Static and dynamic thermal coupling in SOI NMOS current mirrors

Self-heating in SOI MOSFETs has been studied extensively in recent years, both in static and dynamic terms. The focus of these studies have so far been restricted to the behaviour of single devices. However, for circuit design, the issue of self-heating must also be examined in terms of how heat generated in one device affects its neighbours. This is of particular importance in analogue circuits where the operation is based on precise matching. Consider a simple current mirror; for ideal behaviour the output transistor should have exactly the same device characteristics as the reference transistor, and hence isothermal conditions are normally required. Further, for the best possible transistor matching, output and reference devices are normally placed in close proximity. In SOI technology, where heat flow through the buried oxide is poor, it is clear that there may be significant heat transfer from one device to another, depending on the layout of the current mirror