P-N Junction Diodes (N.B. – log into SCHOLAR before viewing) 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. Between each half the electrons and holes combine and cancel each other out This creates the DEPLETION LAYER SCHOLAR LINK

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.

Forward Bias 1 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 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. SCHOLAR LINK

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.

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 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.