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The electron transport characteristics of five n-i-n diodes with (Al/sub x/Ga/sub 1-x/)/sub 0.5/In/sub 0.5/P intrinsic barrier regions of various aluminum composition x were determined from the measured I-V characteristics between 60 and 310 K. From these measurements, three different transport regimes were identified. Fowler-Nordheim tunneling was observed at temperatures below 215, 260, 110, 150, and 120 K for aluminum compositions of x=0.4, 0.5, 0.6, 0.7, and 1.0, respectively, with applied electric fields in excess of 5 MV/m. The temperature dependence of the Fowler-Nordheim tunneling currents is shown in AlGaInP for the first time with direct bandgap AlGaInP exhibiting a strong linear decrease in apparent barrier height with increasing temperature. The measured barrier height using the thermionic emission model yields values close to the expected conduction band offset between the GaInP spacer layers and the AlGaInP intrinsic barriers, as measured using high-pressure photoluminescence, and provides a novel technique for measuring the direct-indirect crossover composition in AlGaInP. It is shown that the lowest lying conduction band in AlGaInP is the dominant barrier to electron transport. This has important implications for the design of AlGaInP laser diodes.