Results on gate-length scaling of the performance of enhancement-mode heterostructure field-effect transistors (HFET's) for gate lengths between 0.4 and 10 μm, are reported. The devices studied were fabricated by a self-aligned gate process. Transconductances as large as 534 mS/mm were achieved with 0.4-μm, devices. We compare two types of pseudomorphic AlGaAs/InGaAs/GaAs heterostructures. One of them is used for modulation-doped FET's and the other for doped-channel FET's. We find that the effects of electron velocity saturation are different for the two types of devices due to the dominance of charge transfer and gate leakage in the conventional modulation-doped device. The experimental results are explained in the framework of a simple charge control model.