
the diode. This category includes metal oxide silicon (MOS)
FETs
Note 1: The name “transistor” is a contraction of the two terms transresistor. Name due to John R. Pierce, Bell Laboratories scientist.
Page 27
© 1997-2004, R.Levine
Transistor Properties
• Transistors can “amplify” electrical signals
– In the normal amplification state, transistors actually
control the flow of electric power, from a battery or other
power source, usually in proportion to the input power
from the signal
– The British name for the vacuum tube (the historical
predecessor of the transistor) was a “valve,” which is a
very good description of what a transistor does in the
amplifying state
– It controls power flow from the power supply like a water
valve controls water flow
• Transistors have 3 electrical terminals, and thus a
separate input and output “port”
– More convenient for processing analog or digital signals
Page 28
© 1997-2004, R.Levine
Junction Transistor
Collector
N
Base
P
N
Emitter
Page 29
In the usual amplifying configuration,
the base is more positive than the emitter,
and the collector is at an even more positive
voltage. The E-B junction is thus ON and the
C-B junction is OFF (reverse biased). The thick
arrow represents the magnitude of electron flow.
Most of the electrons that pass from the Emitter
to the Base are collected by the Collector.
NPN unit is shown.
PNP units also
are made, and use
opposite voltage
polarities from NPN.
The graphic symbol
for a PNP transistor
has the opposite arrow
point direction.
© 1997-2004, R.Levine
Graphic
symbol
C
B
E
Transistor Amplification
• The voltage across the Emitter-Base junction
controls the Emitter current
• A large and almost constant fraction () of the
emitter current is “collected” by the collector
– The ratio iC/iE is traditionally called  (alpha). It depends
mainly on the geometry of the transistor. Since the
neutral region in the base is very narrow, most of the
emitter-base electrons go into the base-collector
junction, where the high electric field propels them out
the collector electrode. A small fraction (1-) leaves via
the base electrode. Typical value for  is 0.99
– The ratio beta  = /(1-) is the ratio of the collector
current to base current. Typical value for  is 99. The
transistor therefore “amplifies” the base current by
approximately 100 and produces a larger current at the
collector.
Page 30
© 1997-2004, R.Levine
One Computational Model
• This simplified circuit model for a junction
transistor uses a current-controlled current
source
– The base current is viewed computationally as the thing
which controls the collector current
C
iC
iB (A current-controlled current source.)
iB
B
E
iE= iB  (1+)
• This model only describes behavior when the collector
junction is in reverse voltage state and emitter junction is in
forward voltage state, typically for amplification purposes.
Page 31
© 1997-2004, R.Levine
Field Effect Transistor
Gate
Electrode
P-gate, N-channel unit
shown.
Depletion layers.
}
Source
Electrode
Drain Electrode
Page 32
The arrow indicates
direction of electron
flow. Narrowing
of arrow suggests
current “strangling”
effect from negative
gate voltage, which
narrows the
neutral N channel.
© 1997-2004, R.Levine
The words “source”
and “drain” are based
on the concept of
positive charge flow.
Notice the “blob” in the
N-side depletion layer due
to electric field interaction
with Drain electrode.
S
Graphic
Symbol
G
D
Two FET Analysis Models
1. Variable resistor between Source and Drain
– Resistance increases when Gate voltage is more
negative
– Physically a good model
• Represents the narrowing of the N-channel
• But computationally non-linear, leads to products of
independent variables like current and voltage
2. Current source between Source and Drain,
controlled by gate voltage
– Not as accurate physically for signals with large voltage
ranges
– But computationally leads to linear equations, which are
easier to solve
Page 33
© 1997-2004, R.Levine
Model 2
• This circuit model uses a voltage-controlled
current source
– The gate voltage is viewed computationally as the thing
which controls the source-drain current
Source
gvG
+ vG
Gate
Page 34
-
iS-D
Drain
The parameter g is the socalled trans-conductance of
the FET. It is the ratio of a change in
iS-D to the causative change in vG.
Note that there is no current path
in this model between the gate and
other parts of the FET. This is due
to assuming that the reverse current
of the gate-body junction is zero. In
fact it is typically a few microamperes.
© 1997-2004, R.Levine
Metal Oxide Silicon (MOS) Transistor
Gate
Electrode
(metal)
Source
Electrode
Drain Electrode
Page 35
© 1997-2004, R.Levine
Also called insulated gate FET
(IGFET). A layer of SiO2 (equivalent to beach sand, shown in
blue on the drawing) electric
insulation here is actually much
thinner than the illustration. No
P-type layer! This still produces
a positive (red) depletion layer
in the N-type part and channel
width is controlled by the gate
voltage. No steady (dc) gate
current flows for either positive
or negative gate voltage.
Multiple
Gate
Transistor
I
Gate
Electrode 1
Gate
Electrode 2
Multiple gate electrodes are
used to implement digital
logic functions (to be discussed
Source
more in a later lecture). This
Electrode form with side-by-side gates
allows some source-drain current
to flow when either gate 1 OR
gate 2 has a positive voltage. This
implements the inclusive OR
logical function with a minimum
number of components, particularly
when implemented in an integrated
circuit.
Drain Electrode
Page 36
© 1997-2004, R.Levine
Multiple Gate Transistor - II
Gate
Electrode 2
Gate
Electrode 1
Source
Electrode
All of these configurations can
be implemented in integrated
circuits, although these pictures
show source and drain electrodes
on the edges.
Drain Electrode
Page 37
Current from source to drain
flows only when both gate 1
AND gate 2 are positive. Note
that there are two places where
negative gate voltage could
pinch off the channel. This
implements the digital logic
AND function.
© 1997-2004, R.Levine
Generic Amplifier
•
•
All these 3-electrode transistor types can be used to build an amplifier
Digitally interesting things happen in the two extreme output voltage
regions of operation, aside from use of such devices for amplifying sound
or radio signals
More details on the operation of the amplifier in the next lecture.
•
Generic “anode”:
(source or collector)
Generic control
electrode: (gate
or base)
input
voltage
source
+
-
RL , “load” resistor (or
Loudspeaker, etc.)
Vpower
output voltage point
This box is replaced
by a particular transistor in a real amplifier.
Generic “cathode”:
(drain or emitter)
Fixed voltage power
supply, shown here
as a battery symbol.
Often the power supply
device and the wire from
power source to ground
is omitted from drawings.
Generic “ground” or “earth”
graphic symbol. Actually represents
the frame or cabinet in most modern
equipment.
Page 38
© 1997-2004, R.Levine
Radiation and Semiconductor
Junctions
• Several important interactions between
absorption and radiation of light and
electromagnetic waves occur in semiconductor
junctions
• These interactions relate to:
–
–
–
–
Temperature dependence of Io (“leakage current”)
Photo-voltaic cells (“electric eyes”)
Light Emitting Diodes
Laser Diodes
• Important to systems reliability and use of diodes
in optical systems
Page 39
© 1997-2004, R.Levine
Temperature Dependence of Io
• The term Io in the formula for diode current:
i = Io (e (qv/kT) -1), is itself temperature dependent
• There is a very high electric field at the very
center of the junction, but usually almost no
conduction electrons are present
– High electric field is due to a combination of excess
electron repulsion and net positive depletion layer
attraction, which both act in the same direction on any
moveable electron which may exist at the junction
center
– A conduction electron can be “created” (electron-hole
“pair” production) in that location when a valence
electron absorbs enough energy so that it reconfigures
its wave function as a conduction electron there.
Page 40
© 1997-2004, R.Levine
Conduction Electron Production
• Energy could come from:
– Thermal kinetic energy
• Interaction with thermal vibration of nuclear cores of atoms
• More thermal energy transfer at higher temperature, leads to
greater Io reverse “leakage” current, exponentially
increasing with temperature
– Direct electron absorption of radiation
• Infra-red, visible or ultraviolet light, or x-rays, cosmic rays,
etc.
• Frequency of radiation must be high enough so E=h•f is
greater than energy gap (where h is Planck’s constant).
Radio frequency radiation is usually too low
– Due to “avalanche” chain reaction
• Secondary effect of thermally created conduction electrons
at Zener breakdown voltage
• Direct electron-electron interactions create even more
conduction electrons via “chain reaction”
Page 41
© 1997-2004, R.Levine
Diode PhotoElectric Devices
• These effects allow reverse-voltage diode
to generate current due to radiation
– Photo-voltaic direct power conversion from
sunlight
• Io proportional to incident light intensity
– opto-electric detector for fiber optic system
receiver
• Avalanche diode is sensitive to very low radiation, due
to “multiplication” of current by the avalanche effect
• Similar phenomena of electron avalanche was used in
historical vacuum tube technology. Electron-multiplier
photocells were used to detect very low levels of light
and in the early Farnsworth “image dissector” TV
camera
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Undesired Radiation Effects
• Devices which use semiconductor junctions
(transistors, etc.) for digital logic and memory
purposes are adversely affected by low-level
ionizing background radiation, cosmic rays, etc.
• Computer memory chips appeared to have
random infrequent but mysterious data errors
until this cause was identified in the 1970s
• Radiation-induced current pulses cause OFF
transistors to suddenly go ON
• Integrated circuit packaging must shield the
silicon chip from external radiation, and must not
itself contain radioactive isotopes
• High purity levels required in plastic encapsulation
as well as interior silicon!
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Light Emission During Forward
Current Flow
• When an electron crosses the junction from N to
P side, its energy changes due to difference in
interior average atomic number of the atom cores
• The electron “cloud” experiences oscillations
during the transition from higher to the lower
energy level
– The frequency f of this oscillation is given by E2-E1=h•f
– Some diodes are made with opaque enclosures so
emitted light is not noticeable. Light may also be in the
Infra-red spectrum and not perceptible to the human
eye! However, radiation is produced by forward current
in a diode.
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Light Emitting Diodes (LEDs)
• Greater difference in energy levels in the P and N
sides of the diode (due to high dopant amounts)
produce greater energy change, higher frequency
light, shorter wavelength
• Earliest light emitting diodes produced infra-red
or visible red light
– LEDs in yellow, green and recently blue visible light
colors are now available
• LEDs are used extensively as indicator lamps,
and as picture elements in color matrix displays
for lap-top computers, etc.
• LEDs are used as electro-optic converters for
multi-mode and graded index fiber optics
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© 1997-2004, R.Levine
Laser Diodes (LDs)
• Fabrication of light emitting junction surrounded by
partially reflecting surfaces which produce a
standing wave electromagnetic field, thus causing
intense emission of approximately mono-chromatic
light (LASER=light amplification by stimulated
emission of radiation)
• More efficient light output than LED
• Narrower, monochromatic, focused beam
– Couples better into small core of single mode optical fiber
than LED
– Less chromatic dispersion (pulse time spreading) in the
fiber, so higher data bit rate is permitted
– Used also for reading/writing reflective spots on CD-ROM
disks
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© 1997-2004, R.Levine