Tunnel Junctions, as addressed in this review, are conductive, optically transparent semiconductor layers used to join different semiconductor materials in order to increase overall device efficiency. The first monolithic multi-junction solar cell was grown in 1980 at NCSU and utilized an AlGaAs/AlGaAs tunnel junction. In the last 4 decades both the development and analysis of tunnel junction structures and their application to multi-junction solar cells has resulted in significant performance gains. In this review we will first make note of significant studies of III-V tunnel junction materials and performance, then discuss their incorporation into cells and modeling of their characteristics. A Recent study implicating thermally activated compensation of highly doped semiconductors by native defects rather than dopant diffusion in tunnel junction thermal degradation will be discussed. AlGaAs/InGaP tunnel junctions, showing both high current capability and high transparency (high bandgap), are the current standard for space applications. Of significant note is a variant of this structure containing a quantum well interface showing the best performance to date. This has been studied by several groups and will be discussed at length in order to show a path to future improvements.

Multiple quantum wells (MQW) lattice matched to GaAs consisting of In_0.14Ga_0.76As wells balanced with GaAs_0.24P_0.76 barriers have been used to extend the absorption of GaAs subcells to longer wavelengths for use in an InGaP/GaAs/Ge triple-junction photovoltaic cell. Thin barriers with high-phosphorus composition are capable of balancing the strain from the InGaAs wells; thus, creating conditions to allow for thicker wells and for carrier tunneling to dominate transport across the structure. As a result, a larger percentage of the depletion region is occupied by InGaAs quantum wells that absorb wavelengths beyond 875 nm and the indium composition is not limited by thermionic emission requirements. Measurements at elevated temperatures and reverse bias suggest that a thermally assisted tunneling mechanism is responsible for transport through the barriers.

Atomic layer epitaxy (ALE) of GaAs with a growth rate as high as 2 μm/h was achieved in a specially designed reactor based on the rotating susceptor concept. High deposition rates are made possible by a unique partition system that permits rapid rotation of substrates between simultaneous streams of columns III and V reactant gases. Mixing of the gas streams at high rotational speeds is avoided by the gas shearing effect provided by the partition system. Background carbon levels in the GaAs films with high growth rates varied from mid 1017 cm−3 at 650 °C to mid 1018 cm−3 at 550 °C. When the growth rate was reduced to 0.3 μm/h, carbon background doping in the 1015 cm−3 was achieved.