Double Heterojunction (DH) LED: Structure, Working, Advantages, and Disadvantages

This article explores the structure, operation, advantages, and disadvantages of the Double Heterojunction (DH) LED.

DH LED Structure and Working Operation

  • Structure: The figure below depicts a GaAs/AlGaAs based DH LED. A thin layer of GaAs is sandwiched between two layers of AlGaAs. The GaAs layer is lightly doped and has a narrower bandgap (Eg1) of about 1.43 eV. The AlGaAs layers have a wider bandgap (Eg2) of about 2.1 eV.

    DH LED, Double Heterojunction LED structure

  • Working Operation:

    • When a forward bias is applied through the top and bottom contacts, electrons are injected from the highly doped (n+) AlGaAs layer to the central active (p-) GaAs layer.
    • The injected electrons are trapped within the middle layer due to the double heterojunction potential barriers (Eg2 > Eg1) existing on both sides of the middle layer. This creates a “quantum well” effect.
    • The electrons are forced to recombine with holes without significant diffusion from the interfaces. They recombine radiatively with energy equal to the band gap of GaAs.
    • Because recombination is limited to the narrower central part, the internal quantum efficiency of such an LED is higher compared to single junction LEDs.
    • Moreover, GaAs and AlGaAs are lattice-matched, resulting in a very small surface density of defects.
    • Emitted photons from the active region are not absorbed by the top and bottom AlGaAs layers due to the Eg2 > Eg1 condition (the AlGaAs is transparent to the light emitted by the GaAs).
    • Due to these factors, the overall performance of the DH LED is improved.

Benefits (Advantages) of DH (Double Heterojunction) LED

  1. Offers higher efficiency with low to high radiance compared to single homojunction (p-n+) LEDs.
  2. Emitting wavelengths of GaAs/AlGaAs based DH LEDs range approximately between 0.8 to 0.9 µm. InP/InGaAsP based LEDs are used for longer wavelength radiation between 0.93 to 1.65 µm due to minimized signal attenuation in optical fibers at these wavelengths.
  3. Both the n-region and p-region are made out of wide bandgap materials (Eg > h*V), acting as optical windows and preventing absorption in these regions.
  4. The n-region and p-region can be highly doped.
  5. Injected electrons and holes are confined in a very narrow active region (i.e., a quantum well) where the n * p product is extremely high. Hence, the radiative recombination rate (R) is also high, leading to efficient light emission.

Drawbacks (Disadvantages) of DH (Double Heterojunction) LED

  1. These LEDs are most effective at low temperatures.
  2. The very small potential barrier encountered by electrons as they reach the p-side of the junction can limit performance.
  3. The growth process is complex and requires precise control over the materials and doping profiles.