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Semiconductor Diodes
Chapter 1
Ch.1 Summary
Diodes
The diode is a 2-terminal device.
A diode ideally conducts
in only one direction.
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Ch.1 Summary
Diode Characteristics
Conduction Region
Non-Conduction Region
The voltage across the diode is 0 V
The current is infinite
The forward resistance is defined as
RF = VF / IF
The diode acts like a short
All of the voltage is across the diode
The current is 0 A
The reverse resistance is defined as
RR = VR / IR
The diode acts like open
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Ch.1 Summary
Semiconductor Materials
Materials commonly used in the development of
semiconductor devices:
Silicon (Si)
Germanium (Ge)
Gallium Arsenide (GaAs)
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Semiconductors
Semiconductors are crystalline materials that are characterized
by specific energy bands for electrons.
Between the bands are gaps; these
gaps represent energies that
electrons cannot posses.
The last energy band is the
conduction band, where
electrons are mobile.
The next to the last band is the
valence band, which is the
energy level associated with
electrons involved in bonding.
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Energy
Conduction band
Valence band
Energy
gap
Energy
gap
Second band
Energy
gap
First
band
Nucleus
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Electron and hole current
At room temperature, some electrons have enough energy to
jump into the conduction band.
After jumping the gap, these electrons are free to drift
throughout the material and form electron current when a
voltage is applied.
Electronhole pair
Energy
For every
electron in the
conduction
band, a hole is
left behind in the
valence band.
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Conduction band
Energy gap
Heat
energy
Valence band
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Electron and hole current
The electrons in the conduction band and the holes in the valence
band are the charge carriers. In other words, current in the
conduction band is by electrons; current in the valence band is by
holes.
When an electron jumps to the conduction band,
valence electrons move from hole-to-hole in the
valence band, effectively creating “hole current” shown
by gray arrows.
Si
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Si
Free
electron
Si
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Ch.1 Summary
Doping
The electrical characteristics of silicon and germanium are
improved by adding materials in a process called doping.
There are just two types of doped semiconductor materials:
n-type
n-type materials contain
an excess of conduction
band electrons.
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p-type
p-type materials contain an
excess of valence band holes.
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Semiconductor doping
•
Semiconductor doping is the process that changes an intrinsic semiconductor to an
extrinsic semiconductor.
•
During doping, impurity atoms are introduced to an intrinsic semiconductor. Impurity
atoms are atoms of a different element than the atoms of the intrinsic semiconductor.
Impurity atoms act as either donors or acceptors to the intrinsic semiconductor,
changing the electron and hole concentrations of the semiconductor.
•
Impurity atoms are classified as donor or acceptor atoms based on the effect they
have on the intrinsic semiconductor.
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Impurities
By adding certain impurities to pure (intrinsic) silicon, more holes
or more electrons can be produced within the crystal.
To increase the number of
conduction band electrons,
pentavalent impurities are added,
forming an n-type semiconductor.
These are elements to the right of Si
on the Periodic Table.
To increase the number of holes,
trivalent impurities are added, forming
a p-type semiconductor. These are
elements to the left of Si on the
Periodic Table.
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III IV V
B C N
Al Si P
Ga Ge As
In Sn Sb
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Ch.1 Summary
p-n Junctions
One end of a silicon or germanium crystal can be
doped as a p-type material and the other end as an
n-type material.
The result is a p-n junction
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Ch.1 Summary
p-n Junctions
At the p-n junction, the excess conduction-band electrons
on the n-type side are attracted to the valence-band holes
on the p-type side.
The electrons in the n-type
material migrate across the
junction to the p-type material
(electron flow).
Electron migration results in a
negative charge on the p-type
side of the junction and a
positive charge on the n-type
side of the junction.
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The result is the formation of a
depletion region around the
junction.
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Ch.1 Summary
Diode Operating Conditions
A diode has three operating conditions:
No bias
Reverse bias
Forward bias
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Ch.1 Summary
Diode Operating Conditions
No Bias
No external voltage is applied: VD = 0 V
There is no diode current: ID = 0 A
Only a modest depletion region exists
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Ch.1 Summary
Diode Operating Conditions
Reverse Bias
External voltage is applied
across the p-n junction in
the opposite polarity of the
p- and n-type materials.
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Ch.1 Summary
Diode Operating Conditions
Reverse Bias
The reverse voltage
causes the depletion
region to widen.
The electrons in the n-type
material are attracted
toward the positive terminal
of the voltage source.
The holes in the p-type material are attracted toward the negative
terminal of the voltage source.
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Ch.1 Summary
Diode Operating Conditions
Forward Bias
External voltage is
applied across the p-n
junction in the same
polarity as the p- and ntype materials.
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Ch.1 Summary
Diode Operating Conditions
Forward Bias
The forward voltage
causes the depletion
region to narrow.
The electrons and holes
are pushed toward the
p-n junction.
The electrons and holes have sufficient energy to cross the p-n junction.
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Diode equation
•
It can be demonstrated through the use of solid-state physics that the general characteristics
of a semiconductor diode can be defined by the following equation for the forward- and reverse-bias
regions:
Where:
ID diode through current
VD voltage across diode
Is is the reverse saturation current.
k is Boltzmann's constant= 1.38 x 10-23 JK-1.
Tk is the working temperature in Kelvin = 273 + temp in ºC.
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Ch.1 Summary
Actual Diode Characteristics
Note the regions for no
bias, reverse bias, and
forward bias
conditions.
Carefully note the scale
for each of these
conditions.
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Ch.1 Summary
Majority and Minority Carriers
Two currents through a diode:
Majority Carriers
The majority carriers in n-type materials are electrons.
The majority carriers in p-type materials are holes.
Minority Carriers
The minority carriers in n-type materials are holes.
The minority carriers in p-type materials are electrons.
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Ch.1 Summary
Zener Region
The Zener region is in the diode’s reverse-bias region.
At some point the reverse bias voltage
is so large the diode breaks down and
the reverse current increases
dramatically.
The maximum reverse voltage that
won’t take a diode into the zener
region is called the peak inverse
voltage or peak reverse voltage.
The voltage that causes a diode to
enter the zener region of operation is
called the zener voltage (VZ).
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Ch.1 Summary
Forward Bias Voltage
The point at which the diode changes from no-bias condition
to forward-bias condition occurs when the electrons and
holes are given sufficient energy to cross the p-n junction.
This energy comes from the external voltage applied across
the diode.
The forward bias voltage required for a:
gallium arsenide diode  1.2 V
silicon diode  0.7 V
germanium diode  0.3 V
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Ch.1 Summary
Temperature Effects
As temperature increases it adds energy to the diode.
It reduces the required forward bias voltage for forwardbias conduction.
It increases the amount of reverse current in the reversebias condition.
It increases maximum reverse bias avalanche voltage.
Germanium diodes are more sensitive to temperature variations
than silicon or gallium arsenide diodes.
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Ch.1 Summary
Resistance Levels
Semiconductors react differently to DC and AC currents.
There are three types of resistance:
DC (static) resistance
AC (dynamic) resistance
Average AC resistance
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Ch.1 Summary
DC (Static) Resistance
For a specific applied DC
voltage (VD) the diode has
a specific current (ID) and
a specific resistance (RD).
VD
RD 
ID
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Ch.1 Summary
AC (Dynamic) Resistance
In the forward bias region:
26 mV
rd 
 rB
ID
The resistance depends on the amount of current (ID) in the diode.
The voltage across the diode is fairly constant (26 mV for 25C).
rB ranges from a typical 0.1  for high power devices to 2  for low
power, general purpose diodes. In some cases rB can be ignored.
In the reverse bias region:
rd  
The resistance is effectively infinite. The diode acts like an open.
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Ch.1 Summary
Average AC Resistance
rav
ΔVd

ΔId
pt. to pt.
AC resistance can be
calculated using the
current and voltage values
for two points on the diode
characteristic curve.
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Ch.1 Summary
Diode Equivalent Circuit
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Ch.1 Summary
Diode Capacitance
When reverse biased, the depletion layer is very large. The diode’s
strong positive and negative polarities create capacitance (CT). The
amount of capacitance depends on the reverse voltage applied.
When forward
biased, storage
capacitance or
diffusion
capacitance (CD)
exists as the diode
voltage increases.
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Ch.1 Summary
Reverse Recovery Time (trr)
Reverse recovery time is the time required for a diode to
stop conducting when switched from forward bias to
reverse bias.
trr: reverse recovery time
ts: storage time
tt:transition time
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Ch.1 Summary
Diode Specification Sheets
Diode data sheets contain standard information, making crossmatching of diodes for replacement or design easier.
1. Forward Voltage (VF) at a specified current and temperature
2. Maximum forward current (IF) at a specified temperature
3. Reverse saturation current (IR) at a specified voltage and temperature
4. Reverse voltage rating, PIV or PRV or V(BR), at a specified temperature
5. Maximum power dissipation at a specified temperature
6. Capacitance levels
7. Reverse recovery time, trr
8. Operating temperature range
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Ch.1 Summary
Diode Symbol and Packaging
The anode is abbreviated A
The cathode is abbreviated K
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Ch.1 Summary
Diode Testing
Diodes are commonly tested using one of these
types of equipment:
Diode checker
Ohmmeter
Curve tracer
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Ch.1 Summary
Diode Checker
Many digital multimeters have a diode checking function.
The diode should be tested out of circuit.
A normal diode exhibits its forward voltage:
Gallium arsenide  1.2 V
Silicon diode  0.7 V
Germanium diode  0.3 V
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Ch.1 Summary
Ohmmeter
An ohmmeter set on a low Ohms scale can be used to test
a diode. The diode should be tested out of circuit.
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Ch.1 Summary
Curve Tracer
A curve tracer displays
the characteristic curve of
a diode in the test circuit.
This curve can be
compared to the
specifications of the
diode from a data sheet.
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Ch.1 Summary
Other Types of Diodes
There are several types of diodes besides the standard
p-n junction diode. Three of the more common are:
Zener diodes
Light-emitting diodes
Diode arrays
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Ch.1 Summary
Zener Diode
A Zener diode is one that
is designed to safely
operate in its zener
region; i.e., biased at the
Zener voltage (VZ).
Common zener diode voltage ratings
are between 1.8 V and 200 V
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Ch.1 Summary
Light-Emitting Diode (LED)
An LED emits light when it is forward biased,
which can be in the infrared or visible spectrum.
The forward bias voltage is usually
in the range of 2 V to 3 V.
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Ch.1 Summary
Diode Arrays
Multiple diodes can be packaged together in an
integrated circuit (IC).
Common Anode
A variety of diode
configurations are
available.
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Common Cathode
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