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We compare the performance of amorphous silicon/silicon nitride field-effect transistors having either nitride or silicon deposited first. In the case of silicon being the first layer, underlying source and drain contacts of aluminium or chromium were compared (as aluminium may react or diffuse more easily than chromium). The best devices required the nitride to be deposited first, following which it was essential to remove impurities such as ammonia from a single chamber system by dilution purging, before depositing silicon. Contributory influences on the poor characteristics of the alternative structure include the higher temperature necessary for nitride deposition, even when this overlies the silicon, and the bombardment of the silicon surface with energetic species from the nitride source gases.