Doping of the confinement regions has a strong influence on the efficiency of double heterostructure LEDs. The resistivity of the confinement regions is one factor in determining the doping concentration in the confinement layers. The resistivity should be low to avoid resistive

heating of the confinement regions.

Another factor is the residual doping concentration in the active region. The active region has a residual doping concentration, even if not intentionally doped. Typical doping concentrations in the active region are in the 1015-1016 cm-3 range. The doping concentration in the confinement layers must be higher than the doping concentration of the active region to define the location of the p-n junction.

The influence of the confinement-layer doping concentration on the internal quantum efficiency was analyzed by Sugawara et al. (1992). The results are shown in Figs. 7.7 and 7.8. The figures reveal that there is an optimum doping range for the confinement regions. For n-type confinement regions, the optimum doping concentration ranges from 1016 to 2 x 1017 cm-3. For p-type confinement regions, the optimum doping concentration ranges from 5 x 1017 to 2 x 1018 cm-3, clearly higher than in the n-type cladding region. The reason for this marked difference could again lie in the larger diffusion length of electrons than that of holes. A high p - type concentration in the cladding region keeps electrons in the active region and prevents them from diffusing deep into the confinement region.

The carrier leakage out of the active region into the p-type cladding layer in double heterostructure lasers was investigated by Kazarinov and Pinto (1994). It was shown that the electron leakage out of the active region is more severe than the hole leakage. The difference is due to the generally higher diffusion constant of electrons compared with holes.

The band diagram of a double heterostructure under forward bias conditions is shown in Fig. 7.9. The figure illustrates that barriers are formed by the depletion layers at the two confinement-active-region interfaces. Compositional grading of these interfaces can be used to reduce these barriers.

The influence of the cladding layer doping concentration on the radiative efficiency of a double heterostructure laser at threshold is shown in Fig. 7.10 (Kazarinov and Pinto, 1994). Inspection of the figure reveals that the confinement layer doping has a severe influence on the