Higher Physics

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Higher Physics

• P-N Junction Diodes
• (N.B. log into SCHOLAR before viewing)

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P-N Junction 1

• Semiconductor material is doped so that half is
  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
  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.

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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 lt Vpn , no current flows
• If Vs gt Vpn , charge carriers (electrons and
  holes) can cross the junction, reducing the
  depletion layer until it disappears, allowing the
  junction to fully conduct.

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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
  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 junctions resistance becomes very large and
  so there is no current in the circuit.

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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.

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Rectification 2

• A better solution is to 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.