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Sensors Technology – MED4
ST10 – Diode
Diode
Lecturer:
Smilen Dimitrov
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ST10 – Diode
Introduction
•
The model that we introduced for ST
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ST10 – Diode
Introduction
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We have discussed
– The units of voltage, current and resistance, from both a microscopic
and macroscopic (electric circuits) perspective
– The definition of an elementary electric circuit, Ohm’s law and Kirschoff
Laws
– Solving and measurement of voltage divider circuit and more
complicated circuits - and applications in sensors
– Resistive based sensors
– AC current, capacitors, and capacitive based sensors
•
This time we discuss the diode as an electronic element, and as a basis for
different types of sensors
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ST10 – Diode
Semiconductors - intrinsic silicon
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•
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Diodes and transistors represent electronic components, which in essence
are semiconductor structures
So far, distinction between: conductor and insulator materials; and
– distinction within conductors, between ideal conductors and (ohmic)
resistive materials. (semiconductors were left out)
In terms of electrical conductance, substances are described as:
– Conductors allow electricity to flow through them (mercury, iron, copper, gold)
– Insulators prevent electricity from flowing through them (sulphur, etc. Paper,
rubber and most plastics)
– Semi-conductors are poor conductors, or poor insulators. They allow the
partial flow of electricity (Carbon, Silicon, Germanium)
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ST10 – Diode
Semiconductors - intrinsic silicon
•
•
Many materials can have free electrons on normal temperatures - however,
in not as great number as metals (resistors); in metals, almost all valence
electrons are free
Some materials have no free electrons (insulators)
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Semiconductors are a class of materials that have conductive properties that
fall somewhere between conductors and insulators
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Dominant characteristic of semiconductors elements:
(Si, Ge) - those are elements with valency 4 – that is,
they have four electrons in the valence shell.
•
This is situation in pure silicon – a semiconductor
that is chemically pure is known as
an intrinsic semiconductor
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ST10 – Diode
Intrinsic silicon - properties
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Silicon has a crystal structure – periodic
arrangement of atoms
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Silicon has a 3D tetrahedral arrangement - the 2D representation is just an
approximation
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ST10 – Diode
Intrinsic silicon - properties
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the fact that a silicon atom might have an unfilled valence bond (that is, it is
a positive ion), here is treated like a positive movable charge, known as a
hole.
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Important notion in semiconductors is that both holes and free electrons
contribute to electric current.
– Holes contribute because they can be seen as movable - as neighbouring
electrons may progressively fill the hole, and create the effect that the hole moves in direction of an electric
field (opposite to the direction of the electrons)
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ST10 – Diode
Doped semiconductors - P and N
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used as so-called doped semiconductors - certain atoms with different
valency are added to the 4-valency intrinsic semiconductor, which then changes the
conductivity of the material. The added material is known as a dopant.
•
If 3-valent dopant is used (such as boron),
then a single covalent bond from the neighbouring 4-valent
silicon remains unfilled, which is a hole. This results with a
semiconductor material which has excess of holes, and is
known as a P-type
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If a 5-valent dopant is used (such as antimony),
then a single electron from the dopant can not participate
in a bond, so it becomes a free electron. This results with a
semiconductor material which has excess of free electrons,
known as a N-type semiconductor.
Impurity atoms with 5 valence
electrons (pentavalent impurities donors)
Impurity atoms with 3 valence
electrons (trivalent impurities acceptors)
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ST10 – Diode
Doped semiconductors - P and N
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When the electron in pure silicon crosses the gap, it leaves behind an electron vacancy
or 'hole' in the regular silicon lattice. Under the influence of an external voltage, both the electron and the hole can
move across the material.
•
•
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In an n-type semiconductor, the dopant contributes extra electrons,
dramatically increasing the conductivity.
In a p-type semiconductor, the dopant produces extra vacancies or holes,
which likewise increase the conductivity.
It is however the behavior of the p-n junction which is the key to the
enormous variety of solid-state electronic devices.
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ST10 – Diode
Diode (PN junction)
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Basic structure - formed by a contact between a P and N type
semiconductor - which is known as a PN junction, or commonly a diode.
•
When p-type and n-type materials are placed in contact with each other, the
junction behaves very differently than either type of material alone.
Current will flow readily in one direction (forward biased) but not in the other
(reverse biased), creating the basic diode. This non-reversing behavior arises from the nature
•
of the charge transport process in the two types of materials.
•
Near the junction, electrons diffuse across to combine with holes, creating a
'depletion region' (zone).
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ST10 – Diode
Diode (PN junction)
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a PN junction is visualised though an energy level sketch (The upward direction in the
diagram represents increasing electron energy.). Conductivity of the junction can be
visualised too
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When reverse voltage [reverse bias or inverse polarization] is applied (that is,
the N-type is on higher positive potential than P-type), then the depletion region grows, and
there is even less possibility of free charges crossing, and current flowing.
When direct voltage [forward bias or
direct polarization] is applied (that is,
P-type is on higher potential than N-type) the depletion
region gets smaller, and free charges gain
enough energy to cross the barries so current can flow. (In this case the
resistance is very small – almost 0.)
•
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ST10 – Diode
Diode (PN junction) - visualisation
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The behavior of the PN junction is visualised on several resources on the
Internet - however, most of them use the energy band diagrams, whose
proper interpretation is beyond the scope of the course
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ST10 – Diode
Diode (PN junction) - Hydraulic analogy
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Very simplified analogy to a diode in relation to water flow - would be a one
way valve
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In electronics, a diode is a component that restricts the direction of
movement of charge carriers. It allows an electric current to flow in one
direction, but essentially blocks it in the opposite direction. Thus the diode
can be thought of as an electronic version of a check valve.
If a water valve has a rubber flap it can be blown out permanently by too
much reverse bias, which is similar to the real thing.
•
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ST10 – Diode
Diode as an electronic element
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Diode - implies a small signal device
with current typically in the milliamp range
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Rectifier - implies a power device,
conducting from1 to 1000 amps or even higher.
•
Many diodes or rectifiers are
identified as 1NXXXX
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A semiconductor diode consists of a PN junction and has two(2) terminals,
an anode (+) and a cathode (-). Current (technical direction!) flows from
anode to cathode within the diode. (diode is a polar element, since it has a polarity [+] or
[-] assigned to its terminals)
•
In order to analyse circuits with a diode, we need a model of the diode
– - a model that will specify the diode branch equation: the dependence
between voltage across, and current through the diode
– (or in other words, the I-U characteristic of the diode).
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ST10 – Diode
Diode as an electronic element
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Commonly used model (branch equation) - Shockley ideal diode equation

I  Is e


VD
VT

 1


where
–
I is the diode current,
–
IS is a scale factor called the saturation current,
–
VD is the voltage across the diode
–
VT is the thermal voltage: 26mV @ T=25°C (room
temp)
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no linear dependency between voltage and current as in ohmic elements so a diode is not an ohmic element
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the Shockley equation doesn't account for the processes involved in reverse breakdown
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in the reverse bias region for a normal diode, the current
through the device is very low (in the µA range) for all reverse
voltages upto a point called the peak-inverse-voltage (PIV).
Beyond this point a process called
reverse breakdown occurs which
causes the device to be damaged
along with a large increase in current.
(though, deliberate for Zener and
avalanche diodes)
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ST10 – Diode
Diode as an electronic element
•
Shockley equation – too complex for quick analysis – so it is simplified: A
diode's I-V characteristic can be approximated by two regions of operation.
– Below a certain difference in potential between the two leads, the
depletion layer has significant width, and the diode can be thought of as
an open (non-conductive) circuit.
– As the potential difference is increased, at some stage the diode will
become conductive and allow charges to flow, at which point it can be
thought of as a connection with zero (or at least very low) resistance.
This simplification lacks the diode
turn-on voltage - one needs to
invest certain energy (that is,
forward voltage bias) in order to
compensate and lower the
depletion zone, so that current
can start flowing.
Once current starts flowing, this turnon voltage can be approximated
to stay constant
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ST10 – Diode
Diode as an electronic element
•
•
In a normal silicon diode at rated currents, the voltage drop across a
conducting diode is approximately 0.6 to 0.7 volts.
– The value is different for other diode types - Schottky diodes can be as
low as 0.2 V and
– light-emitting diodes (LEDs) can be 1.4 V or more depending on the
current.(Blue LEDs can be up to 4.0 V, depending on the type and current.)
Including the diode turn-on voltage in the switch model, we arrive at the socalled large-signal diode model
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ST10 – Diode
Measuring (testing) diodes
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reason for measuring diodes is to find the proper
pins of a diode - and to also make sure
it is working properly
Testing a diode with a DIGITAL multimeter
– Digital multimeters have a special
setting for testing a diode, usually
labelled with the diode symbol.
– Connect the red (+) lead to the anode
and the black (-) to the cathode. The
diode should conduct and the meter will
display a value (usually the voltage
across the diode in mV, 1000mV = 1V).
– Reverse the connections. The diode
should NOT conduct this way so the
meter will display 'off the scale' (usually
blank except for a 1 on the left).
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ST10 – Diode
Diode - construction
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Diodes are produced using a variety of chemical processes.
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ST10 – Diode
Basic circuits - Half wave rectifier
•
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Half wave rectifier is the simplest diode circuit - it is a series connection of a
diode and a resistor.
A rectifier is an electrical device that converts alternating current to direct
current, a process known as rectification.
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ST10 – Diode
Basic circuits - Half wave rectifier
•
Half wave rectifier is the simplest diode circuit - it is a series connection of a
diode and a resistor.
Vi  Vd  Vo  0
->
I
Vi  Vd  R  I
Vi  Vd
R
a) When Vi < VdON (or Vi < 0.7V):
b) When Vi > VdON (or Vi > 0.7V):
–
–
- the diode is directly polarised (forward
biased) so it conducts (it is modelled with
an 0.7V voltage generator)
–
- the current I flowing in the circuit is then
calculated by .
–
- the output voltage Vo = Vi - VdON = Vi 0.7V
- the diode is inversely polarised
(reverse biased) so it does not
conduct (it is modelled with an open
switch)
–
- the current I is zero
–
- the output voltage Vo is zero
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ST10 – Diode
Basic circuits - Half wave rectifier
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One way to visualise this circuits operation, related to a water flow analogy
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Also, Falstad applets
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ST10 – Diode
Clipper
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Clipper – is a simplest implementation of a “distortion” audio effect
a) When Vi < ViON –> ID =
0 so just a voltage
divider
VD 
R2
V  V E  VdON
R1  R2 i
- but how big is ViON?
R2
V VE
R1  R2 i
Vi  VdON  VE 
R1  R2
R2
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ST10 – Diode
Clipper
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Clipper – is a simplest implementation of a “distortion” audio effect
Vi  VdON  VE 
R1  R2
R2
a) When true
b) When false
–
- the diode is directly
polarised (forward biased)
so it conducts (it is
modelled with an 0.7V
voltage generator)
–
- the diode is inversely
polarised (reverse
biased) so it does not
conduct (it is modelled
with an open switch)
–
- the voltage output is fixed
–
- the voltage output is
given by voltage
on Vo = VdON + VE
divider
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ST10 – Diode
Peak (envelope) detector
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Peak detector - very useful circuit - it can help extract the amplitude of an
AC signal.
–
It is useful in getting the envelope of high frequency sugnals as a DC signal - then it is
possible to use high frequency sensor output with low speed DAQ hardware like an Arduino.
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ST10 – Diode
Peak (envelope) detector
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Peak detector
Need Vx – the input voltage when diode starts conducting (will not be solved
here)
a) When Vi > Vx
–
- the diode is directly polarised (forward biased) so it conducts (it
is modelled with an 0.7V voltage generator)
–
- the voltage output is fixed on Vo = Vi - VD = Vi - 0.7
As soon as the input voltage has dropped a little below the maximum kept
by the capacitor, the diode becomes reverse biased, so it turns off and
ID becomes zero.
b) When Vi < Vimax
–
–
- the diode is inversely polarised (reverse biased) so it does not
conduct (it is modelled with an open switch)
Vis
- the voltage output
o  Vi max e

t
RC
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ST10 – Diode
Peak (envelope) detector
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Peak detector in Falstad applet simulator
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ST10 – Diode
Diode logic – AND and OR circuit
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Diodes and resistors can be used for implementing the most basic digital
logic circuits
OR
cir
cui
t
AND
circ
uit
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ST10 – Diode
Light emitting diodes
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Special kind of diodes are the light emitting diodes - or LED's.
– this device converts electric energy into a different physical quantity
(light), and as such it has a definition opposite of a sensor - a LED diode
is an actuator!
•
when LED's are forward biased (and producing light), their resistance is
extremely small, so we can treat it as a closed switch (wire).
– Thus, connecting a LED directly to a source in forward bias, will short
circuit the source and most likely blow up the diode !!!!!!
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ST10 – Diode
Light emitting diodes - interfacing
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A resistor must always be present in series with a LED diode, in order to
limit the current.
– Then, another important thing is to determine the correct polarity of the
LED terminals.
Problem is finding the proper resistor value for the resistor - Typically, LEDs
will need from 20 - 30 mA to shine
A sample calculation of the dropping
resistor is included …
–
Most leds operate at 1.7V although
this is not always the case and it is
wise to check.
– The dropping resistor is simply the
net of supply voltage minus the 1.7V
led voltage, then divided by the led
brightness current expressed as
'amps' (ohms law)
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ST10 – Diode
Sensing application - Photodiode and interfacing
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A photodiode consists of an active p-n junction which is operated in reverse
bias. When light falls on the junction, a reverse current flows which is
proportional to the illuminance.
– Photodiodes can be used under either zero bias (photovoltaic mode) or
reverse bias (photoconductive mode).
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Requirement for reverse current (very small) – makes interfacing more
difficult; must use some sort of amplification/buffering (opamp, transistor)
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