We discuss the interaction of alkali metals with semiconductors with emphasis on the nature of bonding. The particular system chosen to illustrate our ideas is the adsorption of potassium on silicon surfaces. The tendency towards metallization as a function of coverage is shown to differ from that usually believed to occur for alkali metals on transition metals. It is found that alkali metals on semiconductors form a strong ionic bond even up to monolayer coverages. The metallization of the overlayer is shown to be energetically unfavorable due to the presence of the dangling bonds. Thus the overlayer metallization is either suppressed or occurs at rather high coverages. We suggest that the tendency towards metallization at high coverages is the surface analog of the ionic-metal transition discussed by Phillips. © 1988, Pergamon Press plc. All rights reserved.