Light Emitting Diode
A Light-Emitting Diode (LED) is a P-N junction solid-state semiconductor diode that emits light when a current is applied though the device. By scientific
definition, it is a solid-state device that controls current without the deficiency of having heated filaments.
- Figure shows a typical traditional indicator LED. Traditional indicator LEDs utilize a small LED semiconductor chip that is mounted on a reflector cup also known as the anvil, on a lead-frame (whisker).
- This whole configuration is encased in epoxy which also serves the purpose of a lens.
- LEDs have very high thermal resistance with upwards of 200K per Watt.
Principle & Mechanism:
- In figure,The chip is further divided into two parts or regions which are separated by a boundary called a junction.
- The p-region is dominated by positive electric charges (holes) and the n-region is dominated by negative electric charges (electrons).
- The junction serves as a barrier to the flow of the electrons between the p and the n-regions.
- This is somewhat similar to the role of the band-gap because it determines how much voltage is needed to be applied to the semiconductor chip before the current can flow and the electrons pass the junction into the p-region.
- In general, to achieve higher momentum states (with higher velocities), there must bean empty energy state into which the electron may be excited.
- Band-gaps determine how much energy is needed for the electron to jump from the valence band to the conduction band.
- As an electron in the conduction band recombines with a hole in the valence band, the electron makes a transition to a lower-lying energy state and releases energy in an amount equal to the band-gap energy.
- This energy is released in photons. Normally the energy heats the material. In an LED this energy goes into emitted infrared or visible light.
The bandgap energy, Eg is approximately equal to the emitted photon’s energy.
Eg = h ν
where h is the Planck’s constant , h = 6.626 x 10-34
Js =4.135 x 10-15 eVs
The number of photons may be obtained via the following expression
N = E / (hν) = (PΔt )/[h(c/λ)] = (λPΔt)/(hc)
The diode current on the other hand, is related to the band-gap energy via the following formula
J = J1 exp [(e(V-Vg))/kT] for eV/kT >>1