Fur drift field generation. IS structures with a positive interface charge in an insulator compound were developed in the 1980s and employed mainly on field effect solar cells. While there has been a large number of publications about the positively biased insulator-semiconductor structure there has been very little interest in its antipolar counterpart - the negatively biased insulator-semiconductor structure so far. However. by using that structure as a second drift field source for field effect solar cells their conversion efficiency could be improved. The conventional rear surface passivation of silicon active layers by shallow ultrahigh aluminium doping in order to form a back surface field involves a high density of lattice and surface defects which increase recombination rates. The structure introduced herein is put onto a silicon layer and acts thus as an external negative drift field source, thereby avoiding the drawbacks of ultrahigh doping. The layer arrangement aluminiumfluoride-silicon dioxide on silicon seems to be a good candidate for a negative drift field source. Experimental results which led to a fixed interface charge of up to - 1.6 x 10(12) cm(-2) at the interface aluminiumfluoride parallel to silicon dioxide are presented and an interpretation of the fixed negative charge is given.