The Role of the Parasitic BJT Parameters on the Reliability of New Generation Power MOSFET during Heavy Ion Exposure F. Velardi 1 , F. Iannuzzo 1 , G. Busatto 1 , A. Porzio 1 , A. Sanseverino 2 , G. Currò 3 , A. Cascio 3 , F. Frisina 3 1 D.A.E.I.M.I., Università degli Studi di Cassino, 2 D.I.E.T., Università degli Studi di Napoli, 3 ST-Microelectronics, Catania I. INTRODUCTION In this paper we brefly describe an experimental study of the role of the parasitic transistor on the charge generation phenomenon observed during the impact of heavy ions on medium voltage power MOSFET [1-5]. We observed that new generation power MOSFETs reveal, during single event impact, a premature damage of the gate structure before a typical burnout occurrence. The damage is related to the activation of the parasitic BJT but hasn’t to be interpreted as a damage of the drain structure. The damage is detected by an increase of the gate leakage current, but can’t be confused as a typical gate rupture event. Although the gate leakage current seems to recover its original value, the damage is irreversible and leads the gate oxide to the breakdown when the device is subsequently biased. The experimental procedure showed that in this damage mechanism the activation of the parasitic BJT plays a relevant role, so the generated charge and the device robustness could be influenced by the epi- layer parameters. II. THE ROLE OF THE BJT In order to study the role played by the parasitic BJT we performed a comparison between two specifically constructed prototypes having different epi-regions. Both of them have the typical surface lay-out used in 250V VDMOSFET. We used also two analogous diode structures obtained by not performing the source diffusion technological step during their construction. The main parameters of the tested devices are reported in Table.1. MOSFET TYPE SURFACE LAY-OUT EPI-LAYER THICKNESS DIODE A SAME HIGH D A B SAME LOW D B TABLE 1: The prototypes tested Irradiation experiments had been performed at the Laboratori Nazionali di Legnaro – INFN – ITALY, to best simulate the interaction with cosmic rays we have chosen, for the experiments, ions of bromine at 250 MeV [6, 7]. In Fig.1 the comparison between the mean value of the charge generated by the A-type prototype and by its analogous diode D A is reported as a function of the bias voltage. The surplus of charge generated by the MOSFET is attributed to the activation of its parasitic BJT. The A- type MOSFET exhibits gate damages during the exposure for a bias voltage applied V DS =75 V at the point marked by a star point in the Fig.1. The corresponding diode D A , didn’t show any damage up to the maximum blocking voltage. It is worth to outline that diode D A had on its surface the gate structure like the corresponding MOSFET. The diode robustness proves the crucial role played by the parasitic transistor in the failure mechanism detected in the MOSFET. The comparison of the equivalent drain charges versus the bias voltage, obtained for the two prototypes with different epi-thickness under the same irradiation conditions, is reported in Fig.2. FIG.1: Charge vs voltage bias, A-type MOSFET and D A diode FIG.2: Charge vs voltage bias, A-type and B-type MOSFET The generated charge for the devices with smaller thickness is larger than the other devices. The amount of the generated charge is proportional to the current peak of the corresponding pulse, which largely depends on the activation of the parasitic BJT that takes place during the impact with the energetic particle. In this situation the BJT operates in a dynamic condition that is very similar to the high field base push out operation. As a matter of fact the impacting particle causes the formation of a high density electron holes plasma that extends within the epitaxial layer for a depth equal to the range of the particle. In this plasma region the electric field is very small and the LNL Annual Report 2004