There are several ways to connect power leads to the junctions comprising a multijunction stack. These configurations, which provide for varying degrees of electrical isolation of the subcells, are illustrated in Figure 8.4c for a two-subcell stack. In the four-terminal configuration, each subcell has its own two terminals and is electrically isolated from the other subcells. This configuration has the advantage that it sets no constraints on the polarities (p/n vs n/p) of the subcells, or on their currents or voltages. However, the terminals and the electrical isolation between subcells in the four-terminal configuration would be inconvenient to accomplish monolithically, requiring a complicated cell structure and processing. Generally, a four-terminal device is implemented as a mechanical stack, whose complexities of fabrication and assembly make it a significantly less desirable structure than the monolithic device. In addition, unique power controls are needed to keep each cell at its maximum power point and then to sum the power “off-chip”.