FIGURE 3.2 Schematic of p-n junction diode.
holes and improve the likelihood of radiative recombination. This localization is accomplished by introducing quantum wells in the region of the junction. These are thin slivers of lower bandgap-materials that, as their name implies, serve as wells that confine pools of electrons and holes to increase the probability that they will recombine radiatively.
The external view of the typical LED structure is given in Figure 3.3, showing the N-type GaN, the InGaN quantum wells, and the P-type GaN. Most GaN LED devices are formed on a sapphire substrate through the MOCVD process. Typically, one 4-inch-diameter sapphire wafer can produce 5,000 individual devices or “dies.” The 16 percent mismatch in natural lattice size between the sapphire substrate and the GaN overlayers has important consequences on device performance and on the uniformity of the dies grown from a single wafer. This is further discussed in the section “Materials Issues.” In order to connect the device to the outside world, metal contacts must be deposited by evaporation on the N and P regions. Figure 3.3 shows these metal contacts, as well as the transparent and conductive indium tin oxide (ITO) layer that extends the top-side electrical contact over the device surface. Both the sapphire substrate and the ITO spreader contact are transparent to the emitted light, as is necessary for the light to leave the device. High-quality electrical contacts are important to reduce loss due to resistance (R) to current flow (I) in the contact region. This is even more important when the device is operated at high currents or current densities, because loss of power due to resistive heating scales as I2R. In the III-nitride materials, it is a challenge to dope the materials to a sufficient level so that resistances are low, particularly for P-type materials. The formation of the device structure shown in Figure 3.3 is just a starting point for the fabrication of the final solid-state “light-bulb.” An individual device must be further “packaged” to better control its chemical, thermal, and electrical environment and to better integrate it into the final luminaire.
The LED package is the structure in which the LED chip is mounted and through which access to the LED terminals is provided. It is an important part of the finished device. The package serves the following functions: (1) it passivates or protects the active semiconductor material from degradation due to the environment (principally moisture); (2) it integrates an optical lens structure, which determines the optical emission pattern of the structure; (3) it removes heat from the device, protecting against degradation due to overheating; and (4) it protects the device from electrostatic discharge
FIGURE 3.3 A typical GaN LED chip.