Inverted modulation doped structures typically exhibit degraded electrical characteristics. For the AlInAs‐GaInAs heterojunction system, the reduction in electron mobility for two‐dimensional electron gases formed at inverted interfaces can be greater than 50% at 300 K as compared to those formed at normal interfaces. Our data show that the reduction in mobility is due to the movement of Si into the GaInAs channel. The Si movement is found to be dramatically reduced by growing the AlInAs spacer at the inverted interface at a substrate temperature of 300–350 °C. Device structures have been grown using this technique which exhibit the highest conductivity obtained for any 2DEG system.