Diodes
•
•
•
•
•
•
•
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Semiconductors & doping
PN junctions
Reverse bias - no current
Forward bias
Diode IV curves
Uses of diodes
Calculations with diodes in series
Calculations with diodes in parallel
Diodes - 1
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Semiconductors & doping
• Semiconductors have a some conductivity, but
not nearly as good as most metals.
• Silicon is primarily used as a semiconductor.
• Silicon has four valence electrons. Each silicon
atom is covalently bonded to four others.
• Doped silicon has a small (~1 part in a million)
inclusion of other elements with one more or less
electron.
• P type material is made with a group III element one less electron.
• N type material is made with a group V element one more electron.
Diodes - 2
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PN junctions
Electrons move from
the N side to fill in holes
in the P side
A depletion region is formed as
there is now an overall charge
separation & electric field.
P type
N type
Boron atoms form bonds
with electron “holes” in
the Si structure.
Diodes - 3
+
+
+
+
+
+
-
-
-
-
-
-
Phosphorous atoms form
bonds with free electrons
in the Si structure.
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Reverse bias - no current
• In reverse bias, no current can get through the barrier of the depletion
region (unless breakdown occurs at high voltages).
+
P type
N type
-
+
+
+
+
+
+
Depletion region is
strengthened & the
electric field acts as a
barrier to current flow.
Diodes - 4
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Forward bias
• Once the switch on voltage is achieved, the diode conducts current.
• There is a voltage drop as electrons lose energy across the depletion
region.
+
Current
flow
P type
N type
-
Diodes - 5
+
+
+
+
+
+
Depletion region is
reduced & the diode
allows current through.
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Diode IV curves
• Diodes are non-ohmic & have a characteristic voltage current
relationship.
• Once the switch on voltage is reached in forward bias, the potential
difference remains fairly constant, even with increasing current.
• In reverse bias, no current flows until the breakdown voltage is reached.
Current (mA)
Breakdown voltage: may be
kV.
Potential difference (V)
Switch on voltage ~0.7V for
Si diode.
hp://www.flashscience.com/electricity/diodes.htm
Diodes - 6
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Uses of diodes
• Diodes can be used to rectify AC into DC, by blocking the flow of
reverse currents.
• Zener diodes are used as voltage regulators, to keep a constant
potential output.
• LEDs are light emitting diodes - photons of visible light are created as
the electrons lose energy passing across the PN junction. LEDs are
much more energy efficient than traditional incandescent globes.
hp://www.flashscience.com/electricity/smoothing.htm
Diodes - 7
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Calculations with diodes in series
• Assume that the diode is at the switch on voltage eg 0.7V.
• The remaining potential difference is across the resistor(s).
9.0 V
+
8.3 V
0.7 V
1000Ω
500Ω
8.3V
8.3V
II ==
==0.0166A
0.0083A
1000Ω
500Ω
Diodes - 8
16.6mA
8.3
mA
16.6mA
8.3
mA
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Calculations with diodes in parallel
• A diode in parallel with another component will act as a voltage
regulator, limiting the potential difference across the pair.
8.3 V
9.0V
8.3V
I=
= 0.0166A
0.015A
500Ω
600Ω
500Ω
16.6
15 mAmA
0.7
I=
= 0.007A
100Ω
9.0 V
+
0.7 V
100Ω
0.7 V
Diodes - 9
7.0 mA
9.6 mA
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