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.

Tunnel junctions (TJs) have recently been proposed as a solution for several III-nitride current problems and to enhance new structures. Reported III-nitride TJs grown by metalorganic chemical vapor deposition (MOCVD) resulted in backward diodes with rectifying behavior in forward bias, even with Mg and Si doping in 10²⁰ cm⁻³. This behavior limits applications in several device structures. We report a TJ structure based on p⁺In_0.15Ga_0.85N/n⁺In_0.05Ga_0.95N, where the n-side of the junction is co-doped with Si and Mg and with electron and hole concentrations in the mid-10¹⁹ cm⁻³ for both the n and p dopants. Co-doping creates deep levels within the bandgap that enhances tunneling under forward biased conditions. The TJ structure was investigated on both GaN substrates and InGaN templates to study the impact of strain on the TJ I–V characteristics. The resulting TJ I–V and resistivities reported indicate the potential for this TJ approach in several device structures based on III-nitrides. We are not aware of any previous MOCVD grown TJs that show Ohmic performance in both forward and reverse biases.

We present metalorganic chemical vapor deposition-grown III-nitride tunnel junction (TJ) devices showing negative differential resistance (NDR) under forward bias with a peak to valley ratio of 1.3 at room temperature. Previously, NDR in GaN material systems has only been achievable utilizing molecular beam epitaxy or polarization enhanced AlGaN interlayers. The TJ devices presented here utilize structures based on p+InGaN/n+InGaN materials with the n-side of the junction doped with both Si and Mg and with electron and hole concentrations roughly in the 1019 cm−3 range. The Mg precursor flow is maintained at a constant rate during the whole TJ growth. This co-doped technique can eliminate several Mg-related issues such as delayed incorporation, the memory effect, and Mg solid-state diffusion. Structures grown on relaxed InGaN semibulk templates show enhanced hole concentrations and improved TJ performance.