Double Heterojunction DH LED structure, working, advantages, disadvantages
This page covers structure,working operation,advantages and disadvantages of DH (Double Heterojunction) LED. It mentions DH (Double Heterojunction) LED advantages or benefits and DH (Double Heterojunction) LED disadvantages or drawbacks.
DH LED structure | DH LED working operation
• The figure-1 depicts DH LED structure. As shown it is GaAs/AlGaAs based Double Heterojunction LED.
• As shown thin layer of GaAs is sandwiched between two layers of AlGaAs. GaAs is lightly doped and has narrower bandgap (Eg1) of about 1.43 eV. AlGaAs layers have wider bandgap (Eg2) of about 2.1 eV.
• When forward bias is applied through its top and bottom contacts as shown in the figure,
electrons are injected from highly doped (n+) AlGaAs layer to
central active (p-) GaAs layer.
• The injected electrons are trapped within the middle layer due to double heterojunction potential barriers (Eg2 > Eg1) existing on both the sides of the middle layer. The figure depicts energy band diagram when it is forward biased.
• Electrons are forced to recombine with the holes without too much diffusion from interfaces. They recombine radiatively with energy equal to the band gap of GaAs.
• As recombination between electrons and holes is limited to narrower central part, internal quantum efficiency of such LED is higher compare to single junction LED.
• Moreover GaAs and AlGaAs are lattice matched. Hence surface density will be very smaller. In addition, emitted photons from active region is not absorbed by top and bottom AlGaAs layer due to Eg2 > Eg1 condition. Due to above reasons, overall performance of DH LED improves.
Benefits or advantages of DH (Double Heterojunction) LED
Following are the benefits or advantages of DH (Double Heterojunction) LED:
➨It offers higher efficiency with low to high radiance compare to single homojuction (p-n+) LED type.
➨Emitting wavelength of GaAs/AlGaAs based DH LEDs range approx. between 0.8 to 0.9 µm. InP/InGaAsP based LEDs are used for long wavelength radiation between 0.93 to 1.65 µm due to minimum signal attenuation.
➨Both n-region and p-region are made out of wide bandgap materials (Eg > h*V). Hence there is no absorption in these regions. They form optical windows.
➨n-region and p-region can be highly doped.
➨Injected electrons and holes are confined in a very narrow active region (i.e. quantum well) where, the n * p product is extremely high. Hence radiative recombination rate (R) is also high.
Drawbacks or disadvantages of DH (Double Heterojunction) LED
Following are the drawbacks or disadvantages of DH (Double Heterojunction) LED:
➨These LEDs are useful at low temperatures.
➨ It comes from very small potential barrier which electron encounters (in conduction band) when it reaches p-side of the junction.
➨There is complexity involved during growth process.