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Higher Physics
P-N Junction Diodes
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P-N Junction 1
• Semiconductor material is doped so that half is
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p-type (+ve holes) and half is n-type (-ve
electrons).
In this form it is known as a p-n junction diode.
Between each half the electrons and holes
combine and cancel each other out
This creates the DEPLETION LAYER
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P-N Junction 2
• Depletion layer creates ions which produce a
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potential difference of a few hundred millivolts –
POTENTIAL BARRIER
For current to flow through the diode the
potential barrier must be overcome and the
depletion layer removed.
We know from SG Physics that a diode allows
current to flow in only one direction.
However, a p-n junction diode can be connected
to a battery in two ways.
Forward Bias 1
p-type connected to the
+ve side of the battery
and n-type connected to
-ve side.
Diode is forward-biased.
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Forward Bias 2
• junction voltage, Vpn (potential barrier)
opposes the applied voltage from the
supply battery (for silicon diode Vpn ~ 0.6V)
• If Vs < Vpn , no current flows
• If Vs > Vpn , charge carriers (electrons and
holes) can cross the junction, reducing the
depletion layer until it disappears, allowing
the junction to fully conduct.
Reverse Bias 1
p-type connected to the -ve
side of the battery and
n-type connected to +ve
side.
Diode is reverse-biased.
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Reverse Bias 2
• electrons in the n-type material are attracted by
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the +ve terminal of the battery
electrons from the battery will enter the p-type
end of the diode and combine with some of the
holes.
this removes some of the charge carriers from
the diode, increasing the width of the depletion
layer.
the junction’s resistance becomes very large and
so there is no current in the circuit.
Rectification 1
• Common use of
•
diodes is to rectify a.c.
power supplies to
produce d.c outputs
(e.g. phone chargers,
power adapters)
This can be done with
a single diode
• Only the positive half of the supply passes
through, resulting in half wave rectification.
Rectification 2
• A better solution is to
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use a rectifier bridge
(a network of four
diodes)
This effectively
changes the direction
of the negative part of
the a.c. input.
• Both halves of the supply pass through,
resulting in full wave rectification.