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16-03-2015
1
PN Junction
Diode
Table of Contents
What are diodes made out of?____________________slide 3
N-type material_________________________________slide 4
P-type material_________________________________slide 5
The pn junction_________________________________slides 6-7
The biased pn junction___________________________slides 8-9
Properties of diodes_____________________________slides 10-11
Diode Circuit Models ____________________________slides 12-16
The Q Point____________________________________slides 17-18
Dynamic Resistance_____________________________slides 19-20
Types of diodes and their uses ___________________ slides 21-24
Sources_______________________________________slide 25
What Are Diodes Made Out Of?
• Silicon (Si) and Germanium (Ge) are the two most
common single elements that are used to make Diodes.
A compound that is commonly used is Gallium
Arsenide (GaAs), especially in the case of LEDs
because of it’s large bandgap.
• Silicon and Germanium are both group 4 elements,
meaning they have 4 valence electrons. Their
structure allows them to grow in a shape called the
diamond lattice.
• Gallium is a group 3 element while Arsenide is a group
5 element. When put together as a compound, GaAs
creates a zincblend lattice structure.
• In both the diamond lattice and zincblend lattice, each
atom shares its valence electrons with its four closest
neighbors. This sharing of electrons is what ultimately
allows diodes to be build. When dopants from groups
3 or 5 (in most cases) are added to Si, Ge or GaAs it
changes the properties of the material so we are able
to make the P- and N-type materials that become the
diode.
Si
+4
Si
+4
Si
+4
Si
+4
Si
+4
Si
+4
Si
+4
Si
+4
Si
+4
The diagram above shows the
2D structure of the Si crystal.
The light green lines
represent the electronic
bonds made when the valence
electrons are shared. Each Si
atom shares one electron with
each of its four closest
neighbors so that its valence
band will have a full 8
electrons.
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2
N-Type Material
N-Type Material: When extra valence electrons are introduced
into a material such as silicon an n-type
material is produced. The extra valence
electrons are introduced by putting
impurities or dopants into the silicon. The
dopants used to create an n-type material
are Group V elements. The most commonly
used dopants from Group V are arsenic,
antimony and phosphorus.
The 2D diagram to the left shows the extra
electron that will be present when a Group V
dopant is introduced to a material such as
silicon. This extra electron is very mobile.
+4 +4
+5
+4
+4 +4 +4
+4 +4
P-Type Material
P-Type Material: P-type material is produced when the dopant
that is introduced is from Group III. Group
III elements have only 3 valence electrons
and therefore there is an electron missing.
This creates a hole (h+), or a positive charge
that can move around in the material.
Commonly used Group III dopants are
aluminum, boron, and gallium.
The 2D diagram to the left shows the hole
that will be present when a Group III dopant
is introduced to a material such as silicon.
This hole is quite mobile in the same way the
extra electron is mobile in a n-type material.
+4 +4
+3
+4
+4 +4 +4
+4 +4
The PN Junction Steady State1
P n
- - - - - -
- - - - - -
- - - - - -
- - - - - -
- - - - - -
+ + + + + +
+ + + + + +
+ + + + + +
+ + + + + +
+ + + + + +
Na Nd
Metallurgical
Junction
Space Charge Region ionized
acceptors ionized
donors
E-Field
+ + _
_
h+ drift h+ diffusion e- diffusion e- drift = =
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The PN Junction Steady State
P n
- - - - -
- - - - -
- - - - -
- - - - -
+ + + + +
+ + + + +
+ + + + +
+ + + + +
Na Nd
Metallurgical
Junction
Space Charge Region ionized
acceptors ionized
donors
E-Field
+ + _
_
h+ drift h+ diffusion e- diffusion e- drift = = = =
When no external source
is connected to the pn
junction, diffusion and
drift balance each other
out for both the holes
and electrons
Space Charge Region: Also called the depletion region. This region includes
the net positively and negatively charged regions. The space charge region
does not have any free carriers. The width of the space charge region is
denoted by W in pn junction formula’s.
Metallurgical Junction: The interface where the p- and n-type materials meet.
Na & Nd: Represent the amount of negative and positive doping in number of
carriers per centimeter cubed. Usually in the range of 1015 to 1020.
The Biased PN Junction
P n
+ _
Applied
Electric Field
Metal
Contact
“Ohmic
Contact”
(Rs~0)
+ _
Vapplied
I
The pn junction is considered biased when an external voltage is applied.
There are two types of biasing: Forward bias and Reverse bias.
These are described on then next slide.
The Biased PN Junction
Forward Bias: In forward bias the depletion region shrinks slightly in width.
With this shrinking the energy required for charge carriers to cross the
depletion region decreases exponentially.
Therefore, as the applied voltage increases, current starts to flow
across the junction.
The barrier potential of the diode is the voltage at which appreciable
current starts to flow through the diode.
The barrier potential varies for different materials.
Reverse Bias: Under reverse bias the depletion region widens.
This causes the electric field produced by the ions to cancel out the
applied reverse bias voltage.
A small leakage current, Is (saturation current) flows under reverse bias
conditions.
This saturation current is made up of electron-hole pairs being
produced in the depletion region.
Saturation current is sometimes referred to as scale current because of
it’s relationship to junction temperature.
Vapplied > 0
Vapplied < 0
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4
Properties of Diodes Figure – The Diode Transconductance Curve
• VD = Bias Voltage
• ID = Current through
Diode. ID is Negative
for Reverse Bias and
Positive for Forward
Bias
• IS = Saturation
Current
• VBR = Breakdown
Voltage
• V = Barrier Potential
Voltage
VD
ID (mA)
(nA)
VBR
~V
IS
Properties of Diodes The Shockley Equation
• The transconductance curve on the previous slide is characterized by
the following equation:
ID = IS(eVD/VT – 1)
• As described in the last slide, ID is the current through the diode, IS is
the saturation current and VD is the applied biasing voltage.
• VT is the thermal equivalent voltage and is approximately 26 mV at room
temperature. The equation to find VT at various temperatures is:
VT = kT q
k = 1.38 x 10-23 J/K T = temperature in Kelvin q = 1.6 x 10-19 C
• is the emission coefficient for the diode. It is determined by the way
the diode is constructed. It somewhat varies with diode current.
• For a silicon diode is around 2 for low currents and goes down to
about 1 at higher currents
Diode Circuit Models
The Ideal Diode
Model
The diode is designed to allow current to flow in only one
direction.
The perfect diode would be a perfect conductor in one
direction (forward bias) and a perfect insulator in the other
direction (reverse bias).
In many situations, using the ideal diode approximation is
acceptable. Example: Assume the diode in the circuit below is ideal. Determine the
value of ID if a) VA = 5 volts (forward bias) and b) VA = -5 volts (reverse
bias)
+
_ VA
ID
RS = 50 a) With VA > 0 the diode is in forward bias
and is acting like a perfect conductor so:
ID = VA/RS = 5 V / 50 = 100 mA
b) With VA < 0 the diode is in reverse bias
and is acting like a perfect insulator,
therefore no current can flow and ID = 0.
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Diode Circuit Models
The Ideal Diode with
Barrier Potential This model is more accurate than the simple ideal
diode model because it includes the approximate
barrier potential voltage.
Remember the barrier potential voltage is the
voltage at which appreciable current starts to
flow. Example: To be more accurate than just using the ideal diode model
include the barrier potential. Assume V = 0.3 volts (typical for a
germanium diode) Determine the value of ID if VA = 5 volts (forward bias).
+
_ VA
ID
RS = 50 With VA > 0 the diode is in forward bias
and is acting like a perfect conductor
so write a KVL equation to find ID:
0 = VA – IDRS - V
ID = VA - V = 4.7 V = 94 mA
RS 50
V
+
V
+
Diode Circuit Models
The Ideal Diode
with Barrier
Potential and
Linear Forward
Resistance
This model is the most accurate of the three.
It includes a linear forward resistance that is calculated from the
slope of the linear portion of the transconductance curve.
However, this is usually not necessary since the RF (forward
resistance) value is pretty constant.
For low-power germanium and silicon diodes the RF value is
usually in the 2 to 5 ohms range, while higher power diodes have
a RF value closer to 1 ohm.
Linear Portion of
transconductance
curve
VD
ID
Δ VD
Δ ID
RF = ΔVD
Δ ID
+ V
RF
Diode Circuit Models
The Ideal Diode
with Barrier
Potential and
Linear Forward
Resistance
Example: Assume the diode is a low-power diode
with a forward resistance value of 5 ohms. The
barrier potential voltage is still: V = 0.3 volts (typical
for a germanium diode) Determine the value of ID if
VA = 5 volts.
+
_ VA
ID
RS = 50
V
+
RF
Once again, write a KVL equation
for the circuit:
0 = VA – IDRS - V - IDRF
ID = VA - V = 5 – 0.3 = 85.5 mA
RS + RF 50 + 5
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Diode Circuit Models
Values 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 previous
slides where the applied voltage was 5 volts, the barrier
potential was 0.3 volts and the linear forward resistance
value was assumed to be 5 ohms.
The Q Point
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 only needed when the applied
voltage is very close to the diode’s barrier potential voltage.
The example below that is continued on the next slide, shows how the Q point is
determined using the transconductance curve and the load line.
+
_ VA
= 6V
ID
RS = 1000
V
+
First the load line is found by substituting in different
values of V into the equation for ID using the ideal
diode with barrier potential model for the diode. With
RS at 1000 ohms the value of RF wouldn’t have much
impact on the results.
ID = VA – V
RS
Using V values of 0 volts and 1.4 volts we obtain ID
values of 6 mA and 4.6 mA respectively.
Next we will draw the line connecting these two points
on the graph with the transconductance curve. This
line is the load line.
The Q Point
ID (mA)
VD (Volts)
2
4
6
8
10
12
0.2 0.4 0.6 0.8 1.0 1.2 1.4
The
transconductance
curve below is for a
Silicon diode. The
Q point in this
example is located
at 0.7 V and 5.3 mA.
4.6
0.7
5.3
Q Point: The intersection of the
load line and the
transconductance curve.
16-03-2015
7
Dynamic Resistance
The dynamic resistance of the diode is mathematically determined as the
inverse of the slope of the transconductance curve. Therefore, the
equation for dynamic resistance is:
rF = VT
ID
The dynamic resistance is used in determining the voltage drop across
the diode in the situation where a voltage source is supplying a
sinusoidal signal with a dc offset (when a waveform has unequal amounts of
signal in the positive and negative domains).
The ac component of the diode voltage is found using the following
equation:
vF = vac rF
rF + RS The voltage drop through the diode is a combination of the ac and dc
components and is equal to:
VD = V + vF
Dynamic Resistance
Example: Use the same circuit used for the Q point example but change
the voltage source so it is an ac source with a dc offset. The source
voltage is now, vin = 6 + sin(wt) Volts. It is a silicon diode so the barrier
potential voltage is still 0.7 volts.
+
vin
ID
RS = 1000
V
+
The DC component of the circuit is the
same as the previous example and
therefore ID = 6V – 0.7 V = 5.2 mA
1000
rF = VT = 1 * 26 mV = 4.9
ID 5.3 mA
= 1 is a good approximation if the dc
current is greater than 1 mA as it is in this
example.
vF = vac rF = sin(wt) V 4.9 = 4.88 sin(wt) mV
rF + RS 4.9 + 1000
Therefore, VD = 700 + 4.9 sin (wt) mV (the voltage drop across the
diode)
Types of Diodes and Their Uses
PN Junction
Diodes:
Are used to allow current to flow in one direction
while blocking current flow in the opposite
direction. The pn junction diode is the typical diode
that has been used in the previous circuits.
A K
Schematic Symbol for a PN
Junction Diode
P n
Representative Structure for
a PN Junction Diode
Zener Diodes: Are specifically designed to operate under reverse
breakdown conditions. These diodes have a very
accurate and specific reverse breakdown voltage.
A K
Schematic Symbol for a
Zener Diode
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Types of Diodes and Their Uses
Schottky
Diodes:
These diodes are designed to have a very fast
switching time which makes them a great diode for
digital circuit applications. They are very common
in computers because of their ability to be switched
on and off so quickly. A K
Schematic Symbol for a
Schottky Diode
Shockley
Diodes:
The Shockley diode is a four-layer diode while other
diodes are normally made with only two layers.
These types of diodes are generally used to control
the average power delivered to a load.
A K
Schematic Symbol for a
four-layer Shockley Diode
Types of Diodes and Their Uses
Light-Emitting
Diodes:
Light-emitting diodes are designed with a very large
bandgap so movement of carriers across their
depletion region emits photons of light energy.
Lower bandgap LEDs (Light-Emitting Diodes) emit
infrared radiation, while LEDs with higher bandgap
energy emit visible light. Many stop lights are now
starting to use LEDs because they are extremely
bright and last longer than regular bulbs for a
relatively low cost.
A K
Schematic Symbol for a
Light-Emitting Diode
The arrows in the LED
representation indicate
emitted light.
Types of Diodes and Their Uses
Photodiodes: While LEDs emit light, Photodiodes are sensitive to
received light. They are constructed so their pn
junction can be exposed to the outside through a
clear window or lens.
In Photoconductive mode the saturation current
increases in proportion to the intensity of the
received light. This type of diode is used in CD
players.
In Photovoltaic mode, when the pn junction is
exposed to a certain wavelength of light, the diode
generates voltage and can be used as an energy
source. This type of diode is used in the
production of solar power.
A K
A K
Schematic Symbols for
Photodiodes
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9
Diodes Circuits
Key Words:
Diode Limiter
multi diode Circuits
Rectifier Circuits
+ +
-
vi vo vo
-
+
R D
t
t
Von
vi
vo
When vi > Von , D on vo vi;
vi < Von, D off vo = 0。
multiple diodes
Circuits
R
+5V
V2
V1 Vo
D1
D2
V1(V)
V2(V)
Vo(V)
Logic
output 0
0
0.7
0
5
0
0.7
0
0
5
0.7
0
5
5
5
1
16-03-2015
10
Rectifier Circuits
One of the most important applications of diodes is in the
design of rectifier circuits. Used to convert an AC signal into
a DC voltage used by most electronics.
Rectifier Circuits
Simple Half-Wave Rectifier
What would the
waveform
look like if not an ideal
diode?
Rectifier Circuits
Bridge Rectifier
Looks like a Wheatstone bridge. Does not require a center
tapped transformer.
Requires 2 additional diodes and voltage drop is double.
16-03-2015
11
Diode Circuits:
Applications
Applications – Rectifier Circuits
Half-Wave Rectifier Circuits
Applications – Rectifier Circuits
Battery-Charging Circuit
16-03-2015
12
Half-Wave Rectifier with Smoothing Capacitor
Large Capacitance
i=dq/dt or Q = IL T
Q = Vr C
then
C ~ (ILT) / Vr
Half-Wave Rectifier with Smoothing Capacitor
Large Capacitance
Forward bias
charge cycle
Reverse bias
discharge cycle
Start
Vr Peak-to-peak riple voltage
i=dq/dt or Q = IL T
Q = Vr C
then
C ~ (ILT) / Vr
typically :VL ~V m- (Vr /2)
Full-Wave rectifier Circuits
The sources are out of phase
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13
Wave Shaping Circuits Clipper Circuits
Batteries replaced by Zener diodes
+ 600
mV
I flow
below 600 mV
I flow
Above 600 mV
- 600
mV
Half-Wave Limiter Circuits
Current flows thru the resistor until +600 mV is reached, then
flows thru the Diode.
The plateau is representative of the voltage drop of the diode
while it is conducting.
Voltage
divider
Linear Small Signal Equivalent Circuits (1)
When considering electronic circuits in which dc supply voltages are used to bias a nonlinear devices at their operating points and a small ac signal is injected into the circuit to find circuit response:
Split the analysis of the circuit into two parts:
(a)analyze the dc circuit to find the operating point
(b)consider the small ac signal
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14
Linear Small Signal Equivalent Circuits (1)
Since virtually any nonlinear ch-tic is approximately linear (straight) if we consider a sufficiently small
segment
THEN
We can find a linear small-signal equivalent circuit for the nonlinear device to use in the ac analysis
The small signal diode circuit can be substituted by a single equivalent resistor.
Linear Small Signal Equivalent Circuits (2)
dc supply voltage results in operation at Q
An ac signal is injected into the circuit and
swings the instantaneous point of operation
slightly above and below the Q point
For small changes
D
QD
DD v
dv
dii
iD –the small change in diode current from the Q-point
vD –the small change in diode voltage from the Q-point
(diD/dvD) – the slope of the diode ch-tic evaluated at the point Q
Linear Small Signal Equivalent Circuits (2)
dc supply voltage results in operation at Q
An ac signal is injected into the circuit and
swings the instantaneous point of operation
slightly above and below the Q point
For small changes
D
QD
DD v
dv
dii
iD –the small change in diode current from the Q-point
vD –the small change in diode voltage from the Q-point
(diD/dvD) – the slope of the diode ch-tic evaluated at the point Q
1
QD
DD
dv
dir
Dynamic resistance of the diode
D
D
Dr
vi
16-03-2015
15
From small signal diode analysis
q
kTV
nV
vIi
T
T
d
sD
1exp Differentiating
the Shockley eq.
T
D
T
S
D
D
nV
v
nVI
dv
diexp
1
… and following the math on p.452 we can write that dynamic resistance of the diode is
DQ
T
DI
nVr
Linear Small Signal Equivalent Circuits (3)
T
DQ
sDQnV
vII exp~
where
Example - Voltage-Controlled Attenuator
DC control signal
C1, C2 – small or large ?
C in dc circuit – open circuit
C in ac circuit –short circuit
Find the operating point and perform the small signal analysis to obtain the small signal voltage gain
CjZC
1
Example - Voltage-Controlled Attenuator
DC control signal
Dc circuit for Q point (IDQ, VDQ)
DQ
T
DI
nVr Compute at the Q
point (IDQ, VDQ)
16-03-2015
16
Example - Voltage-Controlled Attenuator
The dc voltage source is equivalent to a short circuit for ac signals.
Voltage gain RR
R
v
vA
p
p
in
v
0
Sources
Dailey, Denton. Electronic Devices and Circuits, Discrete and Integrated. Prentice Hall, New
Jersey: 2001. (pp 2-37, 752-753)
2 Figure 1.10. The diode transconductance curve, pg. 7
Figure 1.15. Determination of the average forward resistance of a diode, pg 11
3 Example from pages 13-14
Liou, J.J. and Yuan, J.S. Semiconductor Device Physics and Simulation. Plenum Press,
New York: 1998.
Neamen, Donald. Semiconductor Physics & Devices. Basic Principles. McGraw-Hill,
Boston: 1997. (pp 1-15, 211-234)
1 Figure 6.2. The space charge region, the electric field, and the forces acting on
the charged carriers, pg 213.
Recommended