High-quality two-dimensional electron gases (2DEGs) were regrown onto ex situ patterned substrates and the effect of the distance between the 2DEG and the contaminated regrowth interface was investigated. At a separation of 220 nm mobilities of 7.1 X 10(5) cm2 V-1 s-1 in the dark and 1.3 X 10(6) cm2 V-1 s-1 after illumination with a red light-emitting diode (LED) were measured with corresponding carrier concentrations of 2.4 x 10(11) and 4.3 x 10(11) cm-2. The quality of this 2DEG was equivalent to that attained in conventionally grown 2DEGs at the time, indicating that the 2DEG was not affected by presence of the regrowth interface in this case. For smaller separations a degradation in the 2DEG quality was observed. Thus, at a separation of only 60 mm, a mobility of 4.1 x 10(5) cm2 V-1 s-1 at a carrier concentration of 4.2 X 10(11) cm-2 was measured after illumination. The use of an independently contacted n+ layer in the patterned substrate as a backgate was also demonstrated.