I-V-T data is routinely used to determine the conduction band discontinuity in heterojunction structures. In the present paper, capacitance-voltage and current-voltage-temperature measurements performed on AlGaAs/GaAs isotype heterojunctions are presented and analysed over a wide temperature range (77 K-300 K). Considering thermionic emission alone when analysing I-V-T data resulted in several problems. The Richardson plot $[\ln(J_0/T^2) vs. 1/T]$, in particular while suggesting that the thermally activated process is of importance in the overall conduction mechanism, shows two distinct linear regions of different slope over two temperature ranges. Also the derived activation energies and hence the band discontinuity from I-V-T data is very much lower than the value obtained from C-V profiling which is in very good agreement with values routinely published in literature. However, the results obtained from both I-V-T and C-V data are reconciled when considering a simple analytical expression for the current based on the assumption that thermally assisted tunneling is the dominant current generating mechanism over most of the temperature range.