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What Do You Need for High-Rel & Rad-Hard Applications? Part 3

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):

Figure 1

Recombination of holes trapped in the gate oxide and electrons flowing inside the source-drain channel.

Recombination of holes trapped in the gate oxide and electrons flowing inside the source-drain channel.

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.

Figure 2

The VGS(th) test for a MOSFET power switch.

The VGS(th) test for a MOSFET power switch.

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):

Figure 3

Recombination of electrons and holes trapped in the oxide layer with holes and electrons respectively present in the base and the emitter of the BJT.

Recombination of electrons and holes trapped in the oxide layer with holes and electrons respectively present in the base and the emitter of the BJT.

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):

Figure 4

The β test for a BJT power switch.

The β test for a BJT power switch.

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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7 comments on “What Do You Need for High-Rel & Rad-Hard Applications? Part 3

  1. etnapowers
    December 25, 2013

    Recently I have engineered an IGBT for a submarine application by utilizing a radiation hard package, this is a possible utilization of a qualified radiation hard product.

  2. etnapowers
    December 26, 2013

    The IGBT radiation hard qualified are utilized also for the implementation of power modules in automotive applications that requires a high robustness to temperature stresses, high switching frequency and current capability and an easy driving for the gate terminal ,the hermetic package fits very well for this type of application

  3. samicksha
    December 27, 2013

    If the circuit application applies a reverse voltage to the IGBT, an additional series diode must be used.

  4. etnapowers
    December 28, 2013

    Yes that's correct samicksha, it also depends on the reverse voltage stopping capability of the IGBT.

  5. etnapowers
    December 28, 2013

    Usually a parallel diode of freewheeling is required to dissipate the reverse current when the application requires an high current value to be conducted in a branch of a power module, being the IGBT reverse biased.

  6. etnapowers
    December 28, 2013

    When a freewheeling diode paralleled to the IGBT is required the diode has to be rad-hard qualified as well. This is not easy because many times the diode is not enough robust to radiation.

  7. Netcrawl
    December 30, 2013

    @etnapowers you're right, IGBT has also some advantages over Power MOSFET and BJT, IGBT has a superior current conduction capability and excellent forward and reverse blocking capabilities, plus it can easily controlled in high voltage and hig current applications.

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