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Guard bands minimise MOSFET failures in space (Part 2)

Posted: 24 Dec 2015 ?? ?Print Version ?Bookmark and Share

Keywords:guard band? power MOSFETs? radiation? ATE? ICs?

In Part 1, I tackled the need to choose the correct guard band when selecting power MOSFETs for use in spacecraft power modules. To properly select devices, it's important to check the result of tests where the IC is exposed to radiation at different test conditions and measure how radiation affects RDSON. Figure 3 shows the effects of radiation on RDSON, which causes in increase in VGS.

In Test power ICs to withstand radiation, I evaluated the variation of the RDSON due to the radiation exposure as:

Where A is the active area of the channel conducting the electrical charge inside the MOSFET and ?n is the electronic mobility of electrons inside the power MOSFET. All the remaining terms in the equation can be evaluated by means of a check of the radiation test result, for example RDSON or VGSth can be evaluated by checking the respective radiation curves, which show the variation of the RDSON and VGSth parameters at different levels of impacting radiation (figure 3 and figure 4).

Figure 3: The radiation curve of RDSON of a power MOSFET changes depending on the amount of radiation exposed to the device. The value of RDSON directly affects VGS.

Figure 3: The radiation curve of the threshold voltage of a power MOSFET shows how RDSON changes at different total dose conditions.

Finally, the values of VDSON, ID, VGS, and VGSth in the above equation can be measured directly by the automating testing equipment, here below is reported a testing code to measure VGSth of a power MOSFET. You can use the code below on ATE such as the LTX/Credence ASL1000 series to program guard bands.

By insulating the Qox term from the equation above, we can evaluate the variation of the charge trapped inside the gate oxide of the power MOSFET caused by radiation:

From this equation, we can find the radiation condition of total dose that generates the maximum variation of Qox; that will be the worst case of radiation impacting the IC.

By evaluating the variation of all of the electrical parameters corresponding to the worst case condition, we set the guard-band in the test limits, for example of the RDSON test, by selecting during the testing process the dice whose value of RDSON is in the range:

The procedure can be repeated by varying the type of heavy ion of the impacting radiation. Some ions utilised for radiation purposes are Kr, Xe, Ar at different energies.

The radiation tests and the packages that protect the ICs from radiation are very expensive. These costs are justified by the importance to guarantee that an aerospace module is effective and reliable over a long time.

About the author
Paolo Scalisi is senior test engineer at STMicroelectronics Italy.

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