This paper presents a review of models for direct tunnelling with a view to identifying suitable models for inclusion in a circuit simulator. For thin oxides, the critical quantities required for the derivation of tunnel current are the transparency of the barrier, the oxide field and the supply of carriers for tunnelling. This paper reviews different approaches to the incorporation of these quantities in analytical models. A new model for direct tunnelling, which includes quantum effects in a format suitable for circuit simulation, is outlined. Recent developments in MOSFET models, which include gate current, are briefly discussed. (C) 2001 Elsevier Science Ltd. All rights reserved.