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Diode Biasing

REVERSE BIAS

If the crystal has a negative voltage applied to the p-type and a positive voltage to the n-type crystals, then we REVERSE bias the diode. ( We could use a AA battery to do this! ) The positive holes are attracted AWAY from the junction. Similarly the negative electrons are also attracted AWAY from this region.

We have reinforced the internal E Field with the imposed external field. A large energy difference for electrons in the Conduction Band will now appear across the junction and electrons will fail to cross and merge with holes - it will fail to conduct. ( There is a breakdown point however, diodes deliberately designed to use this are called "Zener diodes" and are used to lock voltages in circuits! )

Similarly a large energy difference in the Valence Band will appear for holes so they will not cross the junction.

FORWARD BIAS

If the p-type is connected to positive and n-type to negative, then the different types of charge will merge at the junction and a full current will flow around the circuit.

The External Electric Field now cancels the internal field.

The Bands will be brought to the same energies and electrons and holes will be able to cross from one type to another. Electrons will then fall into holes and a current is established.

Energy is released at the junction in the form of light /and or heat depending on the types of diode material. Silicon, an Indirect Band-Gap semiconductor, tends to have heat created through change of momentum of the electrons as they drop from Conduction Band to Valence Band. However in Direct Band-Gap semiconductors, each fall of an electron releases a photon whose frequency is given by

ΔECV= hf 

If this light is allowed to escape we have a LED, a light emitting diode.