The aim of the paper is to review certain aspects of the electronic structure and properties of metal-a-semiconductor barriers which are of relevance to the theme of this Conference. The discussion will be confined to amorphous silicon (a-%) prepared by the plasma decomposition of silane, which is of much current interest as an electronically controllable model material. A brief survey of preparation and of some electronic properties of a-Si will be given. We then turn to the model of an ideal Schottky barrier and discuss its application to metal-a-Si contacts where the localised state distribution in the a-semiconductor has an important effect on barrier space charge and profile. In a more realistic model interfacial states at a thin oxide-semiconductor interface are taken into account. In this connection we shall discuss recent experimental work using internal photoemission to investigate the height of metal/a-Si barriers as a function of the metal workfunction. Analysis of these and other results suggests that interfacial state densities on a-Si barriers lie between lOI and 10"' crn~~' eV I, remarkably similar to corresponding values for crystalline Si barriers. The final section of the paper deals with the electronic structure of a-Si p+-i-n+ junctions. The promising photovoltaic
properties of these devices will briefly be considered.