Diodes copy

Kristin Ackerson, Virginia Tech EEKristin Ackerson, Virginia Tech EE
Spring 2002Spring 2002

Table of ContentsTable of Contents
What are diodes made out of?____________________slide 3What are diodes made out of?____________________slide 3
N-type material_________________________________slide 4N-type material_________________________________slide 4
P-type material_________________________________slide 5P-type material_________________________________slide 5
The pn junction_________________________________slides 6-7The pn junction_________________________________slides 6-7
The biased pn junction___________________________slides 8-9The biased pn junction___________________________slides 8-9
Properties of diodes_____________________________slides 10-11Properties of diodes_____________________________slides 10-11
Diode Circuit Models ____________________________slides 12-16Diode Circuit Models ____________________________slides 12-16
The Q Point____________________________________slides 17-18The Q Point____________________________________slides 17-18
Dynamic Resistance_____________________________slides 19-20Dynamic Resistance_____________________________slides 19-20
Types of diodes and their uses ___________________ slides 21-24Types of diodes and their uses ___________________ slides 21-24
Sources_______________________________________slide 25Sources_______________________________________slide 25

What Are Diodes Made Out Of?What Are Diodes Made Out Of?
• Silicon (Si) and Germanium (Ge) are the two mostSilicon (Si) and Germanium (Ge) are the two most
common single elements that are used to make Diodes.common single elements that are used to make Diodes.
A compound that is commonly used is GalliumA compound that is commonly used is Gallium
Arsenide (GaAs), especially in the case of LEDsArsenide (GaAs), especially in the case of LEDs
because of it’s large bandgap.because of it’s large bandgap.
• Silicon and Germanium are both group 4 elements,Silicon and Germanium are both group 4 elements,
meaning they have 4 valence electrons. Theirmeaning they have 4 valence electrons. Their
structure allows them to grow in a shape called thestructure allows them to grow in a shape called the
diamond lattice.diamond lattice.
• Gallium is a group 3 element while Arsenide is a groupGallium is a group 3 element while Arsenide is a group
5 element. When put together as a compound, GaAs5 element. When put together as a compound, GaAs
creates a zincblend lattice structure.creates a zincblend lattice structure.
• In both the diamond lattice and zincblend lattice, eachIn both the diamond lattice and zincblend lattice, each
atom shares its valence electrons with its four closestatom shares its valence electrons with its four closest
neighbors. This sharing of electrons is what ultimatelyneighbors. This sharing of electrons is what ultimately
allows diodes to be build. When dopants from groupsallows diodes to be build. When dopants from groups
3 or 5 (in most cases) are added to Si, Ge or GaAs it3 or 5 (in most cases) are added to Si, Ge or GaAs it
changes the properties of the material so we are able tochanges the properties of the material so we are able to
make the P- and N-type materials that become themake the P- and N-type materials that become the
diode.diode.
SiSi
+4+4
SiSi
+4+4
SiSi
+4+4
SiSi
+4+4
SiSi
+4+4
SiSi
+4+4
SiSi
+4+4
SiSi
+4+4
SiSi
+4+4
The diagram above shows theThe diagram above shows the
2D structure of the Si crystal.2D structure of the Si crystal.
The light green linesThe light green lines
represent the electronicrepresent the electronic
bonds made when thebonds made when the
valence electrons are shared.valence electrons are shared.
Each Si atom shares oneEach Si atom shares one
electron with each of its fourelectron with each of its four
closest neighbors so that itsclosest neighbors so that its
valence band will have a full 8valence band will have a full 8
electrons.electrons.

N-Type MaterialN-Type Material
N-Type Material:N-Type Material: When extra valence electrons are introducedWhen extra valence electrons are introduced
into a material such as silicon an n-typeinto a material such as silicon an n-type
material is produced. The extra valencematerial is produced. The extra valence
electrons are introduced by puttingelectrons are introduced by putting
impurities or dopants into the silicon. Theimpurities or dopants into the silicon. The
dopants used to create an n-type materialdopants used to create an n-type material
are Group V elements. The most commonlyare Group V elements. The most commonly
used dopants from Group V are arsenic,used dopants from Group V are arsenic,
antimony and phosphorus.antimony and phosphorus.
The 2D diagram to the left shows the extraThe 2D diagram to the left shows the extra
electron that will be present when a Group Velectron that will be present when a Group V
dopant is introduced to a material such asdopant is introduced to a material such as
silicon. This extra electron is very mobile.silicon. This extra electron is very mobile.
+4+4+4+4
+5+5
+4+4
+4+4+4+4+4+4
+4+4+4+4

P-Type MaterialP-Type Material
P-Type Material:P-Type Material: P-type material is produced when the dopantP-type material is produced when the dopant
that is introduced is from Group III. Groupthat is introduced is from Group III. Group
III elements have only 3 valence electronsIII elements have only 3 valence electrons
and therefore there is an electron missing.and therefore there is an electron missing.
This creates a hole (h+), or a positive chargeThis creates a hole (h+), or a positive charge
that can move around in the material.that can move around in the material.
Commonly used Group III dopants areCommonly used Group III dopants are
aluminum, boron, and gallium.aluminum, boron, and gallium.
The 2D diagram to the left shows the holeThe 2D diagram to the left shows the hole
that will be present when a Group III dopantthat will be present when a Group III dopant
is introduced to a material such as silicon.is introduced to a material such as silicon.
This hole is quite mobile in the same way theThis hole is quite mobile in the same way the
extra electron is mobile in a n-type material.extra electron is mobile in a n-type material.
+4+4+4+4
+3+3
+4+4
+4+4+4+4+4+4
+4+4+4+4

The PN JunctionThe PN Junction
Steady StateSteady State11
PP nn
- - - - - -- - - - - -
- - - - - -- - - - - -
- - - - - -- - - - - -
- - - - - -- - - - - -
- - - - - -- - - - - -
+ + + + ++ + + + +
++
+ + + + ++ + + + +
++
+ + + + ++ + + + +
++
+ + + + ++ + + + +
++
+ + + + ++ + + + +
++
NaNa NdNd
MetallurgicalMetallurgical
JunctionJunction
Space ChargeSpace Charge
RegionRegionionizedionized
acceptorsacceptors
ionizedionized
donorsdonors
E-FieldE-Field
++++
__ __
h+ drifth+ drift h+ diffusionh+ diffusion e- diffusione- diffusion e- drifte- drift== ==

The PN JunctionThe PN Junction
Steady StateSteady State
PP nn
- - - - -- - - - -
- - - - -- - - - -
- - - - -- - - - -
- - - - -- - - - -
+ + + + ++ + + + +
+ + + + ++ + + + +
+ + + + ++ + + + +
+ + + + ++ + + + +
NaNa NdNd
MetallurgicalMetallurgical
JunctionJunction
Space ChargeSpace Charge
RegionRegionionizedionized
acceptorsacceptors
ionizedionized
donorsdonors
E-FieldE-Field
++++
__ __
h+ drifth+ drift h+ diffusionh+ diffusion e- diffusione- diffusion e- drifte- drift== ==== ==
When no external sourceWhen no external source
is connected to the pnis connected to the pn
junction, diffusion andjunction, diffusion and
drift balance each otherdrift balance each other
out for both the holesout for both the holes
and electronsand electrons
Space Charge Region:Space Charge Region: Also called the depletion region. This regionAlso called the depletion region. This region
includes the net positively and negatively charged regions. The spaceincludes the net positively and negatively charged regions. The space
charge region does not have any free carriers. The width of the space chargecharge region does not have any free carriers. The width of the space charge
region is denoted by W in pn junction formula’s.region is denoted by W in pn junction formula’s.
Metallurgical Junction:Metallurgical Junction: The interface where the p- and n-type materials meet.The interface where the p- and n-type materials meet.
Na & Nd:Na & Nd: Represent the amount of negative and positive doping in number ofRepresent the amount of negative and positive doping in number of
carriers per centimeter cubed. Usually in the range of 10carriers per centimeter cubed. Usually in the range of 101515
to 10to 102020
..

The Biased PN JunctionThe Biased PN Junction
PP nn
++
__
AppliedApplied
Electric FieldElectric Field
MetalMetal
ContactContact
““OhmicOhmic
Contact”Contact”
(Rs~0)(Rs~0)
++
__
VVappliedapplied
II
The pn junction is considered biased when an external voltage is applied.The pn junction is considered biased when an external voltage is applied.
There are two types of biasing: Forward bias and Reverse bias.There are two types of biasing: Forward bias and Reverse bias.
These are described on then next slide.These are described on then next slide.

The Biased PN JunctionThe Biased PN Junction
Forward Bias:Forward Bias: In forward bias the depletion region shrinks slightly inIn forward bias the depletion region shrinks slightly in
width. With this shrinking the energy required forwidth. With this shrinking the energy required for
charge carriers to cross the depletion region decreasescharge carriers to cross the depletion region decreases
exponentially. Therefore, as the applied voltageexponentially. Therefore, as the applied voltage
increases, current starts to flow across the junction.increases, current starts to flow across the junction.
The barrier potential of the diode is the voltage atThe barrier potential of the diode is the voltage at
which appreciable current starts to flow through thewhich appreciable current starts to flow through the
diode. The barrier potential varies for differentdiode. The barrier potential varies for different
materials.materials.
Reverse Bias:Reverse Bias: Under reverse bias the depletion region widens. ThisUnder reverse bias the depletion region widens. This
causes the electric field produced by the ions to cancelcauses the electric field produced by the ions to cancel
out the applied reverse bias voltage. A small leakageout the applied reverse bias voltage. A small leakage
current, Is (saturation current) flows under reverse biascurrent, Is (saturation current) flows under reverse bias
conditions. This saturation current is made up ofconditions. This saturation current is made up of
electron-hole pairs being produced in the depletionelectron-hole pairs being produced in the depletion
region. Saturation current is sometimes referred to asregion. Saturation current is sometimes referred to as
scale current because of it’s relationship to junctionscale current because of it’s relationship to junction
temperature.temperature.
VVappliedapplied > 0> 0
VVappliedapplied < 0< 0

Properties of DiodesProperties of Diodes
Figure 1.10 – The Diode Transconductance CurveFigure 1.10 – The Diode Transconductance Curve22
• VVDD = Bias Voltage= Bias Voltage
• IIDD = Current through= Current through
Diode. IDiode. IDD is Negativeis Negative
for Reverse Bias andfor Reverse Bias and
Positive for ForwardPositive for Forward
BiasBias
• IISS = Saturation= Saturation
CurrentCurrent
• VVBRBR = Breakdown= Breakdown
VoltageVoltage
• VVφφ = Barrier Potential= Barrier Potential
VoltageVoltage
VVDD
IIDD (mA)(mA)
(nA)(nA)
VVBRBR
~V~Vφφ
IISS

The Shockley EquationThe Shockley Equation
• The transconductance curve on the previous slide is characterized byThe transconductance curve on the previous slide is characterized by
the following equation:the following equation:
IIDD = I= ISS(e(eVVDD//ηηVVTT – 1)– 1)
• As described in the last slide, IAs described in the last slide, IDD is the current through the diode, Iis the current through the diode, ISS isis
the saturation current and Vthe saturation current and VDD is the applied biasing voltage.is the applied biasing voltage.
• VVTT is the thermal equivalent voltage and is approximately 26 mV at roomis the thermal equivalent voltage and is approximately 26 mV at room
temperature. The equation to find Vtemperature. The equation to find VTT at various temperatures is:at various temperatures is:
VVTT == kTkT
qq
k = 1.38 x 10k = 1.38 x 10-23-23 J/K T = temperature in Kelvin q = 1.6 x 10J/K T = temperature in Kelvin q = 1.6 x 10-19-19 CC
∀ ηη is the emission coefficient for the diode. It is determined by the wayis the emission coefficient for the diode. It is determined by the way
the diode is constructed. It somewhat varies with diode current. For athe diode is constructed. It somewhat varies with diode current. For a
silicon diodesilicon diode ηη is around 2 for low currents and goes down to about 1is around 2 for low currents and goes down to about 1
at higher currentsat higher currents

MathCAD Example - ApplicationMathCAD Example - Application

Diode Circuit ModelsDiode Circuit Models
The Ideal DiodeThe Ideal Diode
ModelModel
The diode is designed to allow current to flow inThe diode is designed to allow current to flow in
only one direction. The perfect diode would be aonly one direction. The perfect diode would be a
perfect conductor in one direction (forward bias)perfect conductor in one direction (forward bias)
and a perfect insulator in the other directionand a perfect insulator in the other direction
(reverse bias). In many situations, using the ideal(reverse bias). In many situations, using the ideal
diode approximation is acceptable.diode approximation is acceptable.
Example: Assume the diode in the circuit below is ideal. Determine theExample: Assume the diode in the circuit below is ideal. Determine the
value of Ivalue of IDD if a) Vif a) VAA = 5 volts (forward bias) and b) V= 5 volts (forward bias) and b) VAA = -5 volts (reverse= -5 volts (reverse
bias)bias)
++
__
VVAA
IIDD
RRSS = 50= 50 ΩΩ a) With Va) With VAA > 0 the diode is in forward bias> 0 the diode is in forward bias
and is acting like a perfect conductor so:and is acting like a perfect conductor so:
IIDD = V= VAA/R/RSS = 5 V / 50= 5 V / 50 ΩΩ = 100 mA= 100 mA
b) With Vb) With VAA < 0 the diode is in reverse bias< 0 the diode is in reverse bias
and is acting like a perfect insulator,and is acting like a perfect insulator,
therefore no current can flow and Itherefore no current can flow and IDD = 0.= 0.

The Ideal Diode withThe Ideal Diode with
Barrier PotentialBarrier Potential
This model is more accurate than the simpleThis model is more accurate than the simple
ideal diode model because it includes theideal diode model because it includes the
approximate barrier potential voltage.approximate barrier potential voltage.
Remember the barrier potential voltage is theRemember the barrier potential voltage is the
voltage at which appreciable current starts tovoltage at which appreciable current starts to
flow.flow.
Example: To be more accurate than just using the ideal diode modelExample: To be more accurate than just using the ideal diode model
include the barrier potential. Assume Vinclude the barrier potential. Assume Vφφ = 0.3 volts (typical for a= 0.3 volts (typical for a
germanium diode) Determine the value of Igermanium diode) Determine the value of IDD if Vif VAA = 5 volts (forward bias).= 5 volts (forward bias).
++
__
VVAA
IIDD
RRSS = 50= 50 ΩΩ
With VWith VAA > 0 the diode is in forward bias> 0 the diode is in forward bias
and is acting like a perfect conductorand is acting like a perfect conductor
so write a KVL equation to find Iso write a KVL equation to find IDD::
0 = V0 = VAA – I– IDDRRSS - V- Vφφ
IIDD = V= VAA - V- Vφφ = 4.7 V = 94 mA= 4.7 V = 94 mA
RRSS 5050 ΩΩ
VVφφ
++
VVφφ
++

with Barrierwith Barrier
Potential andPotential and
Linear ForwardLinear Forward
ResistanceResistance
This model is the most accurate of the three. It includes aThis model is the most accurate of the three. It includes a
linear forward resistance that is calculated from the slope oflinear forward resistance that is calculated from the slope of
the linear portion of the transconductance curve. However,the linear portion of the transconductance curve. However,
this is usually not necessary since the Rthis is usually not necessary since the RFF (forward(forward
resistance) value is pretty constant. For low-powerresistance) value is pretty constant. For low-power
germanium and silicon diodes the Rgermanium and silicon diodes the RFF value is usually in thevalue is usually in the
2 to 5 ohms range, while higher power diodes have a R2 to 5 ohms range, while higher power diodes have a RFF
value closer to 1 ohm.value closer to 1 ohm.
Linear Portion ofLinear Portion of
transconductancetransconductance
curvecurve
VVDD
IIDD
VVDD
IIDDRRFF == VVDD
IIDD
++
VVφφ RRFF

with Barrierwith Barrier
Potential andPotential and
Linear ForwardLinear Forward
ResistanceResistance
Example: Assume the diode is a low-power diodeExample: Assume the diode is a low-power diode
with a forward resistance value of 5 ohms. Thewith a forward resistance value of 5 ohms. The
barrier potential voltage is still: Vbarrier potential voltage is still: Vφφ = 0.3 volts (typical= 0.3 volts (typical
for a germanium diode) Determine the value of Ifor a germanium diode) Determine the value of IDD ifif
VVAA = 5 volts.= 5 volts.
++
__
VVAA
IIDD
RRSS = 50= 50 ΩΩ
VVφφ
++
RRFF
Once again, write a KVL equationOnce again, write a KVL equation
for the circuit:for the circuit:
0 = V0 = VAA – I– IDDRRSS -- VVφφ - I- IDDRRFF
IIDD = V= VAA - V- Vφφ = 5 – 0.3 = 85.5 mA= 5 – 0.3 = 85.5 mA
RRSS + R+ RFF 50 + 550 + 5

Values of ID for the Three Different Diode Circuit ModelsValues of ID for the Three Different Diode Circuit Models
Ideal Diode
Model
Ideal Diode
Model with
Barrier
Potential
Voltage
Ideal Diode
Model with
Barrier
Potential and
Linear Forward
Resistance
ID 100 mA 94 mA 85.5 mA
These are the values found in the examples on previousThese are the values found in the examples on previous
slides where the applied voltage was 5 volts, the barrierslides where the applied voltage was 5 volts, the barrier
potential was 0.3 volts and the linear forward resistancepotential was 0.3 volts and the linear forward resistance
value was assumed to be 5 ohms.value was assumed to be 5 ohms.

The Q PointThe Q Point
The operating point or Q point of the diode is the quiescent or no-The operating point or Q point of the diode is the quiescent or no-
signal condition. The Q point is obtained graphically and is really onlysignal condition. The Q point is obtained graphically and is really only
needed when the applied voltage is very close to the diode’s barrierneeded when the applied voltage is very close to the diode’s barrier
potential voltage. The examplepotential voltage. The example 33 below that is continued on the nextbelow that is continued on the next
slide, shows how the Q point is determined using theslide, shows how the Q point is determined using the
transconductance curve and the load line.transconductance curve and the load line.
++
__
VVAA
= 6V= 6V
IIDD
RRSS = 1000= 1000 ΩΩ
VVφφ
++
First the load line is found by substituting inFirst the load line is found by substituting in
different values of Vdifferent values of Vφφ into the equation for Iinto the equation for IDD usingusing
the ideal diode with barrier potential model for thethe ideal diode with barrier potential model for the
diode. With Rdiode. With RSS at 1000 ohms the value of Rat 1000 ohms the value of RFF
wouldn’t have much impact on the results.wouldn’t have much impact on the results.
IIDD = V= VAA – V– V φφ
RRSS
Using VUsing V φφ values of 0 volts and 1.4 volts we obtainvalues of 0 volts and 1.4 volts we obtain
IIDD values of 6 mA and 4.6 mA respectively. Nextvalues of 6 mA and 4.6 mA respectively. Next
we will draw the line connecting these two pointswe will draw the line connecting these two points
on the graph with the transconductance curve.on the graph with the transconductance curve.
This line is the load line.This line is the load line.

The Q PointThe Q Point
IIDD (mA)(mA)
VVDD (Volts)(Volts)
22
44
66
88
1010
1212
0.20.2 0.40.4 0.60.6 0.80.8 1.01.0 1.21.2 1.41.4
TheThe
transconductancetransconductance
curve below is for acurve below is for a
Silicon diode. TheSilicon diode. The
Q point in thisQ point in this
example is locatedexample is located
at 0.7 V and 5.3 mA.at 0.7 V and 5.3 mA.
4.64.6
0.70.7
5.35.3
Q Point:Q Point: The intersection of theThe intersection of the
load line and theload line and the
transconductance curve.transconductance curve.

Capacitance and Voltage of PNCapacitance and Voltage of PN
JunctionsJunctions
Diode Operation – AnimationDiode Operation – Animation
Webpage LinkWebpage Link

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