Tunnel junctions (TJ) made of p-Al0.1 Ga0.9As/n-GaAs are used because of their high peak current and low series resistance, but are not fully transparent. The influence of reducing the thickness of these tunnel junctions on the characteristics of InGaP/GaAs tandem cells was investigated. It was found that ultra-thin TJs with excellent performance can be realized. Even for a 7.5/6-nm thick TJ, which is the thinnest possible in our growth reactor, the peak current density is at least 600 A/cm2. The series resistance of the TJs was found to be at a constant level of 0.6 ± 0.2 mΩ cm2 for all total thicknesses of the TJ in the 13.5–40 nm range. Because of a lower absorption in the TJ, a tandem cell with a 7.5/6-nm thick TJ, compared with a cell with a 20/20-nm thick TJ, gained 0.53 ± 0.05 mA/cm2 in short circuit current to a value of 14.8 mA/cm2. Copyright © 2012 John Wiley & Sons, Ltd.