Tunneling and thermionic emission through n+-GaAs-i-Al xGa1-xAs-n-GaAs heterojunction barriers are studied as a function of temperature from 77 to 200 K and as a function of externally applied uniaxial stress up to 10 kbar. A procedure to extract parameters for theoretical calculations is also proposed. The parameters extracted from the I-V characteristics of these heterostructures grown on (100) GaAs substrates with different aluminum mole fractions from 0.3 to 0.8 and thicknesses from 300 to 400 Å agree well with those of previous reports. The dependence of the I-V characteristics on uniaxial stress in the 〈100〉 direction perpendicular to the heterojunction plane has also been measured. The experimental results show good agreement with theoretical calculations assuming there is a linear stress-dependent decrease of the energy-band edges of the longitudinal X valleys (Xl) in AlGaAs with respect to the Γ valley in GaAs. The slope of the decrease is found to be 14±2 meV/kbar. This results in an X-valley shear deformation potential of 9.6±1.8 eV, which is believed to be the most accurate measured value to date.