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In this article, a comparison on the temperature dependence of the electron transport properties of the undoped and modulation-doped AlGaN/GaN heterostructures is presented. The results obtained indicate that the device structure plays an important role in the temperature-dependent transport behavior of the devices. The undoped structure has a nearly constant 2DEG concentration over a wide temperature range. The modulation-doped structure has a temperature-dependent electron concentration. The increase in electron concentration in the modulation-doped structure at high temperatures is due to the thermal activation of Si donors in the AlGaN layer. In addition, the modulation-doped structure shows comparable electron mobility with the undoped structure at high temperatures, indicating that the modulation-doped structure would exhibit a better device performance at high temperatures.