Single-Event Gate Rupture in Power MOSFETs: A New Radiation Hardness Assurance Approach

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Almost every space mission uses vertical power metal-semiconductor-oxide field-effect transistors (MOSFETs) in its power-supply circuitry. These devices can fail catastrophically due to single-event gate rupture (SEGR) when exposed to energetic heavy ions. To reduce SEGR failure risk, the off-state operating voltages of the devices are derated based upon radiation tests at heavy-ion accelerator facilities. Testing is very expensive. Even so, data from these tests provide only a limited guide to on-orbit performance.

   In this work, a device simulation-based method is developed to measure the response to strikes from heavy ions unavailable at accelerator facilities but posing potential risk on orbit.  This work is the first to show that the present derating factor, which was established from non-radiation reliability concerns, is appropriate to reduce on-orbit SEGR failure risk when applied to data acquired from ions with appropriate penetration range.  A second important outcome of this study is the demonstration of the capability and usefulness of this simulation technique for augmenting SEGR data from accelerator beam facilities.  

   The mechanisms of SEGR are two-fold:  the gate oxide is weakened by the passage of the ion through it, and the charge ionized along the ion track in the silicon transiently increases the oxide electric field.  Most hardness assurance methodologies consider the latter mechanism only.  This work demonstrates through experiment and simulation that the gate oxide response should not be neglected.  In addition, the premise that the temporary weakening of the oxide due to the ion interaction with it, as opposed to due to the transient oxide field generated from within the silicon, is validated.  Based upon these findings, a new approach to radiation hardness assurance for SEGR in power MOSFETs is defined to reduce SEGR risk in space flight projects.

   Finally, the potential impact of accumulated dose over the course of a space mission on SEGR susceptibility is explored.  SEGR evaluation of gamma-irradiated power MOSFETs suggests a non-significant SEGR susceptibility enhancement due to accumulated dose from gamma rays.  During SEGR testing, an unexpected enhanced dose effect from heavy-ion irradiation was detected.   We demonstrate that this effect could be due to direct ionization by two or more ions at the same channel location.  The probability on-orbit for such an occurrence is near-zero given the low heavy-ion fluence over a typical mission lifetime, and did not affect SEGR susceptibility.

  The results of this work can be used to bound the risk of SEGR in power MOSFETs considered for insertion into spacecraft and instruments.