Gate performance for observing Coulomb oscillations and Coulomb diamonds are compared for two types of gated sub-µm double-barrier heterostructures. The first type of device contains modulation-doped barriers, whereas the second type of device contains a narrower band gap material for the well and no barriers with doped impurities. Both the Coulomb oscillations and Coulomb diamonds are modified irregularly as a function of gate voltage in the first type of device, while in the second type of device they are only systematically modified, reflecting atom-like properties of a quantum dot. This difference is explained in terms of the existence of impurities in the first type of device, which inhomogeneously deform the rotational symmetry of the lateral confining potential as the gate voltage is varied. The absence of impurities is the reason why we observe the atom-like properties only in the second type of device.