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Higher Physics

Higher Physics. P-N Junction Diodes (N.B. – log into SCHOLAR before viewing). 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.

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Higher Physics

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  1. Higher Physics P-N Junction Diodes (N.B. – log into SCHOLAR before viewing)

  2. 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 SCHOLAR LINK

  3. 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.

  4. p-type connected to the +ve side of the battery and n-type connected to -ve side. Diode is forward-biased. SCHOLAR LINK Forward Bias 1

  5. 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.

  6. p-type connected to the -ve side of the battery and n-type connected to +ve side. Diode is reverse-biased. SCHOLAR LINK Reverse Bias 1

  7. 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 junction’s resistance becomes very large and so there is no current in the circuit.

  8. 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.

  9. 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.

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