Since the early days of SPICE [50] device modeling for circuit simulation - especially of MOSFET circuits - has been a complex topic of intense investigation. Considering the time passed by one might expect that the physics of MOSFETs is perfectly well understood and compact models are readily available for circuit simulation. However, quite the opposite is true: as circuit technique on the one hand and process technology on the other hand incite each other to create faster, smarter, and more complex circuits and devices, an increasing effort is required for device modeling to keep pace with technology, all the more with modern deep-sub-micron and Ultra-Low-Power technologies. One consequence of this situation is a surge of the number of different models which are often specialized for specific applications (e.g., the HSPICE user's manual lists 50(!) different MOSFET models [47]).
This chapter will first discuss the problems associated with device modeling in more detail, giving the motivation for the development of new device models and further refinement of existing ones. Finally, in Section 4.3 a new device model which was developed especially for the simulation of ULP circuits, is presented.