Surface Emitting LED: Structure, Working, Advantages & Disadvantages

This article explains the structure, operation, advantages, and disadvantages of Surface Emitting LEDs. ​Surface Emitting LEDs (Light Emitting Diodes) are widely utilized in optical communication systems due to their straightforward design and efficient optical coupling capabilities.

Surface Emitting LED Structure & Working Operation

• The Surface Emitting LED is a modified form of the Double Heterojunction (DH) LED. In this design, the optical fiber is butt-coupled directly to the LED. (See Figure 1)

• The structure consists of a thin central active layer of p-type GaAs.

• This central layer is bounded by n-type AlGaAs / n+-type GaAs on the top side.

• On the bottom side, the central layer is bounded by p-type AlGaAs / p+-type GaAs.

• The topmost n+-type GaAs and the bottom p+-type GaAs layers are used exclusively to provide low-resistance ohmic contacts.

• The external optical fiber is connected by etching the top layers and shielding with epoxy resin.

• When the refractive indices of both p-type and n-type materials are the same, light is free to exit from all sides of the semiconductor due to the absence of confinement. However, only the active region near the surface emits a significant amount of light while the rest of the device absorbs light. This is why it is known as a “Surface Emitting LED.”

• The output radiation originates from the central thin layer (p-type GaAs layer).

• The maximum optical power (Pc) coupled with the optical fiber is expressed as follows:

  Pc = π * (1-R) * A * Rs * (NA)^2

  Where:

  • R = Reflection loss at the fiber surface
  • A = Fiber cross-section area
  • Rs = Radiance amount from the source
  • NA = Numerical Aperture

Advantages of Surface Emitting LEDs

Here are some of the benefits of using Surface Emitting LEDs:

• High Optical Coupling Efficiency: The optical coupling coefficient between the LED and the external fiber system is relatively high, resulting in better optical coupling efficiency.

• Low Optical Loss: Internal absorption is minimal due to carrier recombination occurring near the top heterojunction.

• Suitable for Long Wavelength Applications: InP/InGaAsP-based LEDs are used for applications requiring long wavelengths.

• High Efficiency & Radiance: Offers higher efficiency and can operate at both low and high radiance levels.

• Low Thermal and Contact Resistance: The top n-GaAs contact layer ensures low thermal resistance and contact resistance. This allows for high current densities and high radiation intensity.

• Good Forward Radiance: The internal absorption within the device is very low because of the larger bandgap confining layers. Furthermore, the high reflection coefficient at the back crystal face results in good forward radiance.

Disadvantages of Surface Emitting LEDs

These are the drawbacks of Surface Emitting LEDs:

• Lower Data Rate: Surface Emitting LEDs typically transmit data at rates lower than 20 Mbps, which is less than that of Edge Emitting LEDs.

• Short Optical Link with Large NA: They are characterized by a short optical link and a large Numerical Aperture (NA). This may limit their suitability for long distance communication.

Conclusion

​Surface Emitting LEDs (Light Emitting Diodes) are semiconductor devices that emit light perpendicular to their surface, offering specific advantages and disadvantages compared to other LED types as mentioned.