Carrier distribution in p-n heterojunctions
All high-intensity light-emitting diodes do not use the homojunction design but rather employ heterojunctions, which have clear advantages over homojunction devices. Heterojunction devices employ two types of semiconductors, namely a small-bandgap active region and a large-bandgap barrier region. If a structure consists of two barriers, i. e. two large-bandgap semiconductors, then the structure is called a double heterostructure (frequently abbreviated as DH).
The effect of heterojunctions on the carrier distribution is shown in Fig. 4.8 (c). Carriers injected into the active region of the double heterostructure are confined to the active region by means of the barriers. As a result, the thickness of the region in which carriers recombine is given by the thickness of the active region rather than the diffusion length.
The consequences of this change are significant. We assume that the thickness of the active region is much smaller than the typical diffusion length. Diffusion lengths may range from 1 to 20 |im. The active region of double heterojunctions may range from 0.01 to 1.0 |im. Thus, carriers in the active region of a double heterostructure have a much higher concentration than carriers in homojunctions, which are distributed over several diffusion lengths. Recalling that the radiative recombination rate is given by the bimolecular recombination equation, i. e.
R = B np (4.20)
it is clear that a high concentration of carriers in the active region increases the radiative recombination rate and decreases the recombination lifetime. For this reason, all high-efficiency LED designs employ double heterostructure or quantum well designs.