  We have fabricated quantum dot single electron transistors, based on
AlGaAs/GaAs heterojunctions without modulation doping, which exhibit clear and
stable Coulomb blockade oscillations. The temperature dependence of the Coulomb
blockade peak lineshape is well described by standard Coulomb blockade theory
in the quantum regime. Bias spectroscopy measurements have allowed us to
directly extract the charging energy, and showed clear evidence of excited
state transport, confirming that individual quantum states in the dot can be
resolved.
