In the structure of light-emitting diode construction, people talk about the active region in between p and n-type semiconductors. Why do we require that active region? Is that active region just the depletion layer of pn junction diode? If so why do we need an active layer in LED?
Can someone help me in understanding this concept?
In principle, the active region of LED could consist of depletion region between n- and p-type layers of the same material. For examples, the first blue LEDs were of this type (GaN codoped with Zn and Si). However, modern LEDs use some type of heterostructure (quantum wells or quantum dots) as an active region sandwiched between n- and p-type material. For example, blue LEDs consist of nGaN, InGaN/GaN quantum wells/barriers (= active region), and pGaN (+ some other layers like AlGaN electron blocking layer etc.). The advantage of this concept is mainly:
1. InGaN alloy has smaller bandgap than GaN. Therefore, the light emitted from InGaN QWs has lower energy than GaN bandgap and is only weakly reabsorbed in the GaN material.
2. InGaN QWs form potential minima for electrons and holes. Therefore, excited carriers tend to accumulate in QWs. High concentration of carriers means high probability of radiative recombination. The non-radiative recombination through deep non-radiative centers is relatively suppressed in comparison to bulk material as the concentration of non-radiative centers inside QWs is limited but the concentration of accumulated carriers is high. On the other hand, high concentration of carriers can lead to non-radiative Auger recombination.
3. In the case of InGaN/GaN heterostructure, the InGaN alloy is very beneficial to suppress the non-radiative recombination through dislocations which are usually present in high concentrations.
https://www.physics-and-radio-electronics.com/electronic-devices-and-circuits/...
In LED, most of the charge carriers recombine at active region. Therefore, most of the light is emitted by the active region. The active region is also called as depletion region.
Dear Bibek S Dhami ,
The light emitting diodes are constructed such that one gets the highest conversion efficiency from electrical energy to photoenergy.
All light emitting diodes are based on the generation of light by making excess electrons and hole recombine with other emitting photons.
The excess electrons and holes are formed by injection across junctions.
These junctions may be homojunctions or heterojunctions.
The light emitting diodes have gone tremendous structural modifications to achieve the highest conversion efficeincy.
At first pn diodes are used.
Then one found that the pin diode are better because they affect double injection form the p side and the n side and the i-region will be flooded by the electrons and holes.
But homojunctions leak larger back injection leading to limiting the electron and hole injection in the i-region. This effect leads to what is called the droop of the emission at high diode currents.
To solve this problem the one used heterojunctions instead of t he homojunctions to suppress the back injection from the active regions to the layers of the injectors.
In addition to confine the electrons and holes inside the active region one built them from quantum wells which help trap the injected and electrons inside the well and preventing them from back injection into the injector region.
This leads to a tremendous improvement in the conversion efficeincy of the diode.
There is now power LEDS lamps used for illumination thanks to the developments in the active region of the LED which is mostly the region of radiation emission.
Best wishes
Depletion layer or region
It is a region or layer between p-type and n-type semiconductor where no charge carriers (free electrons or holes) are present. It is a barrier to current. To overcome this barrier, we need to apply voltage which is greater than barrier potential of this layer.
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