Thermal instability of low voltage power-MOSFET's

This paper analyzes an anomalous failure mechanism detected on last generat ion low voltage power metal oxide semiconductor (MOS) devices at low drain current. Such a behavior, apparently due to a kind of second breakdown phen omenon, has been scarcely considered in literature, as well as in manufactu rer data sheets, although extensive experimental tests show that it is a co mmon feature of modern low voltage metal oxide semiconductor held effect tr ansistor (MOSFET) devices. The paper starts by analyzing some failures, systematically observed on low voltage power MOSFET devices, inside the theoretical forward biased safe o perating area. Such failures are then related to an unexpected thermal inst ability of the considered devices. Experimental tests have shown that in th e considered devices the temperature coefficient is positive for a very wid e drain current range, also including the maximum value. Such a feature cau ses hot spot phenomena in the devices, as confirmed by microscope inspectio n of the failed devices. Finally, it is theoretically demonstrated that the thermal instability is a side effect of the progressive die size and proce ss scaling down. As a result, latest power MOSFET's, albeit more efficient and compact, are less robust than older devices at low drain currents, thus requiring specific circuit design techniques.