4.9. LIGHT EMITTING DIODE (LED) 197
EC
EF
EV
Eg Emission
tQW
Figure 4.32: Band diagram of a single quantum-well LED with the advantages of increased car-
rier density, enhanced confinement and reduced probability of re-absorption of emitted photons
in bulk layers.
4.9.2 Carrier Injection and Spontaneous Emission................
The LED is essentially a forward-biasedp-ndiode, with a quantum well emission region as
shown in figure 4.32. The reason for using a quantum well is to (i) increase the electrons and
hole density in the recombination region increasing the direct recombination rate and leading
to higher light output, (ii) having an emission region that is lower in energy that the injection
(cladding) regions which allows the generated photons to escape without being re-absorbed in the
injection regions, (iii) minimizing the overflow of electrons into the cladding regions where the
injected carriers either recombine non-radiatively or generate light of an undesired wavelength.
The current flow in a p-n junction was discussed in detail earlier in this chapter.The basis of
that derivation was that electrons and holes are injected across the junction and recombine either
in the bulk(long base case) or at contacts (short base case). Neither of those conditions apply
to an LED. Here the current flow occurs via recombination in the quantum well. The turn-on
voltage of the LED is therefore given by the bandgap of the emission region and is not explicitly
related to the built-in voltage of the p-n junction. An example of this is an InGaN LED grown
within GaN p-and n regions. The built-in voltage of this device is close to the bandgap of GaN
(3.4V) though the turn-on voltage is 2.8V close to the emission energy of the photons. The
current flow mechanism is shown in figure 4.33. The current is given byJ=e·Rsponwhere
Rsponis the spontaneous recombination rate in the well. The efficiency of the process is the