diodes podcast notes
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VCE Physics.comDiodes -
Diodes
• 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
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VCE Physics.comDiodes -
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.
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VCE Physics.comDiodes -
PN junctions
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P type N type
Boron atoms form bonds with electron “holes” in
the Si structure.
Phosphorous atoms form bonds with free electrons
in the Si structure.
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.
- -
- -
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+-+-+-+-+-+-
VCE Physics.comDiodes -
Reverse bias - no current
• In reverse bias, no current can get through the barrier of the depletion region (unless breakdown occurs at high voltages).
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P type N type
+-+-+-+-+-+-
+
Depletion region is strengthened & the electric field acts as a barrier to current flow.
VCE Physics.comDiodes -
• Once the switch on voltage is achieved, the diode conducts current.• There is a voltage drop as electrons lose energy across the depletion
region.
Forward bias
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P type N type
+-+-+-+-+-+-
+
Depletion region is reduced & the diode allows current through.
Current flow
VCE Physics.comDiodes -
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.
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Current (mA)
Potential difference (V)
hp://www.flashscience.com/electricity/diodes.htm
Switch on voltage ~0.7V for Si diode.
Breakdown voltage: may be kV.
VCE Physics.comDiodes -
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.
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hp://www.flashscience.com/electricity/smoothing.htm
VCE Physics.comDiodes -
500Ω
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).
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+
1000Ω
0.7 V8.3 V
I = 8.3V
500Ω=0.0166A 16.6 mA
I = 8.3V
1000Ω=0.0083A 8.3 mA 16.6 mA8.3 mA
9.0 V
VCE Physics.comDiodes -
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.
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+500Ω
100Ω
9.0 V
I = 0.7
100Ω=0.007A
0.7 V
0.7 V
8.3 V
I = 9.0V
600Ω=0.015A
15 mA
9.6 mA
7.0 mA
I = 8.3V
500Ω=0.0166A
16.6 mA