In the previous part of this series, we were looking at the effects of irradiation on a power FET and noted how it can affect the gate-source turn-on threshold. See Figure 1 (repeated here from the previous part of this series):
Since irradiation can affect the gate threshold voltage, you should test the irradiated parts to see what their new gate threshold voltage is. See Figure 2.
The effect of radiations depends on the IC considered. For example, the failure mechanism of another type of power switch, the power bipolar junction transistor (BJT) due to the ionizing radiation effect is different respect the failure mechanism of the power MOSFET.
The basic principle of operation of the BJT power switch (for example here we deal a NPN transistor) is a controlled flow of current between the emitter to the collector through the base. Not all the electrons emitted from the emitter will reach the collector, due to a recombination in base. This recombination of electrons inside the base creates a leakage base current and the current gain of the transistor is defined as:
After irradiation of the BJT device has been performed, some e− /p+ couples appear inside the oxide (see Figure 3). An additional parasitic current appears. It is due to the recombination of the additional charges. These are the electrons/holes present in a large number inside the emitter terminal and the base terminal (respectively):
This radiation effect makes, hence, the current gain decreasing:
This results in a new current gain value of the power BJT, so a DC current gain test is recommended, to monitor the effect of the radiation on the switch (see Figure 4):
Another power switch that is widely utilized in the integrated IC circuits, especially in power modules, is the power switch insulated gate bipolar transistor (IGBT). It can be considered a combination of a power BJT and a power MOSFET. To monitor the effect of the radiation on the switch, a DC current gain test and a threshold voltage test is recommended (again, see Figures 3 and 4, above).
Did you experience anything like that? Do you think the test performed before and after irradiation, regarding all the electrical parameters that might be sensitive to the irradiation effects, is effective to check the robustness of the IC to the aerospace environment?
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