Showing posts with label Semiconductor Physics. Show all posts
Showing posts with label Semiconductor Physics. Show all posts
Thursday, 22 December 2011
Wednesday, 21 December 2011
Tuesday, 20 December 2011
Light Emitting Doide
| Almost all LEDs are made of gallium with arsenic, nitrogen and phosphorus additives. A light emitting diode is a standard diode made of semiconductors of the Direct Band-Gap type of material with its junction physically arranged so it freely emits its photons in the desired direction. The diode is encased in transparent coloured or clear plastic often rounded so as to act as a lens. ( ref ) In direct band-gap materials, electrons do not change momentum when moving from conduction band to valence band so no momentum is transferred to the surrounding structure. Heat is not generated but light is. |  |
As a DBG diode conducts in the forward direction, electrons fall from the Conduction Band to the lower energy Valence Band. From basic quantum considerations of atoms, electrons falling inwards under the influence of the Electric Field of the nucleus must emit photons with an energy equal to the difference in energy of the two bands.
ΔECV= hf
Because red light is of lower photonic energy, these were the first to be created as the technology was simplest. Blue LEDs are relatively recent devices ( ref ) using Gallium Nitride as the basic semiconductor as it has the appropriate gap between bands ( band gap ).
From the above equation, red has ΔECV~ 2.0 eV whereas for blue light ΔECV~ 4.0 eV.
Ironically, white LEDs currently are blue/UV LEDs with fluorescent impurities in the casing plastic similar to those used in domestic fluorescent lights. The deep blue/UV components are absorbed by the fluorescents and reemitted across the whole spectrum. This is because the current generation of green LEDs are very inefficient and produce too much heat compared to light.
Semiconductor diode animation link
How exactly charge carries flows inside the diode with different bias voltages are wonderfully explained in this animation . Please refer the below animation for Crystal clear concept in diode.
Semiconductor Physics Doide Biasing
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
