All data provided stem from current-voltage measurements of contacts to AlGaN/GaN heterostructures, following various annealing stages. These data have been used to assess the linearity of contact behaviour, and identify potential carrier transport mechanisms in non-linear regimes. Figure 2: Excel files containing current-voltage data for samples with a 29 nm Al0.28Ga0.72N barrier, with contacts annealed at (a) 700 °C, (b) 750 °C and (c) 800 °C, all for 30 s. Figure 3: Excel files containing current-voltage data for samples with an 8 nm Al0.39Ga0.61N barrier, with contacts annealed at (a) 700 °C, (b) 750 °C and (c) 800 °C, all for 30 s. Figure 4: Excel files containing logarithmic current-voltage data for samples with the thicker AlGaN barrier, annealed at (a) 700 °C and (b) 750 °C for 30 s. Figure 5: Excel files showing the current-voltage data from the sample with 29 nm AlGaN barrier, contacts annealed at 700 °C, after processing in the context of different mechanisms. In (a), the data are processed to a Fowler-Nordheim tunnelling model, and (b) is a subset of (a). In (c) the data are processed to plot in an Arrhenius manner. In (d) the data are processed to a direct tunnelling model. (b), (c) and (d) also contain the linear fits generated by the Origin graph plotting software. Figure S1: Excel file showing current voltage data from the same sample as Figure 5, processed instead to fit a Frenkel-Poole emission model. Figure S2: Excel file showing current voltage data from the same sample as Figure 5, processed instead to fit a trap-assisted tunnelling model.