IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL.15, NO. 3, MAY 2000 575 Thermal Instability of Low Voltage Power-MOSFET’s Alfio Consoli, Fellow, IEEE, Francesco Gennaro, Student Member, IEEE, Antonio Testa, Member, IEEE, Giuseppe Consentino, Ferruccio Frisina, Romeo Letor, and Angelo Magrì Abstract—This paper analyzes an anomalous failure mechanism detected on last generation low voltage power metal oxide semi- conductor (MOS) devices at low drain current. Such a behavior, apparently due to a kind of second breakdown phenomenon, has been scarcely considered in literature, as well as in manufacturer data sheets, although extensive experimental tests show that it is a common feature of modern low voltage metal oxide semiconductor field effect transistor (MOSFET) devices. The paper starts by analyzing some failures, systematically ob- served on low voltage power MOSFET devices, inside the theo- retical forward biased safe operating area. Such failures are then related to an unexpected thermal instability of the considered de- vices. Experimental tests have shown that in the considered devices the temperature coefficient is positive for a very wide drain current range, also including the maximum value. Such a feature causes hot spot phenomena in the devices, as confirmed by microscope inspec- tion of the failed devices. Finally, it is theoretically demonstrated that the thermal instability is a side effect of the progressive die size and process scaling down. As a result, latest power MOSFET’s, al- beit more efficient and compact, are less robust than older devices at low drain currents, thus requiring specific circuit design tech- niques. Index Terms—Average temperature, bipolar devices, FBSOA, MOSFET, thermal instability, threshold voltage. I. INTRODUCTION T HE FORWARD biased safe operating area (FBSOA) defines the recommended operating points of a power device. The FBSOA of a bipolar device is normally limited by the maximum allowable values of voltage, current, and power. Operations inside the second breakdown area must be accu- rately avoided, since current focusing phenomena are otherwise activated causing the immediate failure of the device. Second breakdown mechanism is traditionally absent in power metal oxide semiconductor field effect transistor (MOSFET) devices, although spurious activations of the parasitic bipolar device may theoretically involve some current focusing phenomena [1]. Since the modern technology allows to avoid activation of the parasitic bipolar device, the FBSOA of a power MOSFET normally does not include a second breakdown limitation. Consequently, power MOSFET’s feature a wider operating area if compared with bipolar devices. Apparently in contrast with the general assumption that the FBSOA of a MOSFET does not have the second breakdown Manuscript received May 25, 1999; revised November 22, 1999. Recom- mended by Associate Editor, W. M. Portnoy. A. Consoli and F. Gennaro are with the University of Catania, Catania 95125, Italy. A. Testa is with the University of Messina, Messina 98166, Italy. G. Consentino, F. Frisina, R. Letor, and A. Magrì are with ST-Microelec- tronics, Catania 95100, Italy. Publisher Item Identifier S 0885-8993(00)03384-6. limitation, an anomalous failure mode has been experimentally detected on low voltage power MOSFET devices inside the the- oretical FBSOA at low drain current. Such an anomalous be- havior, looking similar to a second breakdown, is very little con- sidered in literature and in manufacturer data sheets, although extensive experimental tests have shown that it is a common fea- ture of new generations of low voltage ( V) MOSFET de- vices. Moreover, it has been observed that such apparent second breakdown mechanism marginally affects the FBSOA of high voltage MOSFET’s. MOSFET devices equipping switching power converters gen- erally do not operate in the apparent second breakdown area for a time sufficient to activate current focusing mechanisms, that, consequently, do not affect normal operation of the devices. However, MOSFET’s can be forced to function for a relatively long time in the apparent second breakdown area by activation of the current limiting circuits. In such cases the robustness of the devices to current focusing phenomena is essential to avoid a complete converter failure. Analysis of the second breakdown mechanism in modern low voltage power MOSFET devices is of primary concern to en- sure the reliability of switching converters. In the present paper it is demonstrated that the occurrence of second breakdown phe- nomena in low voltage power MOSFET’s paradoxically is re- lated to the same technology improvements that allowed to in- crease the current capability in latest generation devices. In fact, the thermal instability at low drain current is strictly related to the progressive scaling down of power MOSFET die size. As a consequence in modern devices the rated current is reached with a gate voltage very close to the threshold voltage. This extends the range of drain current values in which the temperature coef- ficient is positive, up to include in some cases the rated point. The failure mode analyzed in the present paper is of great concern in the design of future low voltage power MOSFET devices, since any further improvement on current density will be paid in terms of SOA extension and device robustness. The paper starts with a deep examination of unexpected fail- ures of low voltage power MOSFET at low drain current. Close correlation between unexpected failures and key features of the device technology is then carried out by mean of experimental results and theoretical considerations. II. FBSOA OF POWER MOSFET DEVICES It is commonly assumed that, as MOSFET’s are majority car- rier devices, they are immune from thermal runaway and current focusing phenomena causing the second breakdown in bipolar devices. Some second breakdown phenomena can theoretically occur at high drain voltages [2]–[4], caused by the activation of 0885-8993/00 $10.00© 2000 IEEE